Lipids Flashcards
Solubility of lipids
Hydrophobic (water insoluble)
Examples of lipids
Fats, oils, certain vitamins and hormones, and most non-protein membrane components are lipids
What main class of cell molecule is the principle and most concentrated form of storage?
lipids
What is the standard length of fatty acid chains?
C16 or C18
Do most fatty acid chains contain an even or odd number of cabons?
Most fatty acids have even number of carbons
What proportion of fatty acids in plants and animals are unsaturated?
> 50% of fatty acids in plants and animals are unsaturated, often polyunsaturated (PUFAs)
PUFA
polyunsaturated fatty acid
Polyunsaturated fats are lipids in which the constituent hydrocarbon chain possesses two or more carbon–carbon double bonds.[1][2] Polyunsaturated fat can be found mostly in nuts, seeds, fish, algae, leafy greens, and krill. “Unsaturated” refers to the fact that the molecules contain less than the maximum amount of hydrogen. These materials exist as cis or trans isomers depending on the geometry of the double bond.
Where is the first double bond commonly found in unsaturated fatty acids?
Between C9-C10
monounsaturated fat
In biochemistry and nutrition, monounsaturated fatty acids (MUFAs, or more plainly monounsaturated fats) are fatty acids that have one double bond in the fatty acid chain with all of the remainder carbon atoms being single-bonded. By contrast, polyunsaturated fatty acids (PUFAs) have more than one double bond
Stearic acid
18 carbon saturated fatty acid
18:0 Octadecanoic acid
Oleic acid
18 carbon saturated fatty acid
18:1 (∆9) cis-9-Octadecanoic acid
Linoleic acid
18 carbon saturated fatty acid
18:2 (∆9,12) all-cis-9,12-Octadecanoic acid
Omega-6
Omega-6 FAs (linoleic acid) are converted to arachidonic acid [20:4(∆5,8,11,14)]which is converted to pro-inflammatory agents called prostaglandins, eicosanoids, and leukotrienes
α-Linolenic acid
18 carbon saturated fatty acid
18:2 (∆9,12) all-cis-9,12,15-Octadecanoic acid
Omega-3
What are the two conventions for naming fatty acids?
FIGURE 10-1 Two conventions for naming fatty acids.
(a) Standard is Carboxyl carbon = C1, next carbon is α. Number of carbons: number of double bonds, Δ followed by a superscript indicating the position of double bond.
(b) For PUFAs, assigning number 1 to the methyl carbon at the other end of the chain; this carbon is also designated ω.
Archidonic acid
20:4(∆5,8,11,14) all-cis-5,8,11,14-Icosatetraenoic acid
Precursor for lipid signalling molecules
Precursor to prostaglandins (via COX: cyclooxygenase), thromboxanes (via COX: cyclooxegenase), and leulotrienes.
Describe the general trend of the melting point of fatty acids with respect to carbon and double bond number
Melting point goes up with number of carbons
Melting point goes down with number of double bonds
Ex. 12C saturated @ 44.2°C, 24C saturated at 86°C, 24C quadruple unsaturated -49.5°C
What determines the extent of packing among fatty acids?
The extent of packing depends on the degree of saturation.
(a) Saturated FA, stearic acid, 18:0, in extended conformation.
(b) Cis double bond (shaded) in oleic acid, 18:1(Δ9) (oleate), restricts rotation and causes bend in the hydrocarbon tail.
Describe the packing of fatty acids
The packing of fatty acids into stable aggregates (shown). The extent of packing depends on the degree of saturation.
(c) Saturated fatty acids pack into crystalline arrays.
(d) Cis double bonds interferes with this tight packing
What are triacylglycerols?
Esterifications of glycerol and fatty acids
The water formed originates from the H bound to oxygen of glycerol and the -OH of the carboxcylic acid
What is the advantage of using triacylglycerols for energy storage over polysaccharids?
They are more reduced than carbohydrates, so they contain twice as much energy per gram
Because they are hydrophobic, they do not need to carry the weight of water (2g/H2O solvated per carbohydrate) necessary for hydration
Where do trans fats occur naturally and in what amounts?
2-5% of total fat in milk and meat from ruminants
Oleic acid (cis fat) vs Elaidic acid (trans fat) melting point
Oleic acid C18 cis-9 melts at 13.5°C (left)
Eladic acid C18 trans-9 melts at 46.5°C (right)
Membranes prevent passage of what kinds of molecules?
