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L-carnitine is an amino acid derivative (but not a true amino acid because true amino acid must possess an amino group and a carboxyl group at an alpha carbon) with a tertiary amine as a side chain. It is one of the essential compounds needed for fat metabolism. because its beta carbon is a chiral carbon, Carnitine exists in two stereoisomeric forms L and D.
It is the L form that is essential for fat metabolism. L-carnitine is synthesized in the liver and kidney with the help of two amino acids, methionine, and lysine. L-carnitine helps fat metabolism in the sense that free fatty acid released from the fat digestion is first acetylated and then transported into the mitochondrial matrix for further oxidation (beta-oxidation).
Free fatty acids first react with the acetyl CoA Â and ATP to form a Fatty acyl-CoA catalyzed by the Fatty acyl-CoA Â synthetase located on outer mitochondrial matrix. Thus formed fatty acyl-CoA further reacts with L-carnitine to form Fatty acyl-L-carnitine catalyzed by the outer mitochondrial membrane enzyme, carnitine fatty-acyl-transferase I.
Now, fatty acyl-carnitine is ready to be transported to the mitochondrial matrix through the transporter carnitine-acylcarnitine translocase (an antiport that transports carnitine and fatty acyl-L-carnitine in an opposite direction. In this way, one residue of free fatty acid is transported into the mitochondrial matrix with the help of one molecule of L-carnitine.
In the mitochondrial matrix, fatty acyl-L-carnitine is broken down into L-carnitine and fatty acyl-CoA catalyzed by carnitine fatty-acyl-transferase II (inner mitochondrial membrane enzyme) and thus release L-carnitine is transported back to the intermembrane space where it again reacts with a molecule of fatty acyl-CoA to form fatty acyl-L-carnitine to be transported back into the mitochondrial matrix.
This is how L-carnitine aids fatty acid metabolism and, therefore, deficiency of the L-carnitine may impair fatty acid metabolism.
