Chapter 16 (Part 2)

1. Chapter 16 (Part 2) Fatty acid Catabolism (b-oxidation)

3. Beta Oxidation of Fatty Acids • Process by which fatty acids are degraded by removal of 2-C units • b-oxidation occurs in the mitochondria matrix • The 2-C units are released as acetyl-CoA, not free acetate • The process begins with oxidation of the carbon that is "beta" to the carboxyl carbon, so the process is called"beta-oxidation"

4. Fatty acids must first be activated by formation of acyl-CoA • Acyl-CoA synthetase condenses fatty acids with CoA, with simultaneous hydrolysis of ATP to AMP and PPi • Formation of a CoA ester is expensive energetically • Reaction just barely breaks even with ATP hydrolysis DGo’ATP hydroysis = -32.3 kJ/mol, DGo’ Acyl-CoA synthesis +31.5 kJ/mol. • But subsequent hydrolysis of PPi drives the reaction strongly forward (DGo’ –33.6 kJ/mol)

6. Import of acyl-CoA into mitochondria • b-oxidation occurs in the mitochondria, requires import of long chain acyl-CoAs • Acyl-CoAs are converted to acyl-carnitines by carnitine acyltransferase. • A translocator then imports Acyl carnitine into the matrix while simultaneously exporting free carnitine to the cytosol • Acyl-carnitine is then converted back to acyl-CoA in the matrix

7. Deficiencies of carnitine or carnitine transferase or translocator activity are related to disease state • Symptons include muscle cramping during exercise, severe weakness and death. • Affects muscles, kidney, and heart tissues. • Muscle weakness related to importance of fatty acids as long term energy source • People with this disease supplement diet with medium chain fatty acids that do not require carnitine shuttle to enter mitochondria.

8. b-oxidation • Strategy: create a carbonyl group on the -C • First 3 reactions do that; fourth cleaves the "-keto ester" in a reverse Claisen condensation • Products: an acetyl-CoA and a fatty acid two carbons shorter

9. b-oxidation • B-oxidation of palmitate (C16:0) yields 106 molecules of ATP • C 16:0-CoA + 7 FAD + 7 NAD+ + 7 H20 + 7 CoA  8 acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+ 2.5 ATPs per NADH = 17.5 1.5 ATPs per FADH2 = 10.5 10 ATPs per acetyl-CoA = 80 Total = 108 ATPs • 2 ATP equivalents (ATP AMP + PPi, PPi  2 Pi) consumed during activation of palmitate to acyl-CoA • Net yield = 106 ATPs

10. Acyl-CoA Dehydrogenase • Oxidation of the C-C bond • Mechanism involves proton abstraction, followed by double bond formation and hydride removal by FAD • Electrons are passed to an electron transfer flavoprotein, and then to the electron transport chain.

11. Acyl-CoA Dehydrogenase

12. Enoyl-CoA Hydratase • aka crotonases • Adds water across the double bond • Uses substrates with trans-D2-and cis D2 double bonds (impt in b-oxidation of unsaturated FAs) • With trans-D2 substrate forms L-isomer, withcis D2 substrate forms D-isomer. • Normal reaction converts trans-enoyl-CoA to L--hydroxyacyl-CoA

13. Hydroxyacyl-CoA Dehydrogenase • Oxidizes the -Hydroxyl Group to keto group • This enzyme is completely specific for L-hydroxyacyl-CoA • D-hydroxylacyl-isomers are handled differently • Produces one NADH

14. Thiolase • Nucleophillic sulfhydryl group of CoA-SH attacks the b-carbonyl carbon of the 3-keto-acyl-CoA. • Results in the cleavage of the Ca-Cb bond. • Acetyl-CoA and an acyl-CoA (-) 2 carbons are formed

15. b-oxidation of odd chain fatty acids • Odd chain fatty acids are less common • Formed by some bacteria in the stomachs of rumaniants and the human colon. • b-oxidation occurs pretty much as w/ even chain fatty acids until the final thiolase cleavage which results in a 3 carbon acyl-CoA (propionyl-CoA) • Special set of 3 enzymes are required to further oxidize propionyl-CoA • Final Product succinyl-CoA enters TCA cycle

16. b-oxidation of unsaturated fatty acids • b-oxidation occurs normally for 3 rounds until a cis-D3-enoyl-CoA is formed. • Acyl-CoA dehydrogenase can not add double bond between the a and b carbons. • Enoyl-CoA isomerase converts this to trans- 2 enoly-CoA • Now the b-oxidation can continue on w/ the hydration of the trans-D2-enoyl-CoA • Odd numbered double bonds handled by isomerase

17. b-oxidation of fatty acids with even numbered double bonds

18. Ketone Bodies • A special source of fuel and energy for certain tissues • Produced when acetyl-CoA levels exceed the capacity of the TCA cycle (depends on OAA levels) • Under starvation conditions no carbos to produced anpleorotic intermediates • Some of the acetyl-CoA produced by fatty acid oxidation in liver mitochondria is converted to acetone, acetoacetate and -hydroxybutyrate • These are called "ketone bodies" • Source of fuel for brain, heart and muscle • Major energy source for brain during starvation • They are transportable forms of fatty acids!

19. Re-utilization of ketone bodies Formation of ketone bodies

20. Ketone Bodies and Diabetes • Lack of insulin related to uncontrolled fat breakdown in adipose tissues • Excess b-oxidation of fatty acids results in ketone body formation. • Can often smell acetone on the breath of diabetics. • High levels of ketone bodies leads to condition known as diabetic ketoacidosis. • Because ketone bodies are acids, accumulation can lower blood pH.

21. The Glyoxylate Cycle • A variant of TCA for plants and bacteria • Acetate-based growth - net synthesis of carbohydrates and other intermediates from acetate - is not possible with TCA • Glyoxylate cycle offers a solution for plants and some bacteria and algae • The CO2-evolving steps are bypassed and an extra acetate is utilized • Isocitrate lyase and malate synthase are the short-circuiting enzymes

23. Glyoxylate Cycle • Rxns occur in specialized organelles (glycoxysomes) • Plants store carbon in seeds as oil • The glyoxylate cycle allows plants to use acetyl-CoA derived from B-oxidation of fatty acids for carbohydrate synthesis • Animals can not do this! Acetyl-CoA is totally oxidized to CO2 • Malate used in gluconeogenesis