Biological membranes are lipid bilayers that form cell barrier that prevents the passage of polar molecules.
Bilayer lipid polarity
Lipids in bilayers are amphipathic meaning they have a polar part and a hydrophobic part. The hydrophobic part (fatty acids) interact with each other and the hydrophilic parts interact with exterior and cytoplasm
Explain the diversity of membrane lipids
Enormous diversity of membrane lipids due the the various combination of hydrophilic head groups coupled to the various fatty acids
amphipathic
Lipids in bilayers are amphipathic meaning they have a polar part and a hydrophobic part. The hydrophobic part (fatty acids) interact with each other and the hydrophilic parts interact with exterior and cytoplasm
Describe the fluid mosaic model for membrane structure
FIGURE 11-3 FLUID MOSAIC MODEL FOR MEMBRANE STRUCTURE.
Interior fatty acyl chains form a fluid, hydrophobic region. Integral proteins float in this sea of lipid. Both proteins and lipids are free to move laterally in the plane of the bilayer, but movement of from one leaflet of the bilayer to the other is restricted.
Why do bilayers form?
Fatty acids have very hydrophobic alkyl chains, which are surrounded by highly ordered water molecules.
Second law of thermodynamics says that entropy of an isolated system not in equilibrium will tend to increase over time, i. e. will become more random…ordering of water molecules goes against this, therefore, thermodynamically unfavorable.
What happens when lipids are dispersed?
Each lipid molecule forces surrounding H2O molecules to become highly ordered, therefore, entropy is reduced
What happens to entropy when lipids are clustered?
Only the lipid portions at the edge of the cluster force the ordering of water. Fewer H2O molecules are ordered, and entropy is increased
micelle
All hydrophobic groups are sequestered from water; ordered shell of H2O molecules minimised, and entropy is further increased over clusters of lipids or dispersed lipids
Structure of phospholipids
The common glycerophospholipids are diacylglycerols linked to head-group alcohols through a phosphodiester bond. Phosphatidic acid is the parent compound. Each derivative is named for the head-group alcohol (X), with the prefix “phosphatidyl-.”
No triacylglyerides in membrane. Instead, have phospholipids that are amphipathic (hydrophobic and hydrophilic)
What is the convention for naming glycerolphospholipids
Each derivative is named for the head-group alcohol (X), with the prefix “phosphatidyl-.”
Lipid role in signalling
- Some lipids are signaling molecules. Some drugs block the production of these signaling lipids.
- Some are slow acting, steroid hormones that effect gene expression by activating nuclear receptors – testosterone, estrogen, vit. A, E, D.
- Some are fast acting, that bind membrane receptors - prostaglandins, leukotrienes.
steriod
Steriods synthesized from cholesterol.
Signalling molecules
Testosterone - male sex hormone. Estradiol - female sex hormone. Cortisol and aldosterone synthesized in adrenal cortex; regulate glucose metabolism and salt excretion, respectively. Prednisolone and prednisone are synthetic anti-inflammatory agents.
COX
Cyclooxygenase (COX), officially known as prostaglandin-endoperoxide synthase (PTGS), is an enzyme that is responsible for formation of prostanoids, including prostaglandins (COX-2) such as prostacyclin and thromboxane (COX-1).
The abbreviation “COX” is more often encountered in medicine. In genetics, the “PTGS” symbol is officially used for the prostaglandin-endoperoxide synthase (cyclooxygenase) family of genes and proteins, because the stem “COX” was already used for the cytochrome c oxidase family of genes and proteins.
Pharmacological inhibition of COX can provide relief from the symptoms of inflammation and pain. Non-steroidal anti-inflammatory drugs (NSAID), such as aspirin and ibuprofen, exert their effects through inhibition of COX.
In terms of their molecular biology, COX-1 and COX-2 are of similar molecular weight, approximately 70 and 72 kDa, respectively, and having 65% amino acid sequence homology and near-identical catalytic sites. The most significant difference between the isoenzymes, which allows for selective inhibition, is the substitution of isoleucine at position 523 in COX-1 with valine in COX-2. The smaller Val523 residue in COX-2 allows access to a hydrophobic side-pocket in the enzyme (which Ile523 sterically hinders). Drug molecules, such as DuP-697 and the coxibs derived from it, bind to this alternative site and are considered to be selective inhibitors of COX-2.
