β-oxidation of fatty acids occurs in

Question

BlackTom AI · Accepted Answer

Mitochondrial matrix and peroxisomes are the two cellular sites where β‑oxidation of fatty acids occurs.

Relevant knowledge points:
- β‑oxidation is the sequential removal of two‑carbon units (as acetyl‑CoA) from activated fatty acids (acyl‑CoA) by a four‑step enzymatic cycle: dehydrogenation, hydration, a second dehydrogenation, and thiolysis.
- In eukaryotes, most β‑oxidation of physiological long‑chain fatty acids occurs in mitochondria; very‑long‑chain and some branched fatty acids are initially shortened in peroxisomes.
- Peroxisomal dehydrogenation transfers electrons to O2 via acyl‑CoA oxidase producing H2O2 (handled by catalase), whereas mitochondrial dehydrogenation uses FAD and passes electrons to the respiratory chain.

Step‑by‑step reasoning:
1. Activation: Fatty acids are activated to acyl‑CoA (by acyl‑CoA synthetase) at the outer mitochondrial membrane or peroxisomal membrane.
2. Mitochondrial β‑oxidation: Long‑chain acyl‑CoAs are transported into the mitochondrial matrix by the carnitine shuttle. In the matrix the four β‑oxidation steps proceed (acyl‑CoA dehydrogenase → enoyl‑CoA hydratase → 3‑hydroxyacyl‑CoA dehydrogenase → β‑ketothiolase), yielding acetyl‑CoA, FADH2 and NADH for ATP production.
3. Peroxisomal β‑oxidation: Very‑long‑chain (e.g., >20C) or certain branched fatty acids are shortened in peroxisomes by a similar sequence but with acyl‑CoA oxidase as the first enzyme (producing H2O2). Shortened chains (acetyl‑CoA or medium‑chain acyl‑CoAs) are then exported to mitochondria for complete oxidation and ATP production.

Why the correct answer is correct:
- Both mitochondria and peroxisomes conduct β‑oxidation, but they handle different substrate classes and use distinct initial electron acceptors; hence the best choice is “Peroxisome and mitochondria.”

Why the other options are incorrect:
- (a) Peroxisome only: Incorrect because mitochondria are the primary site for most cellular β‑oxidation and ATP generation.
- (c) Mitochondria only: Incorrect because peroxisomes actively β‑oxidize very‑long‑chain and branched fatty acids.
- (d) Peroxisome, mitochondria and endoplasmic reticulum: Incorrect because the ER conducts fatty acid elongation and ω‑oxidation (microsomal), not the standard β‑oxidation spiral.

Conclusion: The correct answer is “Peroxisome and mitochondria,” which matches the provided answer.

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