biochem exam 4 Flashcards
where do LCFAs come from?
released from adipocytes then taken up and converted to acetyl CoA and make NADH and FADH2
how are LCFAs transported?
because they are insoluble, they must be bound to fatty acid binding proteins for transport
what is the significance of serum albumin?
it transports LCFA and can bind up to 6 FA chains
has non-specific binding capacity for several hormones and drugs
how can fatty acids enter cells?
via saturable binding and free diffusion
saturable binding means that a binding protein or receptor can only bind a limited amount of ligand or substrate until all available binding sites are occupied
what happens once the fatty acids have entered the cells?
they are bound to fatty-acid binding proteins that facilitates their transport to the mitochondria, ER, and or peroxisomes
explain fatty acid activation
FA must be activated to ACYL-CoA DERIVATIVES which is facilitated by acyl-CoA synthetase
FA attacks a phosphate from ATP making fatty acyl AMP (via fatty acyl CoA synthetase) and pyrophosphate (hydrolysis of pyrophosphate is favorable which drives this reaction forward) –> CoASH binds to the fatty acid, releasing the bound AMP via another fatty acyl CoA synthetase –> makes a FATTY ACYL CoA molecule
because AMP is generated, reaction uses equivalent of 2 ATP
what is significant about fatty acyl-coA synthetase?
there are at least 4 isoforms that have different affinities for FA of different chain lengths, located in different membranes in cell
VLC, LC, MC, SC
what are the fates of activated fatty acyl-CoA?
it can be converted into ENERGY (via beta-oxidation and ketogenesis), STORAGE (TG), and MEMBRANE LIPIDS (phospholipids and sphingolipids)
where are the locations of the acyl-CoA synthetase when converting fatty acyl-CoA into energy?
mainly in the mitochondria and peroxisomes
- PEROXISOMAL and ER acyl-CoA synthetase recognizes VLCFA
- MITOCHONDRIAL as well as ER acyl-CoA synthetases in all tissues recognize LCFA (located on the outer membrane)
- MCFA acyl-CoA synthase is found in the MITOCHONDRIAL MATRIX of many cell types
- ACETYL-CoA synthetase for SCFA is found in the CYTOSOL of many cell types
explain transport of FA into the mitochondria
acyl-CoA synthetase will activate FA using CoA and ATP, releasing AMP and PPi –> fatty acyl-CoA is transported from the cytosol into the outer mitochondrial membrane –> carnitine:palmitoyl-transferase I (CPTI) cleaves CoA off and binds a carnitine making FATTY ACYLCARNITINE –> fatty acylcarnitine can cross the inner mitochondrial membrane into the matrix via carnitine:acylcarnitine translocase (secondary active transport, antiport; RATE LIMITING STEP) –> CPTII cleaves off the carnitine (transported back to the intermembrane space by carnitine:acylcarnitine translocase for use by CPTI) and rebinds the CoA making fatty acyl-CoA that can now undergo beta-oxidation
where is carnitine found?
in many dietary sources like RED MEAT and can be synthesized from LYSINE by the body
deficiencies are RARE
what happens when there’s genetic defects in CPTI and CPTII?
impairs transporter that facilitates entry of carnitine into muscle cells = PRIMARY CARINITINE DEFICIENCY will cripple FA metabolism
defect in CPTI will cause build up of fatty acyl-CoA in the cytosol and intermembrane space, unable to bind carnitine and convert it to fatty acylcarnitine (build up of fatty acy-CoA may stimulate lipogenesis and can lead to fatty liver disease)
defect in CPTII will cause build up of fatty acylcarnitine in the matrix, unable to cleave carnitine and make fatty acyl-CoA for beta-oxidation, also decreases the amount of carnitine to be used by CPTI (can lead to hypoglycemia)
what are the consequences of L-carnitine and choline?
comes from RED MEAT and can be metabolized by gut microbes to make trmethylamine (TMA) which can be inhibited by antibiotics and allicin (from garlic)
if not inhibited, it will eventually lead to decreases reverse cholesterol transport (RCT) = develop hypercholesterolemia and decrease bile acid synthesis (more cholesterol build up in the body since bile acid is the body’s main source of ridding itself of cholesterol) –> increases risks of atherosclerosis
what is the beta-oxidation spiral?
beta-carbon is oxidized to a carbonyl after the alpha-beta bond is cleaved, producing acetyl CoA
process occurs many times to produce many acetyl-CoA from even-chain-length FA
ex: palmitoyl CoA (16C) makes 8 acetyl CoA
describe the process of beta-oxidation spiral
- fatty acyl CoA undergoes OXIDATION via acyl-CoA DH, making FADH2 (transfers e- to ETC without dissociating from acyl-CoA DH)
- fatty acyl CoA now has a double bond between the alpha and beta carbons = fatty enoyl CoA which undergoes HYDRATION via enoyl CoA hydratase
- beta-hydroxy acyl CoA is formed and undergoes OXIDATION via beta-hydroxy acyl-CoA DH using NAD+ and making NADH (enters ETC)
- beta-keto acyl-CoA is formed and undergoes THIOLYSIS via beta-keto thiolase, using CoASH to make fatty acyl-CoA (can re-enter beta-oxidation) and acetyl CoA (enters TCA or used for synthesis of KB)
what is special about acyl-CoA DH?
there are 4 isozymes of acyl-CoA DH, each with a different substrate specificity
- VLCAD
- LCAD
- MCAD
- SCAD
what is the last step of the beta-oxidation spiral?
the beta-keto fatty acyl-CoA = acetoacetyl-CoA which is important in CATABOLISM of some amino acids and in the SYNTHESIS of KB and CHOLESTEROL
acetoacetyl-CoA can be converted to acetyl CoA, but if the acetyl CoA concentrations are high, the reaction can be reversed by THIOLASE to make acetoacetyl-CoA which can be converted into acetoacetate (KB) –> can be used in fasting and starved state by muscles and brain
what is significant about odd-chain-length FAs?
undergoes multiple rounds of beta-oxidation, resulting in the formation many molecules of acetyl-CoA and one molecule of PROPIONYL-CoA
why is propionyl CoA important? explain this process
propionyl CoA can be converted into succinyl CoA which can enter the TCA cycle (anaplerotic process)
propionyl CoA is converted to D-methylmalonyl Coa via PROPIONYL CoA CARBOXYLASE (uses biotin as a cofactor to transfer CO2 of HCO3- as carboxylate group)
D-methylmalonyl CoA is epimerized via methylmalonyl Coa to L-methylmalonyl CoA
L-methylmalonyl CoA is converted to succinyl CoA via methylmalonyl CoA MUTASE (uses B12 to facilitate radical rearrangement, this enzyme is one of two B12 dependence enzymes in body)
explain unsaturated FAs
beta-oxidation of unsaturated FAs requires assistance of two enzymes = enoyl CoA isomerase and 2,4-dienoyl CoA reductase (also uses NADPH)
basically requires more enzymes for beta-oxidation
most dietary fatty acids are…
long chain
as are the FAs synthesized from excess fuel by the liver and stored in adipocytes
what are other types of dietary fatty acids?
some fatty acids synthesized in EXTRAHEPATIC TISSUES are VLC, MC, or SC and DO NOT FOLLOW STANDARD PATHWAY FOR DEGRADATION/BETA OXIDATION
explain oxidation of SCFAs
they are more WATER SOLUBLE than LCFAs so they are not stored in the adipose tissue but are rather transported directly to the liver (recall that they can enter directly into circulation) or other organs for oxidation
SCFAs are produced primarily by fermentation of DIETARY FIBER by gut microbes (acetate, propionate, butyrate)
- provide health benefits by stimulation immune system and improving metabolic health (anti-diabetes, anti-obesity)
- also primary fuel source for COLONOCYTES (also decreases local inflammation) and can be oxidized by the liver)
SCFAs DO NOT RELY ON CARNITINE TRANSPORT
explain oxidation of MCFAs
also water soluble
MCFA enter the mitochondria via MONOCARBOXYLASE TRANSPORTER then are activated by fatty acyl-CoAs and undergo beta-oxidation (dairy and coconut and palm oils are rich in MCFAs)
acyl-CoA synthetase (in mitochondrial matrix) that recognize MCFAs also recognize other compounds (including several pharmaceutical drugs) that contain CARBOXYL GROUPS –> synthetase forms CoA thioesters, CoA esters converted to acylglycines and excreted
carboxyl –> thioester (via MC acyl-CoA synthetase using CoASH and ATP, releasing AMP and PPi) –> acylglycine (via glycine N-acyltransferase, using glycine and releasing CoASH)
ALTERNATIVE ROUTE IF CPTII IS IMPAIRED
What happens when there’s a disorder in beta-oxidation?
can cause accumulation of acylcarinitines and acylglycines (from MCFAs) in the serum or urine
acylglycine produced by glycine N-acyltransferase
acylcarnitine produced by reverse of CPTII
both reactions are reversible
disorders that cause an increase in mitochondrial [fatty acyl-CoA] will drive formation of fatty acylcarnitines and acylglycines, freeing CoASH for other uses in the cell (primarily TCA where it can be used to make succinyl CoA)
what other organelles can fatty acids be oxidized in?
unusual fatty acids and hydrophobic xenophobics (containing carboxyl groups) are metabolized in cellular PEROXISOMES or in the ER
what kind of fatty acid oxidation do peroxisomes catalyze?
peroxisomes catalyze alpha and beta oxidation
they contain acyl-CoA synthetase that recognizes VLCFA, LCFA, and PHYTANIC ACIDS
fatty acids can be oxidized or used in the synthesis of plasmalogens (class of glycerophospholipids that are important for maintaining cell function and protecting against oxidative damage)
what kind of fatty acid oxidation does the ER catalyze?