NSAID
Nonsteroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen block formation of prostaglandins and thromboxanes from arachidonate by inhibiting the enzyme cyclooxygenase (COX).
Block both COX-1 and COX-2
COX converts _____________ to __________ and _______________, and is inhibited by ____________ such as _____________.
COX converts arachidonate to prostaglandins and thromboxanes, and is inhibited by NSAIDs such as aspirin.
What are the two activities of COX? What activity is inhibited by aspirin and other NSAIDs?
COX has a cyclooxgenase activity and a peroxidase activity, the cyclooxgenase activity is inhibited by NSAIDs.
How is COX inhibited by NSAIDs?
A Ser residue is irreversibly acetylated by the acetyl group of aspirin
acetyl
R-CH(CH3)COO-
thromboxane
A lipid-based signaling factor which increases platelet aggregation and forms a clot…stops bleeding.
However, thromboxane increases blood clots that block blood flow and cause heart attack or stroke, therefore inhibiting it decreases cardiovascular disease.
COX-1 is an enzyme involved in the production of thromboxane
How does aspirin reduce the risk of heart disease?
Aspirin (acetylsalicylic acid) acetylates COX-1 and -2 and inactivates them.
Blocks production of a lipid-based signaling factor called thromboxane (COX-1), which increases platelet aggregation and forms a clot…stops bleeding.
However, thromboxane increases blood clots that block blood flow and cause heart attack or stroke, therefore inhibiting it decreases cardiovascular disease.
COX-1 vs COX-2
COX-1 produces lipids products that protect stomach lining (inhibition causes bleeding), and produces thromboxane, which increases blood platelet aggrigation.
COX-2 produces lipid products that mediate pain and inflammation, also produces prostacyclin, a prostaglandin that prevents platelet aggregation.
Why does selectively blocking COX-2 alone increase the risk of heart disease?
Inhibition of COX-2 alone reduces the body’s production of prostacyclin, a prostaglandin that prevents platelet aggregation and alters the ratio of prostacyclin to thromboxane which is made by COX-1. Therefore, it increases the risk of cardiovascular disease.
Why are omega-3 fatty acids healthy?
- Omega-3 fatty acids-polyunsaturated fatty acids with double bond in the ω-3 position.
- Body cannot synthesize - α-linolenic [18:3 (∆9,12,15)] - essential FA.
- Omega-6 FAs (linoleic acid) are converted to arachidonic acid which is converted to pro-inflammatory agents called prostaglandins, eicosanoids, and leukotrienes.
- Omega-3’s are converted at a slower rate to arachidonic acid [20:4 (∆5,8,11,14)], therefore increasing ω3/ω6 ratio, reduces inflammation.
cholesterol
Cholesterol is a structural component of membranes and precursor for hormones and bile acids
prostaglandin E1
C-8 and C-12 of arachidonate are in five-membered ring
thromboxane
C-8 and C-12 are in 7 member ring with 2 oxygens
leukotriene A4
series of three conjugated double bonds
What is the relative amount of energy of an equal weight of fat vs glycogen in the cell?
Like glucose, fats they are metabolized to CO2 and H2O, but since lower oxidation state, oxidative metabolism of fats yields over twice the energy/dry weight as do carbohydrates or proteins.
Because fats are nonpolar, they are stored in an anhydrous state, whereas glycogen, the storage form of glucose, is polar and stored in a form that contains twice its weight in water.
Hence, fats provide six times the metabolic energy of an equal weight of hydrated glycogen.
Lipids are __________ (soluble/insoluble) while lipases are ________ (soluble/insoluble)
Lipids are insoluble while lipases are soluble, so digestion takes place at the lipid-water interface
Where does digestion of lipids occur? What is it depenent on?
Lipids are not soluble and lipases are, so digestion takes place at the lipid-water interface.
Rate of lipid digestion depends on the surface area of the interface.
What are bile acids, and what do they do?
Bile acids are secreted into the small intestine where they increase the lipid-water surface area due to their detergent activities that cause the formation of lipid micelles
(further reading):
Bile acids are steroid acids found predominantly in the bile of mammals and other vertebrates. Different molecular forms of bile acids can be synthesized in the liver by different species. Bile acids are conjugated with taurine or glycine in the liver, forming bile salts.
