L-carnitine is synthesized from N6-N6-N6-trimethyl-L-lysine, a modified protein-bound L-lysine residue released following lysosomal protein degradation. The first biosynthetic step occurs in mitochondria, with subsequent steps taking place in the cytosol. L-carnitine shuttles cytosolic long-chain fatty acyl-CoAs into the mitochondria for beta-oxidation, and transports medium- and short-chain acyl-CoAs from the peroxisomal matrix into the mitochondria for further oxidation. It regulates the acyl-CoA/free CoA ratio and buffers excess acetyl-CoA through formation of acetylcarnitine. It also facilitates elimination of intermediate acyl metabolites derived from branched-chain amino acid catabolism, as well as partially metabolized or toxic acyl groups. (Adapted from FBrf0247638.)