ER contains enzymes that catalyze omega-oxidation
they contain acyl-CoA synthetase that recognizes LCFA (which undergo omega-oxidation) or are used in the synthesis of membrane phospholipids or TG
method is used when there are disorders in beta-oxidation
what is the order of substances in the cholesterol synthesis pathway?
acetyl CoA –> acetoacetyl CoA –> HMG CoA –> mevalonate –> activated isoprenes (isopentyl PP and dimethylallyl PP) –> squalene (geranyl PP and farnesyl PP) –> lanosterol –> cholesterol
how does peroxisomal beta-oxidation differ from mitochondrial?
- carnitine is not needed for transport across the membrane
- the electron acceptor for the first step is MOLECULAR OXYGEN
- it is regulated only by SUBSTRATE AVAILABILITY
HOWEVER, acetyl CoA, SCFA CoA, and MCFA CoA are all converted to carnitine derivatives before moving into the mitochondria for completion of oxidation (SCFA CoA and MFCA CoA transferred via COT - octanoyltransferase to SCFA-carnitine and MCFA-carnitine, acetyl CoA transferred via CAT - carnitine acetyltransferase to acetyl-carnitine)
what catalyzes the first step in peroxisomal beta oxidation?
FAD-containing oxidase (gives its 2 H2 from FADH2 to O2) that transfers electrons to O2 and forms H2O2
The VLCFA CoA is transported into the peroxisome; it produces H2O2 (via the process above), NADH (which goes to the malate-aspartate shuttle), acetyl CoA, SFCA CoA, and MCFA CoA
explain the methods of regulation of beta oxidation
- FA are released from adipocytes in response to hormones (glucagon, epinephrine, adrenocorticotropic) that signal fasting or increased demand (like exercising or under stress)
- CPT I is INHIBITED BY MALONYL CoA (synthesized from acetyl CoA via acetyl-CoA carboxylase - ACCase –> inhibited by phosphorylation by AMP-dependent protein kinase in muscles and cells, activated by insulin), ACCase catalyzes the first step in FATTY ACID SYNTHESIS, inhibition of CPTI by malonyl Coa prevents simultaneous synthesis and degradation of FA
- when ATP/ADP is HIGH, NADH/NAD+ and FADH2/FAD also HIGH –> disfavor oxidative processes like beta-oxidation (because you already have a lot of energy, don’t want to make more)
why is peroxisomal alpha oxidation used?
it is important for the metabolism of PHYTANIC ACID (degradation product of chlorophyll found in TISSUES of herbivores and dairy products)
beta-oxidation of phytanic acids are NOT POSSIBLE because the presence of a methyl group of the beta carbon, sterically hindering it
it stops the beta-oxidation at beta-hydroxy DH
explain how peroxisomal alpha oxidation occurs?
phytanic acid hydroxylase introduces hydroxyl group on the alpha carbon (with no methyl group) –> then oxidizes the hydroxyl to a ketone and the bond between the two carbonyl carbons is broken
product = PRISTANIC ACID, which can now undergo beta oxidation –> producing many molecules of acetyl CoA, propionyl CoA, and isobutyryl CoA
why is omega-oxidation important?
process occurs in the ER and is only a minor physiological relevance
it is important when DISORDERS OF BETA OXIDATION are present, causing intracellular levels of fatty acids to increase
what are ketone bodies synthesized from?
acetyl CoA in the liver during fasting and starved state
fast progresses –> amount of free fatty acids in serum increases because of increased lipolysis, liver converts some of these FA to KB
after a few days, muscle stops using KB, increasing their concentration, high enough for the brain to use
what occurs in omega-oxidation?
free fatty acids in the ER are oxidized at the methyl end, producing a DICARBOXYLIC ACID which can be further metabolized by beta-oxidation in the MITOCHONDRIA
medium-chain water-soluble dicarboxylic acids may be ELIMINATED, eventually showing up in urine (if this shows up in urine, it indicates that there’s too much FA or beta-oxidation is impaired)
what is acetyl CoA converted to for metabolism KB?
acetoacetate and beta-hydroxybutyrate which can enter circulation and converted back into acetyl CoA for use in the TCA of extrahepatic tissues
beta-hydroxybutyrate ratios are always higher than acetoacetate (especially in a state where [NADH] levels are high because it will reduce the acetoacetate to beta-hydroxybutyrate)
what is omega-oxidation catalyzed by?
a MIXED FUNCTION OXIDASE that requires O2 and NADPH
the FA is oxidized to an alcohol, alcohol is oxidized to an aldehyde via alcohol DH, aldehyde is oxidized to a carboxylic acid via aldehyde DH —> makes dicarboxylic acid on both ends of FA
describe synthesis of ketone bodies
made when [acetyl CoA] is high
Two acetyl CoA is converted to acetoacetyl CoA via THIOLASE –> HMG-CoA via HMG CoA synthase –> acetoacetate via HMG CoA lyase –> beta-hydroxybutyrate and acetone when NADH/NAD+ is high then acetoacetate will be reduced to beta-hydroxybutyrate and both are released into circulation
some acetoacetate will undergo SPONTANEOUS DECARBOXYLATION to form acetone
how is beta oxidation regulated?
- by the RATE OF RELEASE of fatty acids from adipocytes
- by the RATE OF TRANSPORT of fatty acylcarnitines into the mitochondria
- by intracellular NADH/NAD+ and FADH2/FAD ratios
what enzyme does the liver lack that prevents it from using KB as a fuel source?
succinyl CoA:acetoacetate CoA transferase
explain oxidation of ketone bodies
in extrahepatic tissues (NOT RBC) that lack mitochondria –> beta-hydroxybutyrate and acetoacetate are converted back to acetoacetyl CoA and then acetyl CoA
BETA-HYDROXYBUTYRATE to ACETOACETATE via beta-hydroxybutyrate DH (using NAD+ and making NADH for ETC) –> ACETOACETATE to ACETOACETYL CoA via succinyl CoA:acetoacetate CoA transferase - uses a succinyl CoA and makes succinate which costs an ATP (LIVER LACKS THIS ENZYME which is why it can not use KB for fuel) –> ACETOACETYL CoA to 2 ACETYL CoAs via thiolase
how is KB synthesis regulated?
- decreased insulin/glucagon ratio (MORE GLUCAGON = fasting or stress) = increased availability of FA for beta-oxidation in liver
- decreased insulin/glucagon ratio = inactivation of ACCase (phosphorylated) and decrease [malonyl CoA] –> activating CPTI for mitochondrial beta-oxidation
- [ATP] increases because of flux through beta-oxidation, [NADH/NAD+] also increase = SLOW beta-oxidation and TCA causing build-up of MALATE (favors reverse rxn of OAA to malate) which can be exported to the cytosol for gluconeogenesis
- [acetyl CoA] increases due to inability to enter TCA –> drives ketone synthesis of KB
where are FA synthesized?
mainly in the liver, can also occur in adipocytes and lactating mammary glands
glycolysis generates acetyl CoA for FA synthesis and the glycerol backbone for synthesizing TG from FA chains (acetyl CoA can also be generated from excess AA)
pyruvate –> acetyl CoA and OAA make citrate that can leak out of mitochondria into the cytosol and be converted to acetyl CoA then malonyl CoA via ACCase (using NADPH) then palmitate and lastly, FA CoA which combines with glycerol 3-P from DHAP in glycolysis to make TG that are incorporated into VLDL and secreted
where does FA synthesis occur?
in the cytosol but the acetyl CoA needed for this process is generated in the mitochondria HOWEVER acetyl CoA is transported to the cytosol as CITRATE
pyruvate converted to OAA (via pyruvate carboxylase using B7) and acetyl CoA (via pyruvate DH) –> both combine to form citrate –> citrate leaks out to cytosol where citrate lyase converts it to OAA (gets recycled) and acetyl CoA (can be used for FA synthesis)
why is the malic enzyme important?
it generates most of the NADPH needed for FA synthesis (rest comes from PPP)
it is involved in the OAA recycling process to pyruvate (cytosolic malate DH using NADH to convert OAA to malate)
what must acetyl Coa be converted to FIRST to enter FA synthesis?
it must be converted to MALONYL CoA via ACCase using B7 and ATP
malonyl CoA will allow the use of FA synthase
what is the rate-limiting step in FA synthesis?
acetyl CoA to malonyl CoA by ACCase
what stimulates the conversion of acetyl CoA to malonyl CoA?
the elevation of insulin/glucagon ratio
insulin activates the pyruvate DH, which induces citrate lyase and malic enzyme (produces NADPH) –> also activates ACCase to convert acetyl CoA to malonyl CoA
high energy = citrate accumulates because isocitrate DH is inhibited by NADH
explain the regulation of ACCase
insulin activates PP1 which dephosphorylates ACCase to its active form (citrate also activates ACCase)
AMP-dependent protein kinase phosphorylates ACCase to its inactive form due to feedback inhibition of palmitoyl CoA (if there’s too much palmitoyl Coa = enough energy has been produced, no need to make more)
what is FA synthase?