Primary bile acids are those synthesized by the liver. Secondary bile acids result from bacterial actions in the colon. In humans, taurocholic acid and glycocholic acid (derivatives of cholic acid) and taurochenodeoxycholic acid and glycochenodeoxycholic acid (derivatives of chenodeoxycholic acid) are the major bile salts in bile and are roughly equal in concentration.
Bile acids comprise about 80% of the organic compounds in bile (others are phospholipids and cholesterol). An increased secretion of bile acids produces an increase in bile flow. The main function of bile acids is to facilitate the formation of micelles, which promotes digestion and absorption of dietary fat, but they are increasingly being shown to have hormonal actions throughout the body.
cholic acid
Cholic acid is a primary bile acid that is insoluble in water (soluble in alcohol and acetic acid), it is a white crystalline substance. Salts of cholic acid are called cholates. Cholic acid, along with chenodeoxycholic acid, is one of the two major bile acids produced by the liver, where it is synthesized from cholesterol.
Cyclopentanoperhydrophenanthrene ring structure
Cyclopentanoperhydrophenanthrene
Sterane (cyclopentanoperhydrophenanthrenes or cyclopentane perhydro phenanthrene) compounds, also known as are a class of 4-cyclic compounds derived from steroids or sterols via diagenetic and catagenetic degradation and saturation. Steranes have an androstane skeleton with a side chain at carbon C-17. The sterane structure constitutes the core of all sterols.
Pancreatic lipase
Pancreatic lipase catalyzes the sequential hydrolysis of triacylglycerols at their 1 and 3 positions to generate free fatty acids and 2 monoglyceride
pancreatic phospholipase A2
pancreatic phospholipase A2 cleaves C2 fatty acid of phospholipids to yield lysophospholipids, which are detergents due to their amphipathic nature. Hence, they emulsify fat, like bile acids.
What enzyme digests triacylglycerides into fatty acids?
Triacylglycerides are degraded into free fatty acids and glycerol by lipases. Pancreatic lipase catalyzes the sequential hydrolysis of triacylglycerols at their 1 and 3 positions to generate free fatty acids and 2 monoglyceride
What enzyme digests phospholipids into free fatty acids?
Phospholipids are also degraded by pancreatic phospholipase A2, which cleaves C2 fatty acid to yield lysophospholipids, which are detergents due to their amphipathic nature. Hence, they emulsify fat, like bile acids.
lysophospholipid
The term lysophospholipid (LPL) refers to any phospholipid that is missing one of its two O-acyl chains. Thus, LPLs have a free alcohol in either the sn-1 or the sn-2 position. The prefix ‘lyso-‘ comes from the fact that lysophospholipids were originally found to be hemolytic, however it is now used to refer generally to phospholipids missing an acyl chain. LPLs are usually the result of phospholipase A-type enzymatic activity on regular phospholipids such as phosphatidylcholine or phosphatidic acid, although they can also be generated by the acylation of glycerophospholipids or the phosphorylation of monoacylglycerols. Some LPLs serve important signaling functions such as lysophosphatidic acid
What happens to fats ingested in the diet?
Bile salts emulsify dietary fats in the small intestine, forming mixed micelles
Intestinal lipases degrade triacylglycerols
Fatty acids and other breakdown products are taken up by the intestinal mucosa and converted into triacylglycerols
Triacylglycerols are incorporated, with cholesterol and apolipoproteins into chylomicrons
Chylomicrons move through lymphatic system and bloodstream to tissues
Lipoprotein lipase, activated by apoC-II in the capilary, converts triacylglycerols to fatty acids and glycerol
Fatty acids ender cells, where they are oxidised as fuel or reesterified for storage
chylomicron
The surface is a layer of phospholipids. Triacylglycerols inside (yellow) make up more than 80% of the mass. Several apolipoproteins are on surface (B-48, C-III, C-II) and signal uptake of chylomicrons
what is the difference between chylomicrons and micelles?
Chylomicrons have a single phospholipid layer, and the inside is a hydrophobic region dominated by triacylglycerols.
