Beta-Oxidation

Cheatsheet Content

### Overview of Beta-Oxidation - **Definition:** A catabolic process that breaks down fatty acids into acetyl-CoA, NADH, and FADH2. - **Location:** Primarily occurs in the mitochondrial matrix. - **Purpose:** Generates energy (ATP) from stored fats, especially during fasting or prolonged exercise. - **Key Stages:** Activation, Transport, and the Beta-Oxidation Spiral. ### Fatty Acid Activation - **Location:** Outer mitochondrial membrane. - **Enzyme:** Acyl-CoA synthetase (Thiokinase). - **Reaction:** Fatty acid + CoA + ATP $\rightarrow$ Acyl-CoA + AMP + PPi. - **Energy Cost:** Equivalent to 2 ATP molecules (ATP $\rightarrow$ AMP). - **Result:** Converts a fatty acid into its active form, Acyl-CoA, which can then be transported into the mitochondria. ### Carnitine Shuttle (Transport) - **Purpose:** Transports long-chain Acyl-CoA ($>$12 carbons) across the inner mitochondrial membrane. - **Steps:** 1. **CPT-I (Carnitine Palmitoyltransferase I):** Located on the outer mitochondrial membrane. Transfers the acyl group from Acyl-CoA to Carnitine, forming Acylcarnitine. CoA is released. 2. **CAT (Carnitine-Acylcarnitine Translocase):** An antiporter in the inner mitochondrial membrane. Moves Acylcarnitine into the matrix while moving free Carnitine out. 3. **CPT-II (Carnitine Palmitoyltransferase II):** Located on the inner mitochondrial membrane (matrix side). Transfers the acyl group from Acylcarnitine back to CoA, regenerating Acyl-CoA in the matrix and releasing free Carnitine. - **Short-chain fatty acids:** Can diffuse freely into the mitochondria. - **Medium-chain fatty acids:** Can cross the inner membrane without the carnitine shuttle. ### Beta-Oxidation Spiral - **Definition:** A recurring sequence of four enzymatic reactions that shortens the fatty acyl-CoA chain by two carbons with each cycle, producing acetyl-CoA, FADH2, and NADH. - **Each Cycle Produces:** - 1 molecule of FADH2 - 1 molecule of NADH - 1 molecule of Acetyl-CoA - A fatty acyl-CoA shortened by 2 carbons - **Steps (for saturated fatty acids):** #### 1. Dehydrogenation (Oxidation) - **Enzyme:** Acyl-CoA Dehydrogenase (various isoforms based on chain length). - **Reaction:** Acyl-CoA $\rightarrow$ trans-$\Delta^2$-Enoyl-CoA. - **Coenzyme:** FAD is reduced to FADH2. - **Result:** Introduces a double bond between $\alpha$ and $\beta$ carbons. #### 2. Hydration - **Enzyme:** Enoyl-CoA Hydratase. - **Reaction:** trans-$\Delta^2$-Enoyl-CoA + H$_2$O $\rightarrow$ L-$\beta$-Hydroxyacyl-CoA. - **Result:** Adds water across the double bond. #### 3. Dehydrogenation (Oxidation) - **Enzyme:** $\beta$-Hydroxyacyl-CoA Dehydrogenase. - **Reaction:** L-$\beta$-Hydroxyacyl-CoA $\rightarrow$ $\beta$-Ketoacyl-CoA. - **Coenzyme:** NAD$^+$ is reduced to NADH + H$^+$. - **Result:** Oxidizes the hydroxyl group to a ketone. #### 4. Thiolysis (Cleavage) - **Enzyme:** $\beta$-Ketothiolase (Thiolase). - **Reaction:** $\beta$-Ketoacyl-CoA + CoA $\rightarrow$ Acetyl-CoA + (Fatty Acyl-CoA - 2 carbons). - **Result:** Cleaves off a 2-carbon Acetyl-CoA unit and generates a new fatty acyl-CoA, which is 2 carbons shorter and ready for the next cycle. ### Energy Yield (Example: Palmitate 16:0) - **Cycles:** For a 16-carbon fatty acid, 7 cycles are needed (16/2 - 1 = 7). - **Products:** - 8 Acetyl-CoA (from 7 cleavages + final 2-carbon fragment) - 7 FADH2 - 7 NADH - **ATP from products (approximate):** - 1 Acetyl-CoA $\rightarrow$ 10 ATP (via Citric Acid Cycle & Oxidative Phosphorylation) - 1 FADH2 $\rightarrow$ 1.5 ATP (via ETC) - 1 NADH $\rightarrow$ 2.5 ATP (via ETC) - **Total ATP:** - 8 Acetyl-CoA * 10 ATP/Acetyl-CoA = 80 ATP - 7 FADH2 * 1.5 ATP/FADH2 = 10.5 ATP - 7 NADH * 2.5 ATP/NADH = 17.5 ATP - **Gross Total:** 80 + 10.5 + 17.5 = 108 ATP - **Net Total:** 108 ATP - 2 ATP (for activation) = **106 ATP** ### Beta-Oxidation of Unsaturated Fatty Acids - **Challenge:** The presence of double bonds can interrupt the standard four-step cycle. - **Key Enzymes:** - **Enoyl-CoA Isomerase:** Converts cis-double bonds (common in natural unsaturated FAs) to trans-double bonds, allowing enoyl-CoA hydratase to act. - **2,4-Dienoyl-CoA Reductase:** Required for polyunsaturated fatty acids. Uses NADPH to reduce a 2,4-dienoyl-CoA intermediate to a trans-3-enoyl-CoA, which is then isomerized. - **Result:** Requires additional enzymatic steps and sometimes consumes NADPH, slightly altering the ATP yield. ### Beta-Oxidation of Odd-Chain Fatty Acids - **Challenge:** The final cycle leaves a 3-carbon molecule, Propionyl-CoA, instead of Acetyl-CoA. - **Conversion of Propionyl-CoA:** 1. **Propionyl-CoA Carboxylase:** Propionyl-CoA + HCO$_3^-$ + ATP $\rightarrow$ D-Methylmalonyl-CoA. (Requires Biotin) 2. **Methylmalonyl-CoA Epimerase:** D-Methylmalonyl-CoA $\rightarrow$ L-Methylmalonyl-CoA. 3. **Methylmalonyl-CoA Mutase:** L-Methylmalonyl-CoA $\rightarrow$ Succinyl-CoA. (Requires Vitamin B12 - Cobalamin) - **Fate of Succinyl-CoA:** Enters the Citric Acid Cycle. ### Regulation of Beta-Oxidation - **Key Regulatory Point:** Carnitine Palmitoyltransferase I (CPT-I). - **Inhibition:** CPT-I is inhibited by **Malonyl-CoA**. - Malonyl-CoA is an intermediate in fatty acid synthesis. - High levels of Malonyl-CoA signal abundant glucose and active fatty acid synthesis, thus inhibiting fatty acid breakdown. - **Hormonal Control:** - **Glucagon & Epinephrine:** Stimulate beta-oxidation (via cAMP-dependent phosphorylation that inactivates Acetyl-CoA Carboxylase, reducing Malonyl-CoA levels). - **Insulin:** Inhibits beta-oxidation (activates Acetyl-CoA Carboxylase, increasing Malonyl-CoA). - **Availability of Substrates:** High levels of fatty acids promote beta-oxidation. - **ATP/ADP Ratio:** High energy charge inhibits beta-oxidation indirectly by slowing the TCA cycle and ETC.