a homodimeric protein with multiple active sites
growing FA chain is linked to the acyl carrier protein (ACP) via a phosphopantetheinyl arm (B5), passing the substrate from one active site to the next
how is the FAS cycle initiated?
begins with covalent attachment of ACETYL group from acetyl CoA to a CYSTEINE residue
- acetyl group from acetyl CoA is transferred to active site cysteine residue (first attaches to pantetheinyl arm then gets transferred to cysteine residue)
- malonyl CoA transfers its malonyl group (3 carbon, 1 is carboxylate) to pantetheinyl arm of ACP after acetyl group is transferred
- malonyl CoA undergroes beta-DECARBOXYLATION (of the carboxylate), leaving the alpha-carbanion that will attach to the cysteine-linked acetyl group making a BETA-KETOACYL GROUP (move it to the P arm, leaving the cysteine residue free)
explain the reduction of the beta-ketoacyl group
after the malonyl CoA is decarboxylated, it attaches the cysteine-linked acetyl group to the P arm, creating the beta-ketoacyl group (leaving the cysteine residue free)
this beta-ketoacyl group undergoes three reactions to be reduced to an alkane (reverse of beta-oxidation)
- beta-keto group is reduced, using NADPH, to an alcohol
- alcohol is dehydrated, forming double bond (alkene)
- double bond is reduced to an alkane
this FA chain is then transferred to the cysteine residue –> this whole process is then repeated until palmitate (C16) is formed, and then THIOESTERASE hydrolyzes the FA chain from ACP (P arm)
what inhibits CPTI?
malonyl CoA (product of ACCase)
prevents wasteful cycling of newly synthesized FA chains into mitochondria for beta-oxidation and facilitates transport of FA into mitochondria when ACCase activity is low
why is elongation of FA chain needed?
FAS only produces palmitate (16C), and some tissues need longer FA for their own use
explain the process of elongation of the FA chain
FAS produces palmitate, which can be activated by palmitoyl Coa and elongated in the ER using the same reactions as the ones that catalyze FAS in the cytosol (acetyl group attaches, malonyl group attaches, decarboxylation, beta-ketoacyl group, formation of palmitate)
HOWEVER, enzymes ELONGASES are not part of multifunctional complex and FA chain remains attached to CoA instead of being transferred around
what are the predominantly produced FA in the liver?
palmitate and stearate
palmitoyl CoA is reduced to an alcohol, alcohol reduced to alkene, alkene is reduced to alkane, producing stearoyl CoA
why is elongation important for the brain and lactating mammary glands?
the brain contains additional elongation capacities to produce NEURON-SPECIFIC LIPIDS
lactating mammary glands contain SOLUBLE THIOESTERASE that terminate the FAS reaction sequence early, generating SC and MC FA for milk
explain the process of desaturation
desaturation is the introduction of double bonds into fatty acid chains
this process is catalyzed by desaturases (members of mixed function oxidases like omega-oxidation) and occurs in the ER, REQUIRES O2 and NADH
saturated fatty acyl-CoA and NADH reduces O2 to 2H2O (O2 is an oxidizing agent that is reduced to water), producing a monounsaturated fatty acyl-CoA
what carbons does desaturation occur at?
carbons 9, 6, or 5
what FA cannot be synthesized by the body?
linoleic acid (omega 6) = important for synthesis of ARACHIDONIC ACID (precursor of eicosanoid hormones)
alpha-linolenic acid (ALA or omega 3) = important for synthesis of EPA (another precursor of eicosanoid hormones)
these are essential fatty acids that need to be obtained through our diet since the body CANNOT synthesize them because of where double bonds can be made
where do linoleic acid and ALA come from?
produced by plants and are obtained in diet primarily in VEGETABLE OILS (linoleic acid) and LEAFY GREENS, and FISH OILS (ALA)
where are TGs synthesized?
in the liver and adipocytes started with glycerol 3-P from glycerol via glycerol kinase and from DHAP in glycolysis
unlike intestinal epithelial cells which start with 2-MG (pancreatic lipase)
how is glycerol 3-P made in adipocytes?
during the fed state, glucose is taken into adipocytes by GLUT4 (stimulated by insulin) and used to generate DHAP, which is then reduced to glycerol 3-P
LACK GLYCEROL KINASE
how is glycerol 3-P made in hepatocytes?
phosphorylating glycerol using glycerol kinase (not expressed in adipocytes) or from glucose via DHAP
explain the process of synthesis of TAG
begins with glycerol 3-P which combines with 2 FA CoA (FA chains MUST be activated to acyl-CoAs prior to addition to the glycerol backbone, chains may be NEWLY SYNTHESIZED OR BE EXCESS CHAINS BROUGHT TO THE LIVER FROM ADIPOCYTES) –> produces phosphatidic acid (intermediate common to synthesis of TAGs and glycerophospholipids) –> removal of phosphate group makes diacylglycerol –> addition of FA CoA makes TAG which can be released from the liver as VLDL or stored in adipocytes
explain synthesis of VLDL
synthesis of VLDL is similar to chylomicrons; major DIFFERENCE is that ApoB-100 is the apoprotein (not ApoB-48)
Apoproteins are synthesized in the RER with the help of MTP (recall that a deficiency in MTP leads to abetalipoproteinemia), small aggregates of apoproteins, phospholipids, and TAGs are formed
apoprotein complexes move to GOLGI APPARATUS where additional TAG and other lipids are added to form VLDL
Apoprotein made in the RER, moves to golgi apparatus, secreted from golgi as secretory vesicle, and releases VLDL into circulation (needs ApoCII and ApoE to become mature)
how are TAGs stored in adipocytes?
both chylomicrons and VLDL become mature due to the addition of ApoCII and ApoE; ApoCII stimulates LPL which breaks down mature chylomicrons and VLDL to make FA (stored in adipocytes and myocytes) and glycerol (returned to liver)
In fed state –> chylomicrons and VLDL will be delivering FA to adipocytes and myocytes, TAGs hydrolyzed then chylomicrons become chylomicron remnants and VLDL becomes IDL (VLDL remnants)
Recall
- ApoCII activates LPL
- LPL expression and secretion is UPREGULATED by INSULIN (because in fed state you want to break down TG to FA for storage in different tissues)
- adipocytes make glycerol 3-P from glucose (not from DHAP because they can’t use glycerol kinase)
how are FAs released from adipocytes?
hydrolysis of TAG is regulated by HORMONE SENSITIVE LIPASE
hormone-sensitive lipase is activated by phosphorylation catalyzed by PKA; active in fasting state when insulin/glucagon ratio is LOW (high insulin/glucagon = dephosphorylation of hormone-sensitive lipase); PHOSPHORYLATED FORM IS ACTIVE
low insulin/glucagon ratio will activate adenylyl cyclase to convert ATP to cAMP, which will activate PKA to phosphorylate hormone-sensitive lipase to its active form, allowing hydrolysis of TGs (along with other lipases) to 3 FA chains that bind to SERUM ALBUMIN and are transported to tissues while the glycerol is returned to the liver
explain impaired metabolism of Type I diabetes
lack insulin due to the destruction of pancreatic beta cells = failure to release insulin which results in failure of glucose uptake of muscle adipose tissue (mimics STARVED state)
brain will not need KB because there’s enough glucose in the blood to supply its energy, KB will accumulate = DKA
Glut 4 is impaired because there is no insulin to stimulate its activity of transferring glucose into tissues/cells, glucose will accumulate in the blood, leading to HYPERGLYCEMIA (also because of gluconeogenesis due to starved state)
describe the process of acquiring Type I diabetes
exposure to a virus or toxin may begin the process of beta cell destruction in individuals with a genetic predisposition
beta cell destruction will progress over time, leading to decreased production of insulin
insulin production falls below threshold, type 1 diabetes symptoms begin to appear
explain impaired metabolism of Type II diabetes
insulin response is adequate enough because beta cells are still producing insulin, the body is just resistant to it = can prevent massive release of free FA from adipocytes so DKA is rare
HOWEVER, high [glucose] in acute states result in HYPEROSMOLARITY
insulin resistance = there will be more glucose than normal in serum because they’re not being transported into adipocytes and myocytes –> this leads to hyperglycemic and hyperosmolar state (HHS)
insulin resistance is often accompanied by obesity but not always (resistance can also arise from genetics, sedentary lifestyle, and aging)
how can you differentiate between type I and type II diabetes?
lack of insulin due to destruction of beta cells (type I) and still production of insulin from beta cells but the body is insulin resistant (type II)
type I = DKA
type II = hyperosmolar state (or HHH), very strong genetic predisposition
Cholesterol is a precursor for…
steroid hormones (including vitamin D) and bile salts
where is cholesterol obtained and synthesized?
it is obtained from the diet and synthesized in the cytosol and ER of ALL CELL TYPES (all cells have capacity to make cholesterol)
what is the “central hub” of cholesterol metabolism?
the liver
- it processes lipoproteins that carry cholesterol to and from extrahepatic tissues
- synthesizes large quantities of cholesterol for export to other tissues, and bile salts to aid digestion
- excretes cholesterol, bile salts, and phospholipids to intestine (only route for elimination of cholesterol in the body - through feces)
what is important about tissues that produce and excrete steroid hormones?
