LIPID CATABOLISM

1. LIPID CATABOLISM

8. Carnitine

9. Formation of acyl carnitine • Can get transported across inner mitochondrial membrane

11. Summary of one round of the b-oxidation pathway: fatty acyl-CoA + FAD + NAD+ + HS-CoA fatty acyl-CoA (2C less) + FADH2 + NADH + H+ + acetyl-CoA The b-oxidation pathway is cyclic. The product, 2 carbons shorter, is the input to another round of the pathway. If, as is usually the case, the fatty acid contains an even number of C atoms, in the final reaction cycle butyryl-CoA is converted to 2 copies of acetyl-CoA. 

12. Fate of Acetyl CoA • Oxidation in Krebs Cycle • Synthesis of Acetylcholine • Synthesis of Cholesterol • Synthesis of FAs

13. Oxidation in Krebs Cycle • The major proportion of Acetyl-CoA may be directed into TCA for oxidation and production of energy. • Acetyl-CoA forms the central meeting point for oxidation of carbohydrates and fats through Krebs cycle.

14. 2. Synthesis of Acetylcholine: • Acetyl-CoA may be utilized for acetylation reactions. Mainly in brain tissues!!

15. Synthesis of Cholesterol • Acetyl-CoA is the starting material for the biosynthesis of cholesterol. • It condenses with acetoacetyl-CoA to form β-hydroxy-β-methylglutaryl-CoA (HMG CoA) which is an important intermediate in the synthesis of cholesterol.

16. Synthesis of Fatty Acids • Acetyl-CoA units are the building blocks for the synthesis of fatty acids. • Formation of ketone bodies • When the rate of acetyl-CoA formation exceeds the capacity of Krebs cycle to deal with them, the following side reactions takes place…

17. b-Hydroxybutyrate Dehydrogenasecatalyzes reversible interconversion Ketone bodies are transported in the blood to other cells, where they are converted back to acetyl-CoA for catabolism in Krebs cycle, to generate ATP. While ketone bodies thus function as an alternative fuel, amino acids must be degraded to supply input to gluconeogenesis when hypoglycemia occurs, since acetate cannot be converted to glucose.

18. Ketone body synthesis: b-Ketothiolase. The final step of the b-oxidation pathway runs backward. HMG-CoA Synthase catalyzes condensation with a 3rd acetate moiety (from acetyl-CoA). HMG-CoA Lyase cleaves HMG-CoA to yield acetoacetate & acetyl-CoA.

20. Acetone may undergo 2 metabolic changes • It may be further cleaved to yield acetic acid and formic acid. • CH3COCH3  CH3COOH + HCOOH • It can be transformed to propanediol which in turn gets oxidised to pyruvic acid. • CH3COCH3  CH3CHOHCH2OH CH3COCOOH

21. b-Oxidation Pathway: Step 1. Acyl-CoA Dehydrogenase catalyzes oxidation of the fatty acid moietyofacyl-CoA to producea double bond between carbon atoms 2 & 3. There are different Acyl-CoA Dehydrogenases for short (4-6 C), medium (6-10 C), long and very long (12-18 C) chain fatty acids. Very Long Chain Acyl-CoA Dehydrogenase is bound to the inner mitochondrial membrane. The others are soluble enzymes located in the mitochondrial matrix.

22. FAD is the prosthetic group that functions as e- acceptor for Acyl-CoA Dehydrogenase. Proposed mechanism: A Glu side-chain carboxyl extracts a proton from the a-carbon of the substrate, facilitating transfer of 2 e- with H+ (a hydride) from the b position to FAD. The reduced FAD accepts a 2nd H+, yielding FADH2.

23. The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribityl moiety of FAD, giving this part of FAD a role in positioning the substrate and increasing acidity of the substrate a-proton.

24. The reactive Glu and FAD are on opposite sides of the substrate at the active site. Thus the reaction is stereospecific, yielding a trans double bond in enoyl-CoA.

25. The carbonyl O of the thioester substrate is hydrogen bonded to the 2'-OH of the ribitol moiety of FAD, giving the sugar alcohol a role in positioning the substrate and increasing acidity of the substrate a-proton.

26. FADH2is reoxidized by transfer of 2 electrons to an electron transfer flavoprotein (ETF), which in turn passes the electrons to coenzyme Q of the respiratory chain.

