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An acylcarnitine is an ester of carnitine carrying an acyl group. In fatty-acid metabolism, transfer between acyl-CoA and acylcarnitine forms part of the system that moves long-chain acyl units into mitochondria.

The acyl unit is subsequently returned to coenzyme A for further metabolism. The carrier changes during transport; the fatty acid chain need not be broken into smaller pieces to cross the membrane.

Carnitine is a hydroxy-substituted compound with a permanently charged trimethylammonium group and a carboxyl group. Its usual near-neutral aqueous form is a zwitterion. The biological L form has defined stereochemistry; “carnitine” without a stereochemical qualifier does not uniquely specify a pure enantiomer.^[ref]

Transfer of a fatty acyl group to its hydroxyl group forms an acylcarnitine. Reversible transfer between carnitine and coenzyme A participates in the movement and use of fatty-acid-derived groups in cellular metabolism. The chemical point is reversible acyl carriage; it does not imply that all stereoisomers have identical biochemical behaviour.