they require a LARGE quantity of cholesterol (since they are derivatives of cholesterol)
like adrenal glands, testes, and ovaries
what is the common dietary form of cholesterol? and what happens to them?
cholesterol esters that are hydrolyzed by pancreatic esterase prior to uptake of cholesterol and fatty acid by enterocytes
enterocytes also absorb cholesterol released in bile
how is cholesterol absorbed from the intestinal lumen?
through a DIFFUSION-CONTROLLED PROCESS
absorption is inhibited by soluble dietary fibers (because they slow down absorption) and EZETIMIBE or Zetia which inhibits the transporter that moves cholesterol into the liver
how are excess cholesterol and plant sterols eliminated?
unwanted or excess cholesterol and plant sterols can be eliminated from enterocytes and back into the lumen by ABCG5 and ABCG8 (members of ATP-Binding Cassette transporter family)
deficiencies in these transporters results in ELEVATION OF SERUM CHOLESTEROL (hypercholesterolemia) AND PLANT STEROLS (aka phytosterolemia) –> because the transporters are not functioning, they’re not eliminating cholesterol from the serum, causing it to build up and absorption
plant sterols are said to be good because they can help prevent absorption of cholesterol by competing with the transporter, similar effects to EZETIMIBE
why are ABC transporters important?
they play key roles in hepatic drug metabolism and drug resistance in bacteria and cancer cells (resistance to chemotherapy), they also function to just transport things outside of cell (to remove)
recall, this is similar to how we mark drugs for export by adding glycine and glucuronate
how is cholesterol synthesized (basics)?
cholesterol can be synthesized from acetyl-CoA in ALL cells
liver, intestine, adrenal cortex, and reproductive have most active synthesis
cholesterol molecule is highly reduced, requiring NADPH from PPP and malic enzyme
synthesis of cholesterol occurs in CYTOPLASM and will require transport of acetyl-CoA across the mitochondrial membrane in the form of CITRATE
what is the first step of cholesterol synthesis?
begins with synthesis of MEVALONATE from acetyl-CoA
high [acetyl CoA] drives the reaction of beta-oxidation in reverse to make acetoacetyl CoA –> then via HMG CoA synthase (CYTOSOLIC isoform is important in synthesis of cholesterol, MITOCHONDRIAL synthase is important in synthesis of KB), HMG CoA is made –> HMG CoA reductase makes mevalonate (uses 2NADPH and release
statins inhibit HMG CoA reductase = interferes with cholesterol synthesis by decreasing the synthesis of cholesterol because they can’t make mevalonate to continue the process (statins also increase the number of LPL receptors expressed since there is less cholesterol, LPL will decrease cholesterol even more by removing it)
what is the committed/rate-limiting step in cholesterol synthesis?
step catalyzed HMG CoA reductase (to make mevalonate)
how is the transcription of HMG CoA reductase regulated?
by cellular cholesterol concentrations
1. cholesterol binds SCAP (SREBP cleavage-activating protein in the ER membrane), as intracellular [cholesterol] drops, cholesterol is released from SCAP
2. SCAP/SREBP (sterol regulatory element binding protein) complex moves to GOLGI APPARATUS where proteases (S1P and S2P) cleave SREBP DNA-binding domain
3. SREBP DNA-binding domain moves to the NUCLEUS where it stimulates transcription of genes (INCLUDING HMG CoA reductase)
so if you’re low on cholesterol, HMG CoA reductase genes will be transcribed to drive the synthesis of cholesterol
how is the proteolysis of HMG CoA reductase regulated?
by cellular sterol concentrations
activity of reductase is regulated by AMP-activated protein kinase, glucagon, sterols, and insulin
high levels of cholesterol and bile salts in hepatocytes induce a change in oligomerization state of HMG CoA reductase in ER, resulting in proteolysis
AMP-dependent protein kinase is activated by AMP (cholesterol synthesis is sensitive to cellular energy levels), glucagon, and sterols which will phosphorylate HMG CoA reductase to its inactive form because you are in fasting/low energy state and do not need to synthesize cholesterol
insulin (fed/high energy state) will dephosphorylate HMG CoA reductase to its active form to stimulate cholesterol synthesis
what is the second stage of cholesterol synthesis?
mevalonate is converted to activated isoprene in series of steps that requires A LOT OF ATP
isopentyl pyrophosphate and dimethylallyl pyrophosphate are used in synthesis of CoQ (ETC) and dolichol phosphate (N-linked glycoproteins)
if there is a deficiency in either isopentyl PP or dimethylallyl PP then it can impair ETC and and N-linked glycoproteins
what is the third stage of cholesterol synthesis?
6 activated isoprenes are condensed to form 30-carbon molecule = SQUALENE
dimethylallyl PP and isopentyl PP combine to make GERANYL PP (10 C), geranol was first isolated from geraniums –> another dimethylallyl PP is added and PPi is cleaved from geranyl PP to make FARNESYL PP (15C), farnesol was first isolated from flowers of the Farnese acacia tree –> NADPH and another farnesyl PP is added to make squalene
both geranyl PP and farensyl PP are used in PRENYLATION of membrane-associated proteins, some side effects of statins are linked to deficiencies in protein prenylation
what is the fourth stage of cholesterol synthesis?
squalene is oxidized and then cyclized to form LANOSTEROL which undergoes reactions to produce cholesterol
SQUALENE MONOOXYGENASE reduces one O in O2 to H2O using NADPH, other O is added to C2 to form a REACTIVE EPOXIDE
Cyclase binds squalene epoxide to facilitate the closing on the rings to make lanosterol
about 20 steps are required to convert lanosterol to cholesterol
what are the fates of hepatic cholesterol?
both endogenous and exogenous cholesterol
- small fraction is incorporated into hepatic membranes (for its own use)
- LARGE fraction is converted to cholesterol esters and incorporated into lipoproteins (primarily via VLDL), this cholesterol is taken up by extrahepatic tissues that can’t make enough of their own (ACAT or acyl-CoA-cholesterol acyl transferase will use FA CoA to convert cholesterol to cholesterol ester)
- LARGE fraction is secreted in bile to be reabsorbed from the intestine
- fraction is converted to bile salts and secreted
what is the first step of bile salt synthesis?
it is the rate-limiting step and is the conversion of cholesterol to 7 alpha-hydroxycholesterol
NADPH used in this reaction provides electrons to reduce on of the O in O2 to water and other is added C7
7 alpha-hydroxylase is feedback inhibited by bile salts meaning that if there’s enough bile salts in circulation, it will inhibit 7 alpha-hydroxylase from producing more (NADPH transfers it H to cytochrome P450 which will reduce the O2 to H2O via 7 alpha-hydroxylase)
what is the second step of bile salt synthesis?
7 alpha-hydroxycholesterol is converted to CHENODEOXYCHOLIC ACID and CHOLIC ACID
50% of these bile salts will be deprotonated in the intestinal lumen
explain conjugation of bile salts
conjugation means to stick 2 molecules together
cholic acid and chenodeoxycholic acid are conjugated with TAURINE (derivative of cysteine) OR GLYCINE, decreasing their pKa and making their more effective emulsifying agents
carboxyl group on cholic acid or chenodeoxycholic acid must first be activated with CoA then taurine or glycine is exchanged with CoA producing taurocholic acid or glycocholic acid (if chenodeoxycholic was used it would be taurochenodeoxycholic acid or glycochenodeoxycholic acid)
describe bile salt metabolism
intestinal bacteria DECONJUGATE and DEHYDROXYLATE the bile salts at C7 (which was added to make bile salts) forming secondary bile salts
95% of bile salts enter enterohepatic circulation and are recycled and 5% is eliminated through the feces (primary method of ridding cholesterol from body)
secondary bile salts are reconjugated but NOT rehydroxylated
secondary bile salts are LESS soluble and more likely to be excreted (less likely to be recycled)
what are the secondary bile salts?
deconjugation and dehydroxylation of chenodeoxycholic acid and cholic acid produce deoxycholic acid and lithocholic acid
how many conjugated bile salts are there?
8
how can bile salts cause cholelithiasis?
bile contains high concentrations of cholesterol (in addition to phospholipids and bile salts) –> if more cholesterol enters bile than can be solubilized by bile salts and phospholipids, cholesterol will form stones
what are the 4 common causes for the formation of cholesterol stones?
- gross malabsorption of bile salts from intestine (not recycling enough bile salts and not enough bile salts is being made)
- obstruction of biliary duct (can’t release bile salts to break down cholesterol, causing build-up)
- severe hepatic dysfunction resulting in decreased bile salt synthesis
- increased biliary cholesterol secretion
what is another type of gallstone?
pigment stones
formed from excessive bilirubin in the bile
hemolytic anemia will destroy RBCs which contain hemoglobin that will be released into the bloodstream –> the heme part will be metabolized into bilirubin and, excessive amounts can lead to pigment stones
what is the density and size of lipoproteins in relation to TAG content?
density increases and sizes decreases with TAG content
think of TG as fluffy and cholesterol as hard, so when there’s more cholesterol than TG it will be dense
how are lipids transported?
because they are insoluble in the serum, they are transported via lipoproteins
lipoproteins all have the same basic structure but differ in size, identities of apoproteins on their surface (ApoB-48, ApoB-100, ApoCII, ApoE, ApoAI), and proportions of TAG and cholesterol they carry
why are apoproteins important?
they assemble lipoproteins, activate enzymes involved in lipoprotein metabolism, and act as ligands for specific cell surface receptors
what are the functions of the different types of lipoproteins?
chylomicrons = deliver exogenous lipids (least dense)
chylomicron remnants = return exogenous lipids to the liver
VLDL = deliver endogenous lipids
IDL = return endogenous lipids to the liver and also precursor for LDL
LDL = deliver cholesterol to cells
HDL = reverse cholesterol transport (most dense)
what are the functions of the different types of apoproteins?
