OXIDATION OF FATTY ACIDS BETA OXIDATION

Metabolism of Triacylglycerols in Tissues FAs released in the capillaries by lipoprotein lipase are taken up by tissues and either metabolized to CO2 (muscle) or resynthesized into triglycerides (adipocytes).

Mobilization of Stored Fat Hormone-sensitive lipase initiates the hydrolysis of TGs to yield FAs (from either C-1 or C-3) and glycerol. Additional lipases then take over. The FAs can either be exported to other tissues or can be metabolized to CO2. 3) In adipose tissue, the lipase is activated by epinephrine, nor-epinephrine, and glucagon as follows.

Hormones bind to receptors on the plasma membrane and cause: adenylate cylase activity, cAMP, protein kinase A activity, thereby phosphorylating and activating the lipase (Note: analagous to glycogen phosphorylase activation by phosphorylation). Insulin inhibits lipolysis by promoting dephosphorylation of the lipase. Fatty

Note: acetyl CoA carboxylase, the rate limiting enzyme in FA synthesis, is inhibited by hormone mediated phosphorylation. Therefore, if the cAMP-mediated cascade is activated, FA synthesis is turned off and TG degradation is turned on.

Mobilization of Triacylglycerols Stored in Adipose Tissue Liver ≡ hormone-sensitive lipase Modified from Nelson & Cox, Lehninger Principles of Biochemistry, 3rd Edition, Fig. 17-3.

Metabolism of Fatty Acids Activation: A FA must be “activated” before it can either be broken down to CO2 or it can be used for TG synthesis. FA “activation” is catalyzed by by acyl CoA synthetase (also called fatty acid thiokinase). (High energy compound.) Activation reaction occurs in the outer mitochondrial membrane. Reproduced from Stryer, Biochemistry, 4th Edition, p. 607.

Transport of Acyl-CoAs into Mitochondria: < 12 carbon fatty acyl CoAs passively diffuse through the mitochondrial inner membrane; > 12 carbon fatty acyl CoAs are specifically transported across the inner membrane. Basic Strategy: Convert acyl CoA to an acyl carnitine derivative, which is then transported. Then regenerate the acyl CoA within the mitochondrial matrix. Reproduced from Stryer, Biochemistry, 4th Edition, p. 608.

Note: The enzymes carnitine palmitoyl transferase I and II are also called carnitine acyltransferase I and II. Carnitine acyltransferase I is located on the outer mitochondrial membrane (correction page 36). Carnitine acyltransferase II is located on the inner surface of the inner mitochondrial membrane. Outer Membrane Outer Membrane Malonyl CoA is an intermediate in the FA biosynthetic pathway. (Regenerate fatty acyl CoA) Modified from Champe & Harvey, Biochemistry, 2nd Edition, Fig. 17.16.

Regulation: The principal point of regulation of FA Inner Membrane I II Modified from Nelson & Cox, Lehninger Principles of Biochemistry, 3rd Edition, Fig. 17-6. Regulation: The principal point of regulation of FA oxidation is via inhibition of carnitine acyltransferase I by malonyl CoA. Two Important Points: Inhibition occurs at the first committed step in the FA oxidation pathway. ii) Reciprocol regulation of degradative and synthetic pathway. Thus, when FA synthesis occurs (producing malonyl CoA), FA oxidation is inhibited.

Catabolism of Fatty Acyl CoAs in the Mitochondrial Matrix Degradation of FAs proceeds 2 carbons at a time, starting from the carboxyl end. - The reactions occur in the mitochondrial matrix. Oxidation reactions directly feed reducing equivalents to the respiratory chain to make ATP. The released acetyl CoA enters the citric acid cycle which will yield additional ATP.

β-Oxidation of FA Per Cycle: 1 NADH, 1 FADH2, and 1 Acetyl Hydration Thiolysis 3 2 1 Split off the carboxy terminal 2-carbon fragment as acetyl CoA. Fatty Acyl CoA shortened by 2 carbon atoms β α β α The action will be at the β carbon. Per Cycle: 1 NADH, 1 FADH2, and 1 Acetyl CoA are generated. Modified from Nelson & Cox, Lehninger Principles of Biochemistry, 4th Edition, Fig. 17.8.

First 3 cycles of degradation of palmitoyl-CoA: NADH FADH2 Reproduced from Stryer, Biochemistry, 4th Edition, Fig24-6.

Energy Yield of FA Oxidation One Cycle NADH 2.5 ATP FADH2 1.5 ATP Acetyl CoA 10 ATP (generated via citric acid cycle; i.e., 3 NADH + 1 FADH2 +1 substrate level phosphorylation) TOTAL = 14 ATP generated per cycle Example: Palmitate (16 carbons) 7 cycles of oxidation 28 ATP 8 acetyl CoAs produced 80 ATP Fatty acid activation - 2 ATP TOTAL = 106 ATP Initial activation step involves breaking both phosphoanhydride bonds in ATP; This is energetically equivalent to 2 ATP.

6. Oxidation of Unsaturated Fatty Acids For Mono Unsaturated FA Enoyl CoA Cis is the natural configuration. However, only the trans isomer is oxidized via β-oxidation. Importantly, the trans configuration is generated by this step. Trans Reproduced from Stryer, Biochemistry, 4th Edition, Fig24-7. This isomerase converts a cis double bonded carbon to a trans double bonded carbon. The pathway then proceeds as it does for saturated FAs.

Acetyl CoA produced via FA oxidation can enter the TCA cycle only if sufficient OAA exists. When there is insufficient carbohydrate (fasting or diabetes) OAA is consumed to form glucose (via gluconeogenesis). Note: OAA PEP is catalyzed by PEP-carboxykinase, an enzyme which is present in liver, but absent in muscle & heart. Thus, there is no depletion of OAA in muscle & heart and therefore they can handle incoming acetyl CoA. Depletion of TCA cycle OAA in liver causes acetyl CoA to be diverted to ketone body production. Thus, it allows continued FA oxidation even when acetyl CoA is not being oxidized by the cycle.

HMG CoA Synthase is the rate- limiting step and is present in TCA Cycle OR *** β-hydroxy-β-methyl-glutaryl CoA HMG CoA Synthase is the rate- limiting step and is present in significant quantities only in the liver. Reproduced from Champe & Harvey, Biochemistry, 2nd Edition, Fig. 17.22.

high malonyl CoA inhibits high palmitoyl CoA inhibits citrate stimulates TCA Cycle