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Biochemistry: LIPIDS: Fatty Acids, Ketone Bodies, Cholesterol, Triglycerides

11. Outline the steps of fatty acid biosynthesis. Where does this process occur in the cell? What is the most important organ for fatty acid biosynthesis?

Fatty acids are built, for the most part, in the cytosol of liver cells. There are 7 steps to add 2 carbons to the chain. Most fatty acid chains have even numbers of carbons. It takes 7 cycles in the synthesis loop to make palmitate, with 16 carbons.

Step 1 involves converting acetylCoA to malonylCoA. Step 2 adds ACP to an acetylCoa, and Step 3 adds ACP ato malonylCoA. Step 4 combines malonylACP and acetylACP together to make acetoaceytlACP. Step 5 converts that to 3 hydroxybutyrylACP, and Step 6 to crotonylACP. Step 7 results in butyrylACP.

12. What is the key regulatory enzyme for fatty acid biosynthesis? Acetyl CoA Carboxylase. What metabolites are responsible for regulating this pathway? citrate (+) and palmitoyl CoA (-). What are the hormonal regulators? insulin and glucagon

13. What vitamin-derivatives are required for fatty acid synthesis? biotin, niacin and pantothenic acid. What is the role of biotin in fa synthesis? It helps the enzyme in the first step, carboxylation via acetyl CoA carboxylase---biotin carries the CO2. what is the energy inpu required for each cycle of fatty acid biosynthesis? one ATP and two NADPH.

14. Where in the cell does fatty acid desaturation and elongation occur? in the smooth ER, malonyl CoA is added onto palmitate, while the mitochondrial system elongates short & medium chains by adding acetyl CoA. Desaturation is only via oxygenases that are in the sER membrane and require O2 and NADPH. What vitamins are required for these processes? NADPH (niacin) ANY MORE???


15. Be able to lay out the rxn sequence in the conversion of long chain fatty acid via beta-oxidation to acetyl CoA. OK, first, the fatty acid chains must be activated by adding CoA. Two enzymes do this, ER acyl CoA synthetase does it for long chains, and short or medium chains use mitochondrial acyl CoA synthetase. After activation they go into the mitochondrial matrix. Long chains get help from the carnitine shuttle (several steps) and short/med chains can diffuse through the membrane. Then there are four reactions that are repeated, each cycle freeing two carbons from the chain as Acetyl CoA. The reactions are, in order, oxidation, hydration, oxidation, thiolysis. The freed acetyl CoA can go into the Krebs cycle, or be made into ketone bodies in the liver, or go into an assortment of other pathways. It's a key intermediate for sure.

Be able to calculate the energy production from fatty acid oxidation for any fatty acid chain length. OK. Overview, take the # of carbons in the chain, divide by two and subtract one. Multiply that number by 5 and you get how many ATP are generated from beta oxidation. Then you have to figure out the energy from the acetyl CoA's going into the Krebs cycle. Take your original chain length, divide by two and multiply by 12, and you get that number. Add the first two numbers [(#-1)/2]x5 + (#/2)x12 and then subtract the ATP's used for activation (2) and you've got your energy production. Unless it's unsaturated, then you get 2 more ATP's for each double bond.

How does beta-oxidation of an unsaturated fatty acid differ from that of a saturated fatty acid? How does the net energy production differ for these two types of fatty acids? When there's already a double bond in the chain, you don't have to creating any, which costs ATP. So you get a 2 ATP higher energy yield per double bond.

16. Where does beta-oxidation of fatty acids occur in the cell? in the mitochondrial matrix. In what tissues/organs is beta-oxidation prominent? heart, liver and muscle, though it occurs in all cells.

17. What controls the rate of fatty acid oxidation? enzyme: acyl CoA dehydrogenase, also malonyl CoA inhibits it, insulin slows it and glucagon encourages it. What is the role of carnitine in fa oxidation? It's an essential part of the shuttle that gets long chains into the mitochondria. Is carnitine a vitamin derivative? No, it's an amino acid derivative---can be synthesized from lysine or methionine, or consumed in meat or dairy foods.

18. Derivatives of which vitamins are required for fatty acid oxidation? niacin, riboflavin, pantothic acid, biotin, B12 for odd chains and cobalt is in the B12.