ApoAI = in intestine and liver, distribute HDL, activate LCAT (structural component of HDL)
ApoB48 = in intestine, distribute chylomicrons
ApoB100 = in liver, distribute VLDL, IDL, and LDL
ApoCII = in liver, distribute chylomicrons, VLDL, IDL, and HDL (activates LPL)
ApoE = in liver, distribute chylomicron REMNANTS, VLDL, IDL, and HDL (recognized by liver receptors for receptor-mediated endocytosis)
explain chylomicron metabolism
chylomicrons carry exogenous lipids (primarily TAGs but also some cholesterol) from intestine to extrahepatic tissues –> TAGs deplete = chylomicron remnant is processed by liver
intestinal cells secrete nascent chylomicrons –> HDL transfers ApoCII and ApoE to make mature chylomicrons, ApoCII activates LPL and allows break down of TAGs to FA for adipocytes and myocytes as well as glycerol that gets returned to the liver –> ApoCII returns to HDL, become chylomicron remnants where ApoE allows for receptor-mediated endocytosis on the liver (in the liver the TG are further processed)
explain the function of LDL
LDL carries cholesterol esters remaining after the removal of most TAGs from VLDL
50% of LDL particles are taken up by hepatocytes and other tissues by receptor-mediated endocytosis
LDL receptors recognize both ApoE (also in chylomicron remnants and IDL) and ApoB100 (VLDL) and are found in clathrin-coated pits –> LDL receptors are recycled and their contents are hydrolyzed in the lysosomes
explain LDL metabolism
TAGs (from exogenous and endogenous) are packaged into VLDL –> TAG content decreases, they become IDL (that can be taken up by the liver or converted to LDL)
nascent VLDL has ApoCII and ApoE attached, allowing it to activate LPL and break down the TG in VLDL into FA for tissues and into glycerol
DIFFERENT FROM CHYLOMICRONS, VLDL can be converted to IDL (VLDL remnants) –> IDL is endocytosed by hepatocytes via ApoE recognized by receptor-mediated endocytosis and other 50% is recognized by HTGL which removes additional TAG from IDL
TAG-depleted/cholesterol-rich IDL returns ApoCII and ApoE to HDL and become LDL
why are LDL receptors important?
they mediate endocytosis of LDL
they recognize ApoE and ApoB100
N-terminal domain binds LDL (as well as VLDL, IDL, and chylomicron remnants)
C-terminal domain mediates the interaction between the receptor and the clathrin-coated pit
expression of LDL receptor is UPREGULATED BY SRE and SREBP (because the genes encode HMG CoA reductase = drive cholesterol synthesis, needing more LDL receptors to remove cholesterol from circulation)
what is PCSK9?
stimulates lysosomal degradation of LDL receptors
new class of injectable antibodies (aka PCSK9) have potent cholesterol-lowering capacity
PCKS9 binds to epidermal growth factor-like domain of LDLR and stimulates lysosomal degradation (instead of LDLR removing LDL from circulation, LDLR is degraded and LDL builds up) –> HOWEVER antibodies will block PCKS9 from binding to LDLR, allowing LDL to bind and be removed from circulation while also recycling LDLR
what is familial hypercholesterolemia?
caused by deficiencies in LDLR which can lead to elevated serum cholesterol levels and atherosclerosis because cholesterol is not being removed from the serum but is instead building up
there are many classes of familial hypercholesterolemia
people who are homozygous for FH can have serum cholesterol levels around 500-800 mg/dL (normal = 190 mg/dL <)
what are the clinical signs of hyperlipidemia?
xanthomas = lipid accumulates under skin around tendons, joints, and eyes
dyslipidemia
explain the other types of LDL receptors
low-density lipoprotein receptor-related protein (LRP) = binds to ApoE (also some proteins involved in hemostasis), UNLIKE LDLR –> its expression is NOT regulated by cellular [cholesterol] but is UPREGULATED by insulin, consistent with primary responsibility to endocytose chylomicron remnants
scavenger receptors = binds VARIOUS TYPES of molecules, including oxidatively modified LDL (LDL gets exposed to oxidizing agents) –> SR-A1 and SR-A2 are expressed in MACROPHAGES and play a key role in plaque formation in arterial walls (atherosclerosis) because they take up oxidized LDL
how does atherosclerosis occur?
thickening of the arterial walls due to build-up of calcium and lipids (build-up occurs in SUBINTIMAL SPACE)
begins with an injury to the endothelial layer due to:
- arterial hypertension
- elevated LDL, chylomicron remnants, and IDL
- low HDL levels
- cigarette smoking (causes oxidative damage)
- chronic elevations of blood glucose
explain the role that oxLDL plays in plaque formation
when LDL remains in circulation for a long time, it can become oxidized easily and is more readily picked up by scavenger receptors found in macrophages that accumulate at sites of endothelial damage –> leading to hypercholesterolemia
macrophages consume excess modified (oxidized) lipoprotein, becoming FOAM CELLS which stimulate the build-up of plaque
vitamin E and C, beta-carotene, and other antioxidants inhibit the formation of LDL to oxLDL
what happens when the plaque ruptures?
there is a fibrous cap that forms over the luminal surface and if the cap ruptures = a thrombolytic event may result in complete occlusion of the vessel
vascular smooth muscle cells that migrate from tunica into plaque release proteases that weaken the cap at the elbow, when plaque contents come into contact with the blood = clotting cascade is triggered and a clot forms, if thrombus occludes the vessel lumen, tissues distal to the occlusion will become ischemic, resulting in MI or stroke
why is HDL important?
it plays an important role in reverse cholesterol transport (returning excess cholesterol to the liver) and in the maturation of chylomicrons and VLDL (by transferring ApoCII and ApoE)
explain the process of maturation of HDL
nascent HDL particles accumulate phospholipids and cholesterol from cells lining the blood vessels and reverse cholesterol transport which takes cholesterol out of extrahepatic tissues and returns it to the liver
cells move cholesterol from the inner membrane leaflet to the outer leaflet using ABCA1 TRANSPORTER (once in the outer leaflet, cholesterol can diffuse into HDL)
deficiencies in ABCA1 transporter = familial HDL deficiency and Tangier disease –> extremely low serum HDL due to inability to mature (HDL is present but cholesterol is NOT diffusing into HDL)
what role does LCAT play in maturation of HDL?
LCAT esterifies the cholesterol that diffuses from the outer leaflet into HDL, becoming mature HDL and converting cholesterol into cholesterol esterase
LCAT is synthesized and secreted by liver
what is the purpose of esterification by LCAT?
- makes cholesterol even LESS POLAR so that it will pack in the interior of HDL
- make it possible to generate a CHOLESTEROL GRADIENT that allows diffusion of cholesterol out of membranes and into HDL
what is the cofactor for LCAT?
ApoA1
also major lipoprotein associated with HDL
what are the fates of HDL cholesterol?
as they continue to mature, HDL transfers cholesterol esters to VLDL and accept TAG from VLDL
chylomicron ester transfer protein (CETP) catalyzes the swapping of CE and TAG between VLDL and HDL, CE will eventually be taken up by cells as LDL
mature HDL binds to a SCAVENGER receptor on hepatocytes; receptor does not mediate endocytosis but does facilitate transfer of CE into cells –> HDL then returns to circulation
TAG in HDL can be removed by hepatic triglyceride lipase (HTL) ; TG-rich HDL are rapidly delipidated by HTL and then degraded –> hypertriglyceridemia is generally accompanied by lower serum HDL levels (because HDL facilitates the removal of TG, hyperlipidemia cause low serum HDL because CETP will transfer the triglycerides from VLDL making HDL lipid-rich, lipid-rich HDL will be degraded more quickly)
how does HDL function as an apoprotein carrier?
it facilitates the maturation of chylomicrons and VLDL by transferring ApoCII and ApoE to nascent chylomicrons and VLDL and taking them back from their remnants
what are the protective effects of HDL?
mechanisms by which HDL protects against atherosclerosis:
- efflux of cholesterol from lipid-laden macrophages
- vaso-protective effects (anti-inflammatory, anti-oxidative, anti-thrombotic, and anti-apoptotic)
what is the function of chylomicrons?
to deliver dietary (EXOGENOUS) lipids
what is the function of chylomicron remnants?
to RETURN dietary (EXOGENOUS) lipids to the liver via ApoE receptor-mediated endocytosis
what is the function of VLDL?
to deliver ENDOGENOUS lipids
what is the function of IDL?
to RETURN ENDOGENOUS lipids to the liver via ApoE receptor-mediated endocytosis or by converting it to LPL for further processing
what is the function of LDL?
to deliver cholesterol to cells (because their TG content has been depleted, it mainly contains cholesterol)
what is the function of HDL?
reverse cholesterol transport
what is the function of ApoAI?
primary tissue source: intestine and liver
associated with HDL
also activates LCAT because it’s a precursor for it
what does ApoB48 transport?