Vesicles have a lipid bilayer and so the inside is cytoplasmic or hydrophillic
Describe lipolysis
When low levels of glucose in the blood trigger the release of glucagon, (1) the hormone binds its receptor in the adipocyte membrane and thus (2) stimulates adenylyl cyclase, via a G protein, to produce cAMP. This activates PKA, which phosphorylates (3) the hormone-sensitive lipase and (4) perilipin molecules on the surface of the lipid droplet. Phosphorylation of perilipin permits hormone-sensitive lipase access to the surface of the lipid droplet, where (5) it hydrolyzes triacylglycerols to free fatty acids. (6) Fatty acids leave the adipocyte, bind serum albumin in the blood, and are carried in the blood; they are released from the albumin and (7) enter a myocyte via a specific fatty acid transporter. (8) In the myocyte, fatty acids are oxidized to CO2, and the energy of oxidation is conserved in ATP, which fuels muscle contraction and other energy-requiring metabolism in the myocyte.
PKA
The PKA enzyme is also known as cAMP-dependent enzyme because it gets activated only if cAMP is present. Hormones such as glucagon and epinephrine begin the activation cascade (that triggers protein kinase A) by binding to a G protein-coupled receptor (GPCR) on the target cell. When a GPCR is activated by its extracellular ligand, a conformational change is induced in the receptor that is transmitted to an attached intracellular heterotrimeric G protein complex. The Gs alpha subunit of the stimulated G protein complex exchanges GDP for GTP and is released from the complex. The activated Gs alpha subunit binds to and activates an enzyme called adenylyl cyclase, which, in turn, catalyzes the conversion of ATP into cyclic adenosine monophosphate (cAMP) – increasing cAMP levels. Four cAMP molecules are required to activate a single PKA enzyme. This is done by two cAMP molecules binding to each of the two regulatory subunits on a PKA enzyme causing the subunits to detach exposing the two (now activated) catalytic subunits. Next the catalytic subunits can go on to phosphorylate other proteins.
Below is a list of the steps involved in PKA activation:
- Cytosolic cAMP increases
- Two cAMP molecules bind to each PKA regulatory subunit
- The regulatory subunits move out of the active sites of the catalytic subunits and the R2C2 complex dissociates
- The free catalytic subunits interact with proteins to phosphorylate Ser or Thr residues.
Downregulation of protein kinase A occurs by a feedback mechanism: One of the substrates that are activated by the kinase is a phosphodiesterase, which quickly converts cAMP to AMP, thus reducing the amount of cAMP that can activate protein kinase A.
Thus, PKA is controlled by cAMP. Also, the catalytic subunit itself can be down-regulated by phosphorylation.
What two enzymes are involved in the conversion of fatty acid to a fatty acyl-CoA?
fatty acyl-CoA synthetase adds fatty acid to S-CoA in two steps using ATP, inorganic pyrophosphatase converts pyrophosphate (P2O7) into 2Pi
∆G’° for both steps of fatty acyl-CoA synthetase = -15 kJ/mol
∆G’° for inorganic pyrophosphatase = -19 kJ/mol
What is the two-step process by which a fatty acid is converted to fatty acyl-CoA by the enzyme ______________?
The two-step process by which a fatty acid is converted to fatty acyl-CoA by the enzyme fatty acyl-CoA synthetase is:
The carboxylate ion is adenylylated by ATP forming fatty acyl-adenylate (AMP bound to the fatty acid) and PPi (pyrophosphate, degraded to 2Pi by inorganic pyrophosphatase).
Thiol group of CoA attacks the acyl-adenylate (a mixed anhydride), displacing AMP and forming the thioester fatty acyl-CoA.
How do fatty acids enter the mitochondiral matrix?
After fatty acyl–carnitine is formed at the outer membrane or in the intermembrane space, it moves into the matrix by facilitated diffusion through the transporter in the inner membrane. In the matrix, the acyl group is transferred to mitochondrial coenzyme A, freeing carnitine to return to the intermembrane space through the same transporter. Acyltransferase I is inhibited by malonyl-CoA, the first intermediate in fatty acid synthesis. This inhibition prevents the simultaneous synthesis and degradation of fatty acids.
carnitine
Fatty acyl–carnitine formed in outer mitochondrial membrane moves into the matrix by facilitated diffusion through the transporter. In matrix, the acyl group is transferred to mitochondrial coenzyme A, carnitine returns to the intermembrane space through the same transporter.
How are fatty acids oxidised? Where do the products of oxidation go?
Stage 1: Fatty acid is oxidized to acetyl-CoA. This process is called β-oxidation.
