Clark Biochemistry (Exam 2) Flashcards
Define the (3) classes of complex lipids and their subclasses
1) TAG (Triacylglycerol) = glycerol + 3 fatty acids (FA)
2) Glycerophospholipids = glycerol backbone + phosphdiester bond of chemical grp (ex. choline, serine) at C3 position
3) Sphingolipids = ceramide backbone + another grp. (2) subclasses:
- Sphingophospholipids, which only includes Sphingomyelin = ceramide + phosphocholine
- Glycolipids = ceramide + carbohydrate
What is the role of glycerol-3-phosphate (G-3-P), phosphatidic acid, and diacylglycerol (DAG) in TAG and glycerophospholipid synthesis?
Synthesis of TAG and glycerophospholipids begins w/ G-3-P and PA then DAG are intermediates in the process.
- Acetyltransferases take the two -OH (ols) and replace them w/ 2 FA at C1 and C2 to make Phosphatidic acid.
- Then, a phosphatase replaces the C3 phosphate w/ an -OH to make DAG (diacylglycerol) = glycerol + 2 FA + OH
- Finally a phosphodiester bond is formed between either choline, ehtanolamine, serine, or inositol at C3
What are the (2) major mechanisms for the synthesis of glycerophospholipids and the end products for each?
Route 1: starts from DAG. And the head groups choline or ethanolamine are activated by attaching to CDP. This pathway is used to synthesize phosphatidylcholine (PC) or phosphatidylethanolamine (PE)
Route 2: starts with Phosphatidic acid, which FIRST reacts with CTP to form CDP-diacylglyercol. Then that condenses w/ a head grp to make either Phosphatidylinositol (PI) or Cardiolipin.
List the (4) major glycerophospholipids
Phosphatidylcholine (PC)
Phosphatidylethanolamine (PE)
Phosphatidylinositol (PI), and
Cardiolipin
What process is phospholipase C (PLC) involved in? How does the action of PLC generate calcium signaling in the cell?
PLC is one of the enzymes that does glycerophospholipid degradation (i.e. releases fatty acids). PLC cleaves the phosphoester bond at C’3 from PI making DAG + IP3.
IP3 then increases intracellular Ca and calcium signaling!
What is the deficiency in RDS (respiratory distress syndrome)? How do you determine fetal lung maturity based on complex lipid levels?
Dipalmitoylphosphatidylcholine deficiency contributes to RDS, due to its ability to decrease surface tension at the air-water interface.
The PC : sphingomyelin ratio is an indicator for lung maturation. At 34 wks, PC levels rise rapidly while sphingomyelin stays constant or decreases. A ratio of 2.0 or better is a good indicator that lung surfactant levels are sufficient.
What are the (4) sphingolipid classes and members of each class?
1) Phosphosphingolipids. Only member = sphingomyelin
2) Cerebrosides (neutral). Includes Glucocerebrosides, Galactocerebrosides, and Globosides
3) Sulfatide (acidic)
4) Gangliosides. Members = GM1-GM4
What is the fundamental (overall) problem for the sphingolipidoses disorders? What is pattern of inheritance?
Sphingolipidoses are inherited, autosomal recessive disorders that result in a block of breakdown of sphingolipids due to either 1) enzyme deficiency in the degradation pathway, or 2) defect in activator protein of degradative enzyme.
Overall feature = ACCUMULATION of sphingolipids in cells
What are the enzyme deficiencies in Tay-Sachs, Gaucher’s, and Niemann-Pick disorders?
Tay-Sachs = HexA (accum of GM2) Gaucher's = beta-glucosidase (accum of glucocerebrosides) Niemann-Pick = sphingomyleinase (accum of sphingomyelin)
Classic clinical feature of Tay-Sach’s
cherry-red spot on the macula (which indicates the sphingolipid GM2 accumulation due to HexA deficiency)
Classic clinical feature of Gaucher’s
cells w/ cytoplasm that have a “wrinkled tissue paper” appearance (which is consequence of glucocerebroside deposition due to beta-glucosidase [a glucocerebrosidase enzyme] deficiency)
Distinguishing feature of the phosphosphingolipid class of sphingolipids
1 member of this class = Sphingomyelin. Is the ONLY phospho-containing sphingolipid. Has NO carbohydrate
Distinguishing feature of the Cerebroside (Neutral) class of Sphingolipids and (3) members of this class
this class defined by attachment of either glucose or galactose to the ceramide backbone. Since the carb attachment has no charge, these glycosphingolipids (aka “glycolipids”) are uncharged/neutral.