primary tissue source: intestine (because they’re dietary/exogenous)
chylomicrons
what does ApoB100 transport?
primary tissue source: liver (because they’re endogenous, so made in the liver)
VLDL, IDL, and LDL
what does ApoCII transport?
primary tissue source: liver
mature chylomicrons, VLDL, IDL, HDL
also cofactor activator of LPL
what does ApoE transport?
primary tissue source: liver
chylomicron remnants, VLDL, IDL, HDL
what are eicosanoids and why are they released?
paracrine hormones
in response to stimulus (like injury, clotting factors or endocrine hormones) and elicit a response only in NEARBY cells
VERY SHORT LIVED
what do eicosanoids derive from?
arachidonic acids, which can be obtained in the diet or synthesized by elongation and desaturation of omega-6 essential FA (LINOLEATE)
what is the function of eicosanoids?
may regulate many responses like inflammation, blood clotting, and smooth muscle contraction
what is arachidonic acid?
precursor for majority of eicosanoids and is incorporated into MEMBRANE-BOUND PHOSPHOLIPIDS
how is arachidonic acid released?
generally incorporated at C2 of glycerophospholipids
released primarily by PHOSPHOLIPASE A2 –> activity is stimulated upon binding of (converts membrane phospholipids into arachidonic acid) signal molecules to CELL SURFACE RECEPTORS
stimulus for arachidonic acid release and fate of arachidonic acid varies according to cell type
arachidonic acid may also be released by PHOSPHOLIPASE C and DIACYGLYCEROL LIPASE
what inhibits phospholipase A2?
glucocorticoids
anti-inflammatory steroids (corticosteroids) induce EXPRESSION of proteins that INHIBIT some isoforms of phospholipase A2
what can arachidonic acid be converted to?
epoxides (via cytochromse P450) , leukotrienes (via lipoxygenase), thromboxanes and prostaglandins (both via cyclooxygenase or COX)
how is the synthesis of prostaglandins and thromboxanes catalyzed?
by prostaglandin-endoperoxide synthase (PGH synthase)
what are the two catalytic sites of PGH synthase?
- COX active site catalyzed formation of peroxide at C15 and the formation of cyclic peroxide
- peroxidase active site reduces the peroxide at C15 using 2 GSH (used in glutathione)
PGH2 is produced
what are the fates of PGH2?
it can undergo PGI synthase or TXA synthase
PGI synthase –> prostacyclin (PGI2)
- produced by vascular endothelial cells
- INHIBITS platelet aggregation
- causes vasoDILATION
- mainly SLOWS DOWN clotting process (produced as clot formation ends)
TXA synthase –> thromboxanes (TXA2)
- produced by platelets
- STIMULATES platelet aggreagtion
- causes vasoCONSTRICTION
- BLOOD CLOTTING formation (produced at time of endothelial injury)
basically the opposite reactions
what is the function of NSAIDS?
COX inhibitors –> they inhibit PGH synthase = inhibit formation of prostaglandins and thromboxanes
ex: aspirin (irreversible), ibuprofen, acetaminophen, naproxen (acetaminophen and ibuprofen are REVERSIBLE inhibitors)
NSAIDS inhibit both COX-1 and COX-2
long-term and excessive use of NSAIDs can result in problems like gastric bleeding and impaired blood clotting
what are the two isozymes of COX active site in PGH synthase?
COX-1
- constitutively expressed in ALL CELL TYPES and is important for GENERAL CELL FUNCTION
COX-2
- expression is INDUCED in a LIMITED NUMBER OF TISSUES, leading to increased prostaglandin synthesis –> pain, heat, redness, swelling (this is the one you want to inhibit)
why is PGH2 important in prostaglandins?
it is the precursor for all series-2 prostaglandins
what is the structure of prostaglandins?
all contain a CYCLOPENTANE RING
PG in name and subscript indicates the number of double bonds in non-ring portion of structure
derivatives of arachidonic acid will be members of series-2 prostaglandins
why is PGH2 important in thromboxanes?
it is the precursor for all series-2 thromboxanes
what is the structure of thromboxanes?
all contain a 6-MEMBERED CYCLIC ETHER
naming and subscript is like prostaglandins
TXA2 stimulates platelet aggregation and formation of a thrombus
LOW-DOSE aspirin therapy for prevention of MI works by inhibiting TXA2 formation and preventing thrombus formation at ruptured atherosclerotic plaques (platelets lack nuclei –> WIL NOT be able to replace inactivated COX)
explain the lipoxygenase pathway
eicosanoids are also substrates for various lipoxygenases –> which oxidize them first to a peroxide –> peroxide is reduced to an alcohol –> formation of leukotrienes
what is the structure of leukotrienes?
always have AT LEAST 3 DOUBLE BONDS, none closer to the methyl end than omega-6
will have hydroxyl or epoxide and may have glutathione or cysteine attached
subscript of leukotrienes indicate number of double bonds
what is the function of leukotrienes?
they are mediators of allergic response and inflammation
in humans, activation of 5-LIPOXYGENASE in leukocytes stimulates production of leukotrienes that provoke BRONCHOCONSTRICTION and INFLAMMATION IN ASTHMA (many drugs that treat asthma inhibit leukotriene synthesis or leukotriene receptors)
describe the eicosanoid pathway
linoleate –> arachidonic acid –> membrane phospholipids –> arachidonic acid via phospholipase A2 (inhibited by glucocorticoids) –> can make epoxides, leukotrienes, thromboxanes, or prostaglandins
what can sustained inflammation cause?
it is thought to be a significant contributor to several diseases including cardiovascular disease and even some types of cancer
what are the inflammatory eicosanoids?
prostaglandins and leukotrienes, synthesized from arachidonic acid (omega 6 FA)
how can inflammatory response be resolved?
certain eicosanoid-like lipids, many derived from omega 3 FA, are KEY to resolving inflammatory response
omega 3 FAs may be beneficial because they are competitive inhibitors of arachidonic acid in eicosanoid synthesis –> slowing the formation of inflammatory lipids and/or producing less effective ones
omega 3 FAs are also used to make hormones with specific ANTI-INFLAMMATORY activities
what is the function of resolvins and protectins?
anti-inflammatory lipids produced from omega-3 FAs
many appear to be synthesized using the same enzymes used in the synthesis of prostaglandins and leukotrienes
what are the two common omega-3 FAs and where can you find them?
eicosapentaenoic acid (EPA) and docosahexenoic acid (DHA)
found in leafy vegetables (kale, broccoli), some nuts, and fatty fish
omega-3 FAs are thought to have many health benefits, somewhat related to their anti-inflammatory properties and important for neuronal development
can be made by the body from ALA (alpha-linolenate)
what is the expected ratio of omega-6 and omega-3 in the diet?
we consume them at 10:1 to 30:1 but it should be, at OPTIMAL, 1:1 to 4:1
where are omega-6 found?
high quantities in vegetable oils
what are the subclasses of glycerolipids?
triacylglycerols, glycerophospholipids, and ether glycerolipids
what are the subclasses of phospholipids?
glycerophospholipids, ether glycerophospholipids, and sphingophospholipids
what are the subclasses of sphingolipids?
sphingophospholipids and glycolipids
explain glycerophospholipids
they’re major components of cell membranes, lipoproteins, bile, and lung surfactant
they have glycerol backbone, 2 FA chains, and a head group linked to glycerol backbone by a PHOSPHATE
choline, ethanolamine, serine, inositol, and glycerol are possible head groups (cardiolipin contains 2 glycerophospholipids linked COVALENTLY by a glycerol head group)
phosphatidylcholine and phosphatidylethanolamine are both PROTONATED at serum pH
phosphatidic acid, phosphatidylserine, phosphatidylinositol (PI), and phosphatidylglycerol are all NEUTRAL at serum pH
what is the additional role of phosphatidylinositol?
is important in membrane bilayers, but also important in cell signaling pathways
precursor for second messengers DAG (diacylglycerol) and IP3 (inositol triphosphate)
PI and be phosphorylated to form PI-4,5 bisP –> PI-4,5-bisP is substrate recognized by PHOSPHOLIPASE C which is activated by G-alpha Q –> phospholipase C generates DAG and IP3 (2 second messengers)
explain respiratory distress syndrome (RDS)
RDS of PREMATURE INFANTS is somewhat related to a deficiency in the synthesis of lung surfactant (composed of dipalmitoylphosphatidylcholine - DPPC, phosphatidylglycerol - PG, and apoproteins
lung maturity can be measured by determining ratio of DPPC to sphingomyelin (which gives you a standard comparison to see whether or not there’s too little or too much DPPC) in the amniotic fluid –> administration of synthetic corticosteroids 48-72 hours before delivery can INDUCE SYNTHESIS of DPPC and decrease risks of RDS
without lung surfactant, lung will collapse because it is used to reduce surface tension
how are glycerophospholipids degraded?