27. Step 2. Enoyl-CoA Hydratase catalyzes stereospecific hydration of the trans double bond produced in the 1st step, yielding L-hydroxyacyl-Coenzyme A.

28. Step 3. Hydroxyacyl-CoA Dehydrogenase catalyzes oxidation of the hydroxyl in the b position (C3) to a ketone. NAD+ is the electron acceptor.

29. A cysteine S attacks the b-keto C. Acetyl-CoA is released, leaving the fatty acyl moiety in thioester linkage to the cysteine thiol. The thiol of HSCoA displaces the cysteine thiol, yielding fatty acyl-CoA (2 C less). Step 4. b-Ketothiolase catalyzes thiolytic cleavage.

30. Overall Per Beta Oxidation cycle • 1 FADH2…………………………1.5 ATP • 1 NADH………………………….2.5 ATP • 1 Acetyl CoA to Krebs • 3 NADH X 2.5 ATP / NADH………7.5 ATP • 1 FADH2…………………………….1.5 ATP • 1 GTP………………………………..1.0 ATP Total =14.0 ATP

31. Triacylglycerols (triglycerides) are the most abundant dietary lipids. They are the form in which we store reduced C for energy. Each triacylglycerol has a glycerolbackbone to which are esterified 3 fatty acids Most triacylglycerols are “mixed.” The 3 fatty acids differ in chain length & number of double bonds.

32. Lipid digestion, absorption, transport will be covered separately. Lipases hydrolyze triacylglycerols, releasing 1 fatty acid at a time, yielding diacylglycerols, & eventually glycerol.

33. Glycerol, arising from hydrolysis of triacylglycerols, is converted to the Glycolysis intermediate dihydroxyacetone phosphate, by reactions catalyzed by: 1 Glycerol Kinase 2 Glycerol Phosphate Dehydrogenase.

34. Free fatty acids, which in solution have detergent properties, are transported in the blood bound to albumin, a serum protein produced by the liver. Several proteins have been identified that facilitate transport of long chain fatty acidsinto cells, including the plasma membrane protein CD36.

35. Fatty acid activation: Acyl-CoA Synthases (Thiokinases) of ER & outer mitochondrial membranes catalyze activation of long chain fatty acids, esterifying them to coenzyme A. This process is ATP-dependent, & occurs in 2 steps. There are different Acyl-CoA Synthases for fatty acids of different chain lengths. 

36. Summary of fatty aid activation: • fatty acid + ATP  acyladenylate + PPi PPi 2 Pi • acyladenylate + HS-CoA  acyl-CoA + AMP Overall: fatty acid + ATP + HS-CoA  acyl-CoA + AMP + 2 Pi b-Oxidation pathway: For most steps of the b-oxidation pathway, there are multiple enzymes specific for particular fatty acid chain lengths.

37. A 16-C fatty acid with numbering conventions is shown. Most naturally occurring fatty acids have an even number of carbon atoms. The pathway for catabolism of fatty acids is referred to as the b-oxidation pathway, because oxidation occurs at the b-carbon (C-3).

38. Beta Oxidation on 16 C fatty Acid

39. Beta Oxidation on 16 C fatty Acid • 7 rounds of Beta oxidation (bottom numbers) • Form 8 acetyl Co A (top numbers)

43. Omega Oxidation

44. Alpha Oxidation

45. Alpha-oxi CASTEELS MARIA REINHILDE • Knowledge of alpha-hydroxylation and alpha-oxidation of 3-methyl-branched fatty acid as phytanic acid has progressed substantially in recent years. It is not known however what the role is of these enzyme in the synthesis and degradation of alpha-hydroxy-fatty acids in brain. Some findings seem to indicate that in brain these processes are catalysed by different enzymes. The role of alpha-hydroxylation in myelinisation will also be studied.

46. Odd-numbered carbons

48. Ketone Bodies • Starvation causes accumulation of acetyl CoA • not enough carbohydrates to keep Kreb’s Cycle going • acetyl CoA forms acetoacetate, b-hydroxybutyrate, and acetone.

49. This impedes entry of acetyl-CoA into Krebs cycle. Acetyl-CoA in liver mitochondria is converted then to ketone bodies, acetoacetate & b-hydroxybutyrate. During fasting or carbohydrate starvation, oxaloacetate is depleted in liver due to gluconeogenesis.