(3) members = Glucocerebrosides (glucose only), Galactocerebrosides (galactose only), and Globosides (both glucose and galactose)
Distinguishing feature of Gangliosides class of Sphingolipids and nomenclature for gangliosides
contain actylated sugars GlcNac, GalNac, and a NANA attachment to galactose. Members = GM1-GM4.
G = ganglioside then M (mono), D (di), or T (tri) = # of NANA attachments
What is the function of hormone sensitive lipase (HSL)?
enzyme that removes FFA from TAG. This is a major control point for FA beta-oxidation in the liver and extrahepatic tissues (muscle)
Regulation of HSL with glucagon v. insulin. Name (3) additional means of regulating HSL
Glucagon phosphorylates and stimulates HSL. Insulin inhibits HSL (indirectly through a phosphodiesterase).
Also:
-Catecholamines, NE > Epi stimulate HSL
-Transcriptional reg: Glucocorticoids and Thyroid hormone increase HSL mRNA and protein levels
-Caffeine stimulates HSL and counters insulin action
How is it that HSL activity in ADIPOSE tissue controls fatty acid beta-oxidation and ketone body (KB) synthesis in the LIVER?
Acetyl-coA produced in the mito as a product of FA-beta-oxidation = substrate for KB synthesis. Glucagon stimulates HSL and lipolysis in adipose tissue, which releases FFA from TAG for beta-oxidation. Therefore, ketone body synthesis in the liver is dependent upon TAG metabolism in adipose.
How do LCFA (long chain fatty acids) get transported into the mitochondria for FA-beta oxidation? Name (4) enzymes that complete this.
First, the FA is activated to Acyl-coA by Acyl-coA synthetase (outer membrane). Then, through a reversible exchange of CoA for carnitine, the FA is driven into the mito matrix. (3) enzymes that accomplish this, in order =
-CPT I (outer membrane), Carnitine acylcarnitine translocase (inner), and CPT II (inner)
What are Acyl-CoA dehydrogenases? How does a defect in acyl-coA dehydrogenase lead to hypoglycemia?
A family of enzymes that distinguish between FA based on chain length. Are the first step in beta-oxidation. So mutations in genes encoding acyl-coA dehydrogenases lead to deficiencies in beta-oxidation. Ex. MCAD.
Hypoglycemia results from decreased ability to use FA for energy during fasting state. So muscles and other extrahepatic tissues that would be using FA now take in additional glucose leading to hypoglycemia.
What are ketone bodies (KB) and what do they do? Name the 2 ketone bodies.
KB are short, 4-carbon carboxylic acids composed of two acetyl-coAs. Function = shuttle Acetyl-coA from the liver to extrahepatic tissues as a way to supply energy during fasting. Those tissues oxidize KB back to acetyl-coA to enter the TCA cycle and make ATP.
(2) KB = Acetoacetate and Beta-hydroxybutyrate.
How is the synthesis of ketone bodies (KB) dysregulated in T1DM?
T1DM leads to OVERproduction of ketone bodies. Due to less insulin, missing the inhibition of HSL. So continued activity of HSL means increased levels of FA to generate Acetyl-coAs. These acetyl-coAs are substrates for KB synthesis, so T1DM leads to increased level of KB = Ketoacidosis.
Only place in body where ketone bodies are synthesized
Liver. Liver is required to supply both glucose and ketone bodies.
Cellular location of FA synthesis v. degradation
Synthesize FA in cytoplasm and degrade (by beta-oxidation) in the mitochondria. This makes sense b/c degradation generates Acetyl-coA, which can go right into the TCA cycle in the mito
Basic cause of NAFLD (non-alcoholic fatty liver disease)
insulin resistance. When insulin resistant, adipose tissue is NOT responding to insulin, so there is continual hydrolysis and the FA pool keeps coming back to the liver. The increase in FA exceeds the liver’s capacity of VLDL synthesis, so get accumulation of fat droplets.
What is the purpose of FA beta-oxidation?
to generate acetyl-coA for use in the TCA cycle
Insulin signaling. Name (3) forms of storage it promotes and (2) catabolic processes it shuts down
Insulin is controlling hormone in Fed state. Promotes glucose utilization and nutrient storage in forms of glycogen, TAG, and protein.
At same time, insulin signaling shuts down catabolic processes for glycogen and TAG breakdown (inhibits HSL)
What does the citrate shuttle do? Why?
Transports acetyl-coA out of the mito into the cytosol. Why = FA biosynthesis. Acetyl-coA is made inside mito from pyruvate of glycolysis. But must be shuttled out into the cytoplasm for FA synthesis.