Stage 2: Acetyl groups oxidized to CO2 via the citric acid cycle.
Stage 3: Electrons derived from the oxidations of stages 1 and 2 pass to O2 via the respiratory chain, providing the energy for ATP synthesis by oxidative phosphorylation.
β-oxidation I
A double bond is formed between the α and β carbons in the fatty acyl-CoA by acyl-CoA dehydrogenase, transferring 2H+ and 2e- to FAD, reducing it to FADH2. FADH2 enters the ETC.
β-oxidation II
The C=C double bond between carbons α and β is hydrated by enoyl-CoA hydratase, forming a β-hydroxy-acyl-CoA
β-oxidation III
The β-hydroxy acyl-CoA formed in step II is dehydrogenated, forming a β-ketoacyl-CoA, transferring 2 H+ and 2 e- to NAD+, reducing it to NADH + H+. NADH enters the ETC.
β-oxidation IV
C1 and C2 of the fatty-acyl-CoA, along with CoA, are cleaved from the rest of the fatty acid by acyl-CoA acetyltransferase (thiolase), another molecule of CoA-SH is bound to the remaining fatty acid, the end effect is shortening the chain by 2 carbons. Acetyl-CoA enters the TCA cycle.
How many molecules of acetyl-CoA are formed through the β-oxidation of a 14 carbon fatty acid?
7 molecules of acetyl-CoA, two carbons are broken off every four steps.
(number of carbons/2)
What are the products of one round of the β-oxidation pathway of an n carbon fatty acid?
1 FADH2 from step I, 1 NADH from step III, Acetyl-CoA and (n-2) Acyl-CoA from step IV.
Which carbons of a fatty acid are the first to be removed during β-oxidation? The last?
C1 and C2 are the first to be removed, during step IV another molecule of CoA-SH enters and attatches to C3 of the chain. When a fatty acid is fully β-oxidised, the last two carbons (C19 and C20 of a 20-cabon fatty acid, for example) are the last to be removed as acetyl-CoA
What are the reactions of the β-oxidation pathway similar to?
The first three steps are similar to the reactions that converty succinate to oxaloacetate in the TCA cycle
How much ATP can one molecule of palmitoyl-CoA (16 carbon fatty acyl-CoA) produce?
108!
Palmitoyl-CoA
Palmitoyl-CoA is part of the carnitine shuttle system, which transports other fatty acyl-CoA molecules into the mitochondria for β-oxidation.
16-carbon fatty acyl-CoA
Summarise the flow of electrons and protons through the ETC
Electrons reach Q through Complexes I (oxidation of NADH to NAD+) and II (oxidation of succinate to fumarate). The reduced Q (QH2) serves as a mobile carrier of electrons and protons. It passes electrons to Complex III, which passes them to another mobile carrier, cytochrome c. Complex IV then transfers electrons from reduced cytochrome c to O2. Electron flow through Complexes I, III, and IV is accompanied by proton flow from the matrix to the intermembrane space.
How are unsaturated fatty acids metabolised?
∆3, ∆2-enoyl-CoA isomerase converts β, γ cis bond to α, β trans bond
Oleic acid, as oleoyl-CoA (∆9), is the example used here. Oxidation requires an additional enzyme, ∆3, ∆2-enoyl-CoA isomerase, to reposition the double bond, converting the cis isomer to a trans isomer, a normal intermediate in β oxidation (enters β-oxidation II).
If acetyl-CoA is not used for the TCA cycle, what other metabolic pathyway can it enter?
Besides being oxidized via the citric acid cycle, acetyl Co-A has another fate in liver mitochondria.
In a process called ketogenesis, acetyl CoA is converted to acetoacetate or β-hydroxybutyrate. These compounds along with acetone are known as ketone bodies, even though β-hydroxybutyrate is not a ketone!
Serve as important fuels for heart, skeletal muscle, kidney cortex and the brain during starvation. Not used by liver.
acetone
acetoacetate
D-β-hydroxybuterate
Why would acetyl-CoA accumulate?
- Oxaloacetate is limiting because it is used for glucose in gluconeogenesis.
- NADH levels are high, so PDC, isocitrate dehydrogenase and alpha ketoglutarate DeH are shut down.