phospholipase located in cell membranes or in lysosomes degrade glycerophospholipids
each phospholipase is SPECIFIC for which bond it hydrolyzes
- phospholipase A1 hydrolyzes C1
- phospholipase at A2 hydrolyzes C2
- phospholipase C cleaves carbon 3 BEFORE the phosphate is added
- phospholipase D cleaves carbon 3, more specifically, the bond between the head group and the phosphate
phospholipases A1 and A2 are important in REMODELING reactions where FA are exchanged –> important for removing oxidatively damaged FA chains
arachidonic acid is often incorporated at C2, phospholipase A2 is important for releasing it prior to eicosanoid synthesis
explain PLA2 activity of venoms
most snake and bee venoms contain PLA2 (phospholipase A2) enzymes that are similar to human PLA2
each type of venom targets glycerophospholipids in different tissues (each venomous PLA2 contains unique targeting domain)
many venoms are NEUROTOXIC or MYOTOXIC, some cause HEMOLYSIS
what happens when you hydrolyze glycerophospholipids?
results in formation of lysophospholipids, disrupting membrane structure
many cases, results in influx of Ca2+ that can cause cellular dysfunction
what are the ETHER glycerophospholipids?
plasmalogens and platelet-activating factor
what are sphingolipids?
they contain sphingosine backbone
ONLY subclass is SPHINGOMYELIN which is found in high concentration in MYELIN SHEATH
all glycolipids (cerebrosides, sulfatides, globosides, and gangliosides) are sphingolipids –> important in cell recognition and intercellular communication
explain the synthesis of sphingolipids
synthesized by addition of head groups to ceramide
ceramide is synthesized from PALMITATE, SERINE, and FA CoA
phosphatidylcholine can donate its head group to form sphingomyelin and release DAG
sugar nucleotides are used to provide head groups of cerebrosides and gangliosides to ceramide
ex: UDP-galactose = galactocerebroside, UDP-glucose = glucocerebroside –> can be used to make ganglioside (all gangliosides contain at least ONE NANA residue)
explain sulfatide synthesis
found primarily in the brain and require for PAPS for sulfation
galactocerebrosize via PAPS –> sulfatide
3’-phosphoadenosine 5’-phosphosulfate (PAPS) donates SULFATE groups for this reaction
why are AAs important?
they serve as precursors for nitrogen-containing compounds and are building blocks for protein synthesis
may be oxidized to provide energy or be converted to FAs or glucose
fasting = proteolysis/ protein hydrolysis
high insulin/glucagon = protein synthesis
AA carbon skeletons may be used for FA or glucose synthesis
what are the common transamination pairs?
alanine (alpha-AA) and pyruvate (alpha-keto acid)
aspartate (alpha-AA) and OAA (alpha-keto acid)
glutamate (alpha-AA) and alpha-ketoglutarate (alpha-keto acid)
what are the steps in AA catabolism?
aminotransferases transfer amino groups to alpha-KG and convert them to glutamate
glutamate DH oxidatively deaminates some glutamate to regenerate alpha-KG and releases free NH4+ –> will react with glutamate via glutamine synthesis to form glutamine (contains 2Ns) –> transported to liver, kidney, and intestine (eventually into urea cycle)
in muscle tissue –> alanine aminotransferase transfers amino groups from glutamate to pyruvate (makes alalnine) –> transported as alanine to the liver
explain transamination
process of moving amino group from an alpha-amino acid to an alpha-keto acid
many different aminotransferases, each with different specificities for the alpha AA (like aspartate, alanine, and glutamate)
reaction from aspartate to OAA is catalyzed by aspartate aminotransferase (AST) where amino group from aspartate is transferred to alpha-KG to make glutamate, products = OAA and glutamate (reaction is REVERSIBLE)
alpha-keto acid is usually an alpha-KG
what are the ONLY AA that DO NOT undergo transamination?
thr and lys
how do transaminases/aminotransferases catalyze reactions?
they catalyze near-equilibrium reactions which means that the reaction will be determined by the conc. of glutamate, alpha-KG, alpha-AA, and alpha-keto acid
reaction:
alpha-KG + alpha-AA (PLP –>) ⇌ glutamate + alpha-keto acid
PLP (pyridoxal phosphate/B6) helps aminotransferases catalyze the reaction
AA catabolism (fasting or exercise) = high [AA] so process will shift right toward glutamate synthesis –> glutamate is consumed in other reactions so it prevents the reverse of transamination
AA anabolism (fed, positive nitrogen balance, growing child, pregnant women) = low [AA] so process will shift left toward AA synthesis
why is PLP important?
it is used as a COFACTOR by aminotransferases during CATALYSIS, also required for reaction catalyzed by glycogen phosphorylase
aka B6
how are aminotransferases used as a diagnostic tool?
they are INTRAcellular enzymes which are present in high concs. in the LIVER and MUSCLE TISSUE –> released into bloodstream upon cellular damage due to physical trauma or disease
serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are elevated in almost all LIVER DISEASES
other serum markers of HEPATIC DAMAGE are alkaline phosphatase and bilirubin
explain the glutamate DH
catalyzed the deamination of glutamate to produce alpha-KG (releases NH4+) or amination of alpha-KG to produce glutamate
GDH can used NADP+ or NAD+
when AAs are being CATABOLIZED, transamination rxns INCREASE [glutamate] (recall the transaminase rxn) –> GDH rxn will favor OXIDATIVE DEAMINATION of glutamate –> produces NADPH, NH4+, and alpha-KG
when AAs are being MADE, transamination rxns DECREASE [glutamate] –> HOWEVER, Km for NH4+ is about 1 mM so REDUCTIVE AMINATION (producing NADP+ and glutamate) is ONLY observed when NH4+ levels are HIGH
formation of alpha-KG allows continued transamination of AA –> NADH can provide energy or reducing equivalents for GLUCONEOGENESIS which may sometimes occur with AA catabolism
explain ammonia toxicity
high [NH4+] will drive GDH in reductive amination, adding amino groups to alpha-KG to produce glutamate
high [NH4+] in tissues = depletion of alpha-KG for the TCA because it is being used to make glutamate –> results in depletion of CELL ENERGY (damaging to nervous system)
increased [glutamate] = increased [glutamine] (uses glutamine synthetase) –> result in OSMOLAR IMBALANCE (problematic in brain, leading to coma)
metabolism defects in nitrogen metabolism = cause increase in serum ammonia (hyperammonemia) and glutamine levels (SO CAN LIVER DISEASE)
where does high [NH4+] occur?
in PERIVENOUS REGION OF THE LIVER (where hepatic portal vein is)
NH4+ is produced in large amounts from GLUTAMINE by ENTEROCYTES and from UREA by INTESTINAL MICROBES
transported to liver via portal vein (cells in this region are adapted to metabolize high [NH4+])
glutamine –> glutamate –> alpha-KG (uses deamination)
how is ammonia transported using alanine?
because of it’s toxicity, we CANNOT deaminate glutamate in tissues where it’s produced during AA catabolism so it is carried in serum in form of ALANINE or GLUTAMINE
muscle tissue (ONLY) will use ALANINE as its nitrogen carrier
AA transfers its amino group via aminotransferase to alpha-KG and produces glutamate –> glutamate transfers its amino group to pyruvate via alanine aminotransferase (ALT) –> alanine is transported out of the muscle to the liver where it is deaminated to pyruvate which is used for gluconeogenesis or converted nitrogen which will enter urea and be excreted in the urine = GLUCOSE-ALANINE CYCLE
the pyruvate that was used in gluconeogenesis in liver produces glucose that will be used by the muscle to make more pyruvate via anaerobic glycolysis
alpha-KG in this reaction is REGENERATED for use in additional aminotransferase reactions
how ammonia transported using glutamine?
muscles and other tissues will use glutamine as nitrogen carrier
ammonia is generated by many metabolic processes involving AA and nucleic acids –> if cellular [NH4+] are high = glutamate will be converted to glutamine by GLUTAMINE SYNTHETASE which uses ATP
LIVER, KIDNEY, and INTESTINAL EPI CELLS express GLUTAMINASE
KIDNEY will excrete NH4+ in the URINE, ENTEROCYTES will used glutamine as a fuel, NH4+ is sent to the liver
rxn:
alpha-KG (glutamate DH, uses NADPH) –> glutamate (glutamine synthetase, uses ATP) –> glutamine –> goes to liver where glutaminase converts it to glutamate and released amino group to urea –> GDH converts glutamate to alpha-KG, releasing another amino group to urea which will be excreted through the urine
what is the urea cycle?
a process that removes toxic ammonia from the body and converts it into urea, a substance that can be excreted in urine
what does the liver do with all the nitrogen?
some will be released as free ammonia and others will be incorporated into ASP, both will donate nitrogen to the urea cycle
aminotransferase transfers an amino group from an AA onto alpha-KG to produce glutamate –> glutamate can undergo GDH to release NH4+ which can go into urea cycle (regenerate alpha-KG) –> aspartate aminotransferase will transfer the amino group from glutamate to OAA to make aspartate and alpha-KG, aspartate will going into the urea cycle
other reactions –> NH4+ also generated by many other hepatic enzymes, as well as by gut microbes and enterocytes will enter urea cycle, LIVER GLUTAMINASE released NH4+ from glutamine which will enter urea cycle
explain step one of the urea cycle
carbamoyl phosphate synthase I (CPSI), in urea cycle, requires 2 ATP to catalyze the addition of ammonia and phosphate to a molecule of bicarb to make an activated carbamoyl group (analogous to pyruvate DH in TCA to make activated acetyl group for acetyl-CoA)
bicarb + NH4+ (via CPSI using 2 ATP) –> CARBAMOYL PHOSPHATE
FIRST TWO REACTIONS OF UREA CYCLE OCCUR IN MITOCHONDRIA
also a CPSII in cytosol that functions in NUCLEOTIDE SYNTHESIS (helps detect and diagnose certain diseases)
explain step two and the rest of the urea cycle
ORNITHINE TRANSCARBAMOYLASE transfers amide from carbamoyl phosphate to ornithine, forming CITRULLINE –> from this point on the later steps occur in the cytosol
- citrulline is converted to argininosuccinate via argininosuccinate SYNTHETASE using ASP and ATP
- argininosuccinate is converted to ARGININE via argininosuccinate LYASE, releasing fumarate for TCA
- arginine is converted to UREA (has two amino groups - one from ammonia and one from asp) and ornithine via arginase (ONLY THE LIVER EXPRESSES THE LAST ENZYME IN THE UREA CYCLE)
there is COTRANSPORTER of ornithine (in) and citrulline (out) of the mitochondria to continue this process
ornithine is lysine missing one methyl group, reaction is analogous to transfer of acetyl group from acetyl-CoA to OAA in TCA
what is the ornithine transcarbamoylase deficiency?