19. How does the complete oxidation of odd-chain length fatty acids differ from oxidation of even chain? Well you get a 3 carbon propionyl CoA left over, and you need a couple of special enzymes to finish breaking it down. One of them requires biotin again, and you get succinylCoA which is an intermediate in the Krebs cycle. Than answers the next Q: what intermediate does the Krebs cycle and beta-oxidation of odd chain fatty acids have in common (succinyl CoA). Identify a source of odd chain fa's in human nutrition: dairy from cows has 5C chains and other odd lengths due to the microbes that are in cow guts.

20. What hormones promote lipolysis and beta-oxidation? glucagon and glucagon.


21. What three compounds are called ketone bodies? acetoacetate (first formed), acetone (the least useful) and beta-hydroxybutyrate (the most important one). What is the organ of ketone body formation? the liver. In what cellular compartment are KB's made? the mitochondrial matrix. Under what condition are KB's made? when there is a lot of fatty acid oxidation going on, and the acetyl CoA levels get high. Why are ketone bodies produced? Because it's a way to make use of the acetyl CoA when it piles up, and because it can be used as a water soluble energy source for the periphery, esp the brain during a fast. By what mechanism do KB's enter the circulation? diffusion (they're small)

22. What is the precursor of KB's? acetyl CoA. What is the product of KB breakdown? same. Why might the breath of an untreated diabetic have the odor of acetone? Because the diabetic can't process glucose and fatty acid oxidation is in high geare, producing excessive acetyl CoA. This in turn depletes NAD+ and oversupplies NADH, which depresses the Krebs cycle, so the excess acetyl CoA can't be processed that way. Betahydroxybutyrate is used by the tissues but uptake of acetone isn't as good, so it accumulates in the blood and gets breathed out via the lungs. What orgon has a heavy dependence on KB's during starvation? brain.


23. Describe the formation of triglycerides. Well let's see, they can be made from glycerol or from glucose via DHAP. It costs some energy (an ATP for glycerol or an NADH for DHAP) to activate the precursor to G3P, glycerol-3-phosphate. Add to that a couple of fatty acylCoA's, then take away the phosphate and add one more acylCoA, the acylCoA ester bond is busted and the three fatty acid chains are added to the glycerol to make triacylglycerol, which is then sent out to the body via the liver as VLDL's, or is deposited in adipose.

Where are TB's stored in adipocytes? fatty droplets in the cytosol

What is the rate-limiting enzyme for breakdown of TG's? hormone-sensitive lipase, activated via cAMP 2nd messenger system. How is triglyceride breakdown regulated hormonally? (+) by glucagon & epinephrine, which activate the 2nd messenger system. (-) by insulin.

24. In which organ (liver?), in which pathway (?), and at which step (?) does the glycerol liberated by the hydrolysis of a trigrlyceride enter intermidiary metabolism??? Why isn't the glycerol utilized by adipocytes? because they lack glycerol kinase, the enzyme. What advantages do triglycerides have over carbs as an energy storage form? They are much more energy dense: 9kcal/gram vs 4.


1. What roles are played by steroids in human physiology? lots! glucocorticoids (cortisol) and mineralocorticoids (aldosterone) together are known as corticosteroids. sex hormones (androgens, estrogens, progestins) all made from steroid base. What is the parent compound of steroids in the body? cholesterol.

2. What organ is responsible for most cholesterol biosynthesis? liver.

3. What is the soucr of carbons in cholesterol? acetyl group of acetyl CoA. Why is cholesterol synthesis a high energy requiring process? You're building four interconnected rings: it's a high E molecule.????

4. What are the products of the four statges of cholesterol biosynthesis? 1. mevalonate 2. activated isoprene (isopentenyl pyrophosphate) 3. squalene (30c) 4. lanosterol ---to cholesterol.

5. What is the rate limiting step (which stage) in cholesterol synthesis? stage 1. What enzyme catalyzes this step? HMG CoA reductase. Describe the regulation of cholesterol synthesis. OK. If you don't eat, you don't build cholesterol, it's done in the "fed condition". Eating carbohydrates elevates synthesis, while eating cholesterol actually reduces synthesis. Hormonally, insulin boosts production and glucagon inhibits it.

In what other pathway does HMG CoA appear? in KB synthesis. Do these pathways intersect? maybe so. ???

In what form is cholesterol stored? esterified with a long fatty acid. What two enzymes are involved in storage? LCAT and ACAT. How is cholesterol removed from the body? In poop by way of bile in the gut, in sweat, or in shed skin.