Conditions that promote gluconeogenesis (untreated diabetes, severely reduced food intake) slow the citric acid cycle (by drawing off oxaloacetate) and enhance the conversion of acetyl-CoA to acetoacetate. The released coenzyme A allows continued β oxidation of fatty acids
(further reading)
The acetyl-CoA produced by β-oxidation enters the citric acid cycle in the mitochondrion by combining with oxaloacetate to form citrate. This results in the complete combustion of the acetyl-CoA to CO2 and water. This is the fate of acetyl-CoA wherever β-oxidation of fatty acids occurs, except under certain circumstances in the liver.
In the liver oxaloacetate is wholly or partially diverted into the gluconeogenic pathway during fasting, starvation, a low carbohydrate diet, prolonged strenuous exercise, and in uncontrolled type 1 diabetes mellitus. Under these circumstances oxaloacetate is hydrogenated to malate which is then removed from the mitochondrion to be converted into glucose in the cytoplasm of the liver cells, from where it is released into the blood. In the liver, therefore, oxaloacetate is unavailable for condensation with acetyl-CoA when significant gluconeogenesis has been stimulated by low (or absent) insulin and high glucagon concentrations in the blood. Under these circumstances acetyl-CoA is diverted to the formation of acetoacetate and β-hydroxybutyrate. Acetoacetate, β-hydroxybutyrate, and their spontaneous breakdown product, acetone, are known as ketone bodies. The ketone bodies are released by the liver into the blood. All cells with mitochondria can take ketone bodies up from the blood and reconvert them into acetyl-CoA, which can then be used as fuel in their citric acid cycles, as no other tissue can divert its oxaloacetate into the gluconeogenic pathway in the way that the liver does this. Unlike free fatty acids, ketone bodies can cross the blood-brain barrier and are therefore available as fuel for the cells of the central nervous system, acting as a substitute for glucose, on which these cells normally survive. The occurrence of high levels of ketone bodies in the blood during starvation, a low carbohydrate diet, prolonged heavy exercise and uncontrolled type 1 diabetes mellitus is known as ketosis, and in its extreme form in out-of-control type 1 diabetes mellitus, as ketoacidosis.
How are ketone bodies produced?
Normally, ketone body formation is low.
When acetyl-CoA accumulates (as in starvation or untreated diabetes), thiolase catalyzes the condensation of two acetyl-CoA molecules to acetoacetyl-CoA and then to HMG-CoA, the parent compound of the three ketone bodies.
Ketone body formation occur in the matrix of liver mitochondria.
β-hydroxy-β-methylglutaryl-CoA (HMG-CoA)
Basically, what is HMG-CoA?
Three acetyl groups connected to CoA-SH
How are ketone bodies used in the cells that recieve them?
Once inside target cell, the reaction that forms ketone bodies is reversed
D-β-Hydroxy-butyrate from liver travels through blood to other tissues, where it is converted in three steps to acetyl-CoA.
What enzyme crucial to ketone body metabolism is missing in the liver? Why?
β-ketoacyl-CoA transferase, which hydrolyses succinyl-CoA into succinate and CoA-SH. This is required to catabolise ketone bodies. It is missing in the liver because the liver makes ketone bodies for other organs that cannot recieve fatty acids directly to catabolise, such as the brain.
What is D-β-hydroxybutyrate broken down into in cells to allow it to enter the TCA cycle?
2 acetyl-CoA molecules
malonyl-CoA
Malonyl-CoA
Describe the difference between active and inactive acetyl-CoA carboxylase
PKA phosphorylation causes shift from polymers (active) to protomers (inactive).
What are the three functional regions of acetyl-CoA carboxylase?
biotin carrier protein (gray);
- biotin carboxylase, which activates CO2 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction.
- transcarboxylase, which transfers activated CO2 (shaded green) from biotin to acetyl-CoA, producing malonyl-CoA.
Where does the carbon that is used to form fatty acids ultimately come from?
Bicarbonate (HCO3-)
biotin carrier protein
Holds activated CO2 and transfers it between catalytic regions of the acetyl-CoA carboxylase enzyme
what is the first catalytic site of acetyl-CoA carboxylase?
Biotin carboxylase, which activates CO2 by attaching it to a nitrogen in the biotin ring in an ATP-dependent reaction.
What is the second catalytic site in acetyl-CoA carboxylase?
Transcarboxylase, which transfers activated CO2 from biotin to acetyl-CoA, producing malonyl-CoA.