How is FA synthesis directly related to carbohydrate (and protein) intake?
From fact that decarboxylation of pyruvate makes Acetyl-coA (in mito), and acetyl-coA is substrate for FA synthesis (in cytoplasm). Pyruvate levels maintained by glyolysis and amino acid metabolism activated by insulin in fed state. Thus, greater the carb (protein) intake, the more FA synthesized.
Levels of which molecule control citrate movement to the cytoplasm from the mitochondria?
ATP. Increase in ATP leads to increased cytosolic citrate levels to be used for more FA synthesis.
(2) enzymes of FA synthesis
ACC = acetyl-coA carboxylase (regulated step!) FAS = fatty acid synthase
What is ACC and how is it regulated?
ACC = acetyl-coA carboxylase. It is the first step in FA synthesis. Regulated multiple ways:
- Hormonal: Insulin stimulates ACC. Glucagon and Epi inhibit it.
- Allosteric: citrate binding promotes ACC activity
- Long-term diet-induced: chronic high calorie diet will increase transcription of ACC and vice versa.
Name the (2) essential fatty acids. What makes them essential?
omega-3-linoleNic and omega-6-linoleic acid. Humans lack the desaturase enzymes that catalyze the reduction of the fatty acid chain within 6 carbons of the omega end. Since we can’t synthesize omega-3 or -6, must get from diet. These 2 FA are required for precursors of Eicosanoid hormones.
(4) stages and the major intermediates in cholesterol biosynthesis
1) Regulated step. HMG-CoA synthase converts Acetyl-coA into HMG-CoA. Then, HMG-CoA reductase (REGULATED STEP) converts HMG-CoA into Mevalonate.
2) Energy expended step. 3 ATP required to turn mevalonate into C5 isoprene building blocks
3) Relevant lipid intermediates step. Condensing the C5 isoprenes make Geranyl and Farnesyl pyrophosphate. More condensation makes Squalene.
4) Final enzyme/SLOS step. Squalene into Lanosterol. Then, 7-dehydrocholesterol reductase (encoded by DHCR7 gene) turns lanosterol into cholesterol.
Name (2) biologically relevant lipid intermediates in the cholesterol synthesis pathway and how they relate to Ras
Farnesyl and Geranyl pyrophosphate. Both serve as lipid anchors for membrane proteins (“protein prenylation”).
Ras = protein oncogene that is prenylated and attached to cell membrane. Since Ras is overactivated in a lot of cancers, there are clinical trials trying to inhibit the enzymes that attach farnesyl or geranyl (lipid anchors) to the membrane protein (Ras)
What is major (non-transcriptional) regulation of HMG-CoA reductase?
Intracellular cholesterol levels through SREBP-2
Low cholesterol = high expression of HMG-CoA reduct
High cholesterol = low expression of HMG-CoA reduct
What does SREBP-2 do? What 2 genes it is associated with?
SREBP-2 is an integral membrane protein in the ER that works as a transcription factor for (2) genes: HMG-CoA reductase and LDL receptor. When cholesterol levels are low, it moves to the nucleus and promotes transcription of the 2 genes = increased cholesterol synthesis. High cholesterol keeps SREBP in the ER so that no transcription occurs.
How do glucagon and insulin (post-transcriptionally) regulate HMG-CoA reductase activity?
Phosphorylation of HMG-CoA reductase INACTIVATES it:
- Glucagon phosphorylates and inactivates it (b/c cells are in catabolic state to meet energy demands, so cholesterol synthesis is suppressed.)
- Insulin phosphorylates and ACTIVATES it!
Overall effect of statin drugs on HMG-CoA reductase
INHIBIT HMG-CoA reductase by acting as competitive inhibitors and binding to it to halt its activity
Outline the (4) steps of LDL receptor-mediated endocytosis
1) Binding: LDL binds to receptor on the plasma membrane
2) Internalization: complex of LDL + its receptor are internalized into the lysosome where LDL then dissociates from the receptor
3) Hydrolysis: LDL degraded by lysosome into cholesterol esters (CE). Then CE hydrolyzed to free cholesterol
4) Delivery: cholesterol delivered to ER w/in the cell and LDL receptors RECYCLED back to plasma membrane to take up more LDL
(3) regulatory actions that increased intracellular cholesterol has inside a cell after LDL-receptor mediated endocytosis
- decreased HMG-CoA reductase activity;
- INCREASED ACAT activity (which catalyzes formation of cholesterol esters); and
- decreased LDL receptors
