Carnitine acyltransferase I is an enzyme outside the mitochondrion that transfers the acyl group on coenzyme A to carnitine in the reaction below. This is the reversal of the reaction catalyzed by Carnitine acyltransferase II inside of the mitochondrion and is essential for transport of the acyl group into the mitochondrion.

Acyl-CoA + Carnitine <=> Acyl-Carnitine + CoASH