Fatty acid oxidation import step: The carnitine shuttle transports acyl groups into mitochondria via formation of which intermediate?

Introduction / Context:
Long-chain fatty acids require specialized transport to enter the mitochondrial matrix for beta-oxidation. The carnitine shuttle ensures regulated import and prevents futile cycles with fatty acid synthesis.

Given Data / Assumptions:

• Fatty acids are activated in the cytosol/outer mitochondrial membrane to acyl-CoA.
• Inner mitochondrial membrane is impermeable to CoA esters.
• Carnitine acyltransferases mediate reversible transfer between CoA and carnitine.

Concept / Approach:
Carnitine palmitoyltransferase I (CPT I) on the outer membrane forms acyl-carnitine from acyl-CoA and carnitine. A translocase shuttles acyl-carnitine across the inner membrane. In the matrix, CPT II regenerates acyl-CoA and releases carnitine to return to the cytosolic side. Malonyl-CoA inhibits CPT I to coordinate oxidation with synthesis.

Step-by-Step Solution:
Activate fatty acid: FA + CoA + ATP → acyl-CoA + AMP + PPi.CPT I reaction: acyl-CoA + carnitine → acyl-carnitine + CoA.Translocase moves acyl-carnitine into matrix.CPT II reaction: acyl-carnitine + CoA → acyl-CoA (matrix) + carnitine.

Verification / Alternative check:
Inborn errors of CPT I/II or carnitine deficiency impair beta-oxidation, causing hypoketotic hypoglycemia and muscle weakness; supplementation and dietary management target this shuttle.

Why Other Options Are Wrong:

• 3 acetyl-CoA: A downstream product after multiple beta-oxidation cycles, not the transport intermediate.
• Acyl-CoA only: Cannot cross the inner membrane efficiently without the carnitine intermediate.
• None / Malonyl-CoA: Malonyl-CoA regulates entry but is not the transported intermediate.

Common Pitfalls:
Confusing activation (acyl-CoA formation) with transport (acyl-carnitine intermediate), and overlooking malonyl-CoA’s inhibitory role.

Final Answer:
Acyl-carnitine.