β-Oxidation of fatty acids is a critical metabolic pathway that breaks down fatty acids to generate energy in the form of acetyl-CoA, NADH, and FADH₂. This process occurs primarily in the mitochondria of eukaryotic cells and involves several tightly regulated steps.

This article clarifies important aspects of β-oxidation, focusing on the committed regulatory step, the location of key intermediates, transport mechanisms, and the cofactors involved.

Understanding β-Oxidation of Fatty Acids

β-Oxidation is a cyclic process that shortens fatty acid chains by two carbons at a time, producing acetyl-CoA units that enter the TCA cycle for further oxidation and ATP generation. The process includes four main enzymatic steps:

1. Dehydrogenation by acyl-CoA dehydrogenase producing FADH₂.

2. Hydration by enoyl-CoA hydratase.

3. Oxidation by 3-hydroxyacyl-CoA dehydrogenase producing NADH.

4. Thiolysis by β-ketothiolase releasing acetyl-CoA.

Evaluating the Given Statements

(1) Formation of Malonyl CoA is the Committed and Regulatory Step

• Malonyl CoA formation is not part of β-oxidation; it is a key step in fatty acid synthesis.

• The enzyme acetyl-CoA carboxylase catalyzes the formation of malonyl CoA from acetyl CoA.

• Malonyl CoA acts as a potent inhibitor of carnitine palmitoyltransferase I (CPT1), regulating fatty acid entry into mitochondria and thus indirectly regulating β-oxidation.

• Therefore, malonyl CoA formation is the committed step in fatty acid synthesis, not β-oxidation.

(2) Formation of Acetoacetyl ACP Occurs in Mitochondria

• Acetoacetyl-ACP formation is part of fatty acid synthesis, not β-oxidation.

• This reaction occurs in the cytosol on the fatty acid synthase complex.

• β-Oxidation occurs in the mitochondrial matrix, but acetoacetyl-ACP is not an intermediate in this process.

(3) Cytosolic Acyl CoA Is Transported into Mitochondria

• True.

• Fatty acids are first activated in the cytosol to form acyl-CoA.

• Long-chain acyl-CoA molecules cannot cross the inner mitochondrial membrane directly.

• They are converted to acyl-carnitine by carnitine palmitoyltransferase I (CPT1) on the outer mitochondrial membrane.

• Acyl-carnitine is transported into the mitochondrial matrix by a translocase.

• Inside, carnitine palmitoyltransferase II (CPT2) converts acyl-carnitine back to acyl-CoA, which then undergoes β-oxidation.

(4) Use of NADH + H⁺ as Reducing Equivalent

• False.

• β-Oxidation uses NAD⁺ as the oxidizing agent, which is reduced to NADH + H⁺ during the third step.

• The pathway does not use NADH as a reducing equivalent; rather, it produces NADH.

• The reducing equivalents generated (NADH and FADH₂) feed into the electron transport chain to produce ATP.

Summary Table

Conclusion

The correct statement regarding β-oxidation of fatty acids is:

(3) Cytosolic acyl CoA is transported into mitochondria