L-Carnitine (3-hydroxy-4-N-trimethylaminobutyrate) is a quaternary ammonium compound that serves as an essential cofactor in mitochondrial fatty acid metabolism. It facilitates the transport of long-chain fatty acids across the inner mitochondrial membrane, a process that is impermeable to free long-chain acyl-CoAs.

The mechanism involves the carnitine shuttle system:

• Carnitine palmitoyltransferase I (CPT1), located on the outer mitochondrial membrane, catalyzes the transesterification of long-chain acyl-CoA to form acylcarnitine, releasing free coenzyme A (CoA).
• Acylcarnitine is translocated across the inner membrane by the carnitine-acylcarnitine translocase (CACT).
• Inside the mitochondrial matrix, carnitine palmitoyltransferase II (CPT2) regenerates acyl-CoA from acylcarnitine, releasing free carnitine for recycling.

This enables entry of long-chain fatty acyl moieties into the matrix for β-oxidation, leading to sequential chain shortening, production of acetyl-CoA, and subsequent ATP generation via the tricarboxylic acid cycle and oxidative phosphorylation.

By mediating acyl group transfer, L-carnitine modulates the intramitochondrial acyl-CoA/CoA ratio, preventing accumulation of potentially toxic acyl-CoA esters and maintaining CoA availability for other metabolic reactions.