Carnitine, which is derived from lysine, carries activated long chain fatty acids across the inner mitochondrial membrane and is essential for fatty acid oxidation. The fatty acid is transferred from the sulfur atom of an acyl-CoA molecule by the enzyme carnitine acyltransferase I to the hydroxyl oxygen atom of carnitine, forming an acyl carnitine. A translocase enzyme then transfers the acyl carnitine across the mitochondrial membrane where it transfers the fatty acid back to CoA by the enzyme carnitine acyltransferase II. Smal-l and medium-chain fatty acids do not need carnitine to enter the mitrochondrion. Carnitine is an essential amino acid for some animals.