many urea cycle deficiencies, like the OTC deficiency, that cause build-up of carbamoyl phosphate (which can leak out into cytosol) will result in accumulation of OROTIC ACID (key diagnostic to identify OTC deficiency) IN URINE
carbamoyl phosphate is also used in SYNTHESIS OF PYRIMIDINES, using CPSII in cytosol
accumulated mitochondrial carbamoyl phosphate can diffuse into the cytosol and be used to make orotate (intermediate in pyrimidine synthesis) that is excreted in urine when produced in excess
urea cycle impaired = develop hyperammonemia which is very toxic to babies
what are the fates of fumarate?
fumarate is generated in step 3 of urea cycle and has 3 possible fates –> all involve initial HYDRATION to malate and transfer to mitochondria
- malate –> OAA –> glucose (gluconeogenesis)
- malate –> OAA –> asp –> urea cycle
malate –> OAA –> transferring the amino group from glutamate to OAA makes asp which can enter the urea cycle - malate –> TCA to generate energy
explain regulation of the carbamoyl phosphate synthase
CPSI is activated by N-ACETYLGLUTAMATE
N-ACETYLGLUTAMATE can only be formed when cell concs of glutamate and acetyl-CoA are HIGH (when there’s lots of nitrogen/glutamate and source of acetyl CoA)
N-ACETYLGLUTAMATE SYNTHASE is activated by ARGININE (component of urea cycle that will be present at HIGH CONCS when cycle is RUNNING)
glutamate + acetyl-CoA is converted to N-acetylglutamate via N-acetylglutamate synthase (activated by ARGININE) –> N-acetylglutamate ACTIVATES CPSI to make carbamoyl phosphate
N-acetylglutamate is only made when we want to do the urea cycle
how is the urea cycle regulated?
urea cycle is essential for detoxification of ammonia so it is regulated largely by SUBSTRATE AVAILABILITY (ammonia)
- N-acetylglutamate is KEY ALLOSTERIC REGULATOR of carbamoyl phosphate synthase, only present when cell has high levels glutamate, acetyl-CoA, and arginine
- urea cycle enzymes are induced when delivery of ammonia or AA to liver increases like in high protein diets or during starvation (when body spares AA and begins using more KB)
what happens to the urea cycle during fasting?
in fasting, AA is used for gluconeogenesis and urea cycle plays an important role
fast progresses = KB become main source of energy for brain, sparing muscle tissue to provide AA carbon skeleton for gluconeogenesis
so urea-nitrogen begins to decline since AA releases NH4+ that is converted to urea and excreted through urine
what are some treatments for people with hyperammonemia?
can be treated by administration of BENZOATE or PHENYLBUTYRATE (phenylacetate) –> these compounds covalently bind to GLYCINE or GLUTAMINE and are then excreted using MC acyl-CoA synthetase to activate the xenobiotics then glycine N-acetyltransferase to make excretion product
oral administration of LACTOBACILLUS ACIDOPHILUS and/or LACTULOSE (commonly used in adults)
Lactobacillus acidophilus is normal, non-pathogenic gut microbe that LACKS the enzyme UREASE (which breaks down urea into CO2 and ammonia), it is predominant intestinal microbe which minimizes intestinal production of ammonia
lactulose = lactose analog in which glucose is replaced by FRUCTOSE, humans cannot digest it, so it is metabolized by gut microbes to form LACTIC ACID –> decreasing colonic pH and protonating ammonia produced in gut (protonated ammonium CANNOT CROSS membranes)
where does protein digestion begin?
it begins in the stomach and is completed in the intestine
what is the process of protein digestion?
the acidity of the stomach denatures the dietary proteins, making peptide bonds more accessible to digestive proteases
additional proteases are secreted by the pancreas (trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidases A and B) and aminopeptidases, digesting protein to di- and tri- peptides and individual AA which are taken up by intestinal epithelial cells (enterocytes) –> also express peptidases, some on brush border and some in cytosol
di and tri-peptidases digest di- and tri- peptides into AA
what are digestive proteases and their function?
they’re secreted as ZYMOGENS (inactive enzyme precursor) –> this protects the pancreas and chief cells of the stomach from SELF-DIGESTION and lengthens the half-lives of the digestive enzymes (allows them to stay in circulation longer) –> secreted into lumen of GI and become activated
each protease has its own substrate specificity, increasing probability of being able to break down dietary protein into smaller peptides (aminopeptidases = N-terminal and carboxypeptidases = C-terminal are exopeptidases while pepsin, trypsin, chymotrypsin, and elastase are endopeptidases)
zymogens are ACTIVATED by proteolysis (EXECPT pepsinogen which undergoes autoproteolysis)
- pepsinogen is activated by H+ to pepsin (in stomach)
- trypsinogen is activated by enteropeptidase to trypsin
- chymotrypsinogen is activated by trypsin to chymotrypsin
- proelastase is activated by trypsin to elastase
- procarboxypeptidases are activated by trypsin to carboxypeptidases
how are AA absorbed?
they are absorbed from the intestinal lumen by SEMISPECIFIC Na+-dependent transporters (with some overlapping specificities) –> these same transporters are found in the renal epithelium and facilitate reabsorption of AA (so if intestinal transporter is impaired, so is the renal)
enterocytes can absorb and proteolyze small peptides
process:
AA and Na+ from the intestinal lumen are brought into the enterocyte via a cotransporter –> AA is transported by a FACILITATED DIFFUSION on the serosal side while the Na+ is transported in the Na+/K+ ATPase aka active transporter (low intracellular [Na+] is generated by this ATPase)
what happens to the amount of protein digested and absorbed each day?
amount of protein that is digested and absorbed each day from digestive juices and cells released into intestinal lumen may be EQUAL or GREATER than amount of protein in diet –> body will try to reabsorb as much of this protein as it can
what are the disorders of intestinal and renal amino acid transport?
CYSTINURIA and HARTNUP disease = two genetic disorders involving deficiencies in intestinal and renal amino acid transporters
both disorders = specific AA are not absorbed in the intestine and are not resorbed by kidney (not returned to the serum from the renal filtrate) –> AA may be low in the serum and accumulate in the urine
what is cystinuria?
cystine (oxidized form of cysteine) and basic AAs (COAL - cystine, ornithine, arginine, and lyse) are not transported
cys, orn, and arg can be synthesized by the body but lys (essential FA) is OBTAINED from peptide absorption, so dietary resorption is not noticeable
cystine stones form in renal filtrate and cause pain and genitourinary bleeding (occurs when cystine is not reabsorbed, leading to build up)
what is the hartnup disease?
NONPOLAR AA are not transported –> results in trp deficiency which leads to similar symptoms of PELLAGRA since trp is used to make niacin
treatment = dietary niacin and high protein diet (to restore trp)
what happens after AA is released into circulation?
they are take up by all tissues
they are transported from serum into cells by Na+-dependent cotransporters that are similar to those found in the intestine
once inside cell –> AA are used to synthesize proteins or other N-containing compounds or metabolized to carbon skeletons for energy production or fuel storage (specific to urea cycle)
protein synthesis and degradation is important process of AA (some proteins have shorter half lives = in cell for short time rather than cells with long half-live which stay for a long time)
what is lysosomal protein turnover?
cellular proteins can undergo AUTOPHAGY (engulfed in a vesicle that fuses with lysosomes)
autophagy is a complex, tightly regulated process –> STIMULATED when cell energy levels are LOW and INHIBITED by INSULIN (high energy state)
autophaged proteins undergo proteolysis catalyzed by CATHEPSINS in lysosomes –> PURPOSE: to release free AAs that can be used for energy production in the cell or gluconeogenesis in the liver
insulin inhibits lysosomal protein turnover and protein synthesis
explain the ubiquitin-proteasome pathway
specific proteins are targeted for degradation via ubiquitin-proteasome pathway
many proteins rich in PRO, GLU, SER, and THR (PEST sequences) are targeted for covalent modification with UBIQUITIN (small protein that covalently attaches to LYS side chains)
ubiquitinated proteins are degraded in proteasome complex (more ubiquitin = quicker degradation)
what are the two forms of HMG CoA synthase and what are their functions?
cytosolic HMG-CoA synthase = synthesis of cholesterol
mitochondrial HMG-CoA synthase = synthesis of KB
what do chief cells of the STOMACH secrete?
pepsin (an acid-stabile) digestive protease