6. What intermediate in cholesterol synthesis is the starting point for a multitude of products, including lipid-soluble vitamins, dolichols and ubiquinone? activated isoprene (isopentenyl pyrophosphate). What are the roles of dlichol and ubiquinone? Dolichol is a long thing that spans the ER membrane multiple times, helps make glycoproteins. Ubiquinone is CoQ10, an antioxidant and part of the electron transport chain.

7. What are bile acids? cholic and chenodeoxycholic acid, 24 carbons each, pK ~ 6, based on steroids, used to make bile. Where are bile acids made in the body? In the liver, several organelles but esp the ER. What role do bile salts play in digestion? they emulsify fats. Where are bile salts produced? in the liver via conjugation with either taurine or glycine.

Discuss the enterohepatic circulation of bile acids. Well I don't know why but they go around in circles. Here's the route. First primary BA's are made in the liver. They are changed from primary to secondary in the gut, by the bacteria there. Then they are taken back to the liver where they are conjugated with glycine or taurine to make bile salts (not acids anymore). Back to the gut, where they are deconjugated. Back to the liver, where they are reconjugated, then secreted via bile back into the gut. Back to the liver via the lymphatic portal system where they are reconjugated. Back to the gut where they are deconjugated. etc etc etc. Why? Not fully clear yet.

8. What steroid is the common precuros (branch point) of the steroid hormones? pregnenolone. Which tissues and organs are most active in steroid hormone biosynthesis? adrenals, gonads, thyroid, etc.


9. Detail the process of digestion and transport of lipids from the gut following a meal. Well let's see....the bile salts emulsify the fats as they are being broken down in the duodenum, and the enterocytes (intestinal absorbtion cells) eventually take in all the bits of fat and bile too. From there they get transported into lymphatic chyle as chylomicrons where the portal takes the fat globs into circ via the left subclavian, and to the tissues and liver. Anyway the chylomicrons get to the liver where they are processed into TAG's and packed into VLDL's which are sent via circ to the body, shrinking the VLDL's to IDLs, LDLs as the TAG's are removed. Eventually they become HDL's which pick up cholesterol from the tissues and bring it back to the liver.

What are the 3 types of lipases required for breakdown of dietary lipids? Synthesized where?
There are the acid lipases in saliva (from glands in the mouth) and gastic juice (secreted from the stomach) that break down small and medium chain lipids. The pancreatic lipases (made in pancreas?) cleave the ester bonds of triglycerides, with the help of colipase which helps the enzyme attach to micelles. The pancreas also secretes phospholipase, so if pancreatic is the 2nd category I don't know yet what the 3rd is. ??? The lipoprotein lipase frees fatty acids from VLDL's at the tissues, but I don't think that's breaking down dietary lipids, exactly.

10. What is the function and site of formation of each of the lipoprotein particles? Also match lipoproteins to their lipoprotein particles, the Apos: E, B-100, B-48, C11, A. The chylomicron is the largest particle, and is formed by the fats that are absorbed from the intestines into the lymphatic system. It has all the apos except B-48. The VLDL is made in the liver, and is the next largest and next least dense particle due to the large amount of TAG's inside it. It has B-100, C and E apos. The LDL lipoprotein is what's left over after the VLDL has been to the tissues and dropped off its TAG's. It has B-100. And the HDL is the smallest and densest lipoprotein, I'm not sure where it is made but it could be created in the circulation from the LDL's. It has A-1 and A-II, and is a transient reservoir for C-II and E apos.

11. What is the purpose of receptor-mediated endocytosis of LDL? To absorb fats & chol into cells, for use or for storage???? What role might this process play in hypercholesterolemia? Wellllllll. When cells bind to the apoB and E on the lipoprotein they swallow the glob. When they have more receptors for those apos, the levels of LDL and cholesterol in the circulation are lowered. When the cell has enough cholesterol it reduces the number of receptors, which increases the amount of cholesterol in the circulation. So hypercholerolemia could be caused by downregulation of receptors due to cells having enough cholestol already, thanks anyway.

12. Outline the proposed scheme for development of arteriosclerosis? LDL's are circulating too long, system is not health, LDL's get oxidized. Once they are oxidized they get eaten by macrophages with not off switch, and the macrophages get engorged and become foam cells in the subendothelium of the circulatory vessels. Smooth muscle cells migrate in to cover the deposits of foam cells, and gradually those deposits of lipid and dead macrophages can accumulate bony material and become quite rigid. Voila, sclerosis.
Tags: biochemistry, blood, cholesterol, diabetes, fasting, hormones, lipids, liver, metabolism, obesity, vitamins

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