What is the difference between the yeast and animal version of fatty acid synthase vs. bacterial versions? What is the difference between the yeast and animal version of fatty acid synthase and mammalian fatty acid synthase?
Fatty acid synthesis involves seven enzymatic reactions that are carried out in bacteria by seven unique enzymes.
In yeast and animals all activities as well as ACP are found in one protein.
The mammalian enzyme (shown) consists of two identical large polypeptide chains. It is a dimer.
The mammalian enzyme likely evolved from fusion of the individual bacterial genes.
When fatty acid synthase is primed, what groups are bound to what subunits?
As process starts, acetyl (or longer acyl) group bound to β-ketoacyl-ACP synthase and malonyl bound via a thioester bond to acyl carrier protein, both are delivered to the protein complex by CoA-SH
What binds malonyl-CoA and acetyl-CoA to the fatty acid synthase complex?
malonyl/acetyl transferase
Fatty acid synthase step I
Ketoacyl ACP synthase condenses acyl group and two carbons derived from malonyl-CoA, with elimination of CO2, extends chain by two carbons.
The β-keto product of this condensation is then reduced in three more steps nearly identical to the reactions of β-oxidation, but in the reverse sequence.
Fatty acid synthase step II
the β-keto group is reduced to an alcohol with electrons coming from NADPH by β-ketoacyl-ACP-reductase. (β-carbon is reduced!)
Fatty acid synthase step III
elimination of H2O by β-hydroxacyl-ACP dehydratase creates a double bond.
NOTE THAT THE CHEMISTRY IS THE REVERSE (NEARLY) OF FATTY ACID BREAKDOWN.
Fatty acid synthase step IV
Enoyl-ACP recductase reduces the double bond, again with NADPH, to form the corresponding saturated fatty acyl group
How is the fatty acid synthase complex recharged after the addition of 2 carbons to the growing fatty acid?
Translocation of growing fatty acid to Cys residue on β-ketoacyl-ACP synthase from acyl carrier protein (ACP), then addition of a new malonyl-CoA to acetyl carrier protein synthase to begin addition of another 2 carbons
Overview of how fatty acid chains grow from fatty acid synthase
The fatty acyl chain grows by two-carbon units donated by activated malonate, with loss of CO2 at each step. The initial acetyl group is shaded yellow, C-1 and C-2 of malonate are shaded pink, and the carbon released as CO2 is shaded green. After each two-carbon addition, reductions convert the growing chain to a saturated fatty acid of 4, 6, 8 carbons etc. until (16:0) (palmitate)
The first acetate added to a growing chain of palmitate ends up where in the final molecule?
At the end of the chain, the fatty acid is grown from “highest” carbon number to lowest, or towards the carboxyl group. The original methyl of acetate is the methyl of palmiate.
Acyl carrier protein
The prosthetic group is 4′-phosphopantetheine, which is covalently attached to the hydroxyl group of a Ser residue in ACP.
Phosphopantetheine contains the B vitamin pantothenic acid, also found in the coenzyme A.
SH group is the site of entry of malonyl groups during fatty acid synthesis
Where does the NADPH required for fatty acid synthase come from?
The pentose phosphate pathway
How are acetyl groups passed from the mitochondria to the cytosol?
Acetyl groups pass out of the mitochondrion as citrate. Citrate lyase produces acetyl-CoA for fatty acid synthesis. Oxaloacetate is reduced to malate. Malate is oxidised by malic enzyme to generate cytosolic NADPH.
What is the rate limiting step in fatty acid synthesis?
Acetyl-CoA carboxylase is the rate limiting enzyme of fatty acid synthesis. Both allosteric regulation and hormone-dependent covalent modification influence the flow of precursors into malonyl-CoA.
What inhibits fatty acid synthesis, and what enzyme is controlled in the process?
glucagon and epinephrine trigger phosphorylation/inactivation of acetyl-CoA carboxylase
Palmitoyl-CoA (part of carnitine shuttle system, later stage product of fatty acid synthesis) inhibits acetyl-CoA carboxylase
What activates fatty acid synthesis, and what enzyme is controlled in the process?
insulin triggers activation of citrate lyase, which produces acetyl-CoA, and citrate triggers activation of acetyl-CoA carboxylase, which produces malonyl-CoA from acetyl-CoA
