Carnitine O-acetyltransferase also called carnitine acetyltransferase (CRAT, or CAT) () is an enzyme that encoded by the CRAT gene that catalyzes the chemical reaction acetyl-CoA + carnitine CoA + acetylcarnitine where the acetyl group displaces the hydrogen atom in the central hydroxyl group of carnitine. Thus, the two substrates of this enzyme are acetyl-CoA and carnitine, whereas its two products are CoA and O-acetylcarnitine. The reaction is highly reversible and does not depend on the order in which substrates bind. Different subcellular localizations of the CRAT mRNAs are thought to result from alternative splicing of the CRAT gene suggested by the divergent sequences in the 5' region of peroxisomal and mitochondrial CRAT cDNAs and the location of an intron where the sequences diverge. The alternatively splicing of this gene results in three distinct isoforms, one of which contains an N-terminal mitochondrial transit peptide, and has been shown to be located in mitochondria. Nomenclature This enzyme belongs to the family of transferases, to be specific those acyltransferases transferring groups other than aminoacyl groups. The systematic name of this enzyme class is acetyl-CoA:carnitine O-acetyltransferase. Other names in common use include acetyl-CoA-carnitine O-acetyltransferase, acetylcarnitine transferase, carnitine acetyl coenzyme A transferase, carnitine acetylase, carnitine acetyltransferase, carnitine-acetyl-CoA transferase, and CATC. This enzyme participates in alanine and aspartate metabolism. Structure In general, carnitine acetyltransferases have molecular weights of about 70 kDa, and contain approximately 600 residues1. CRAT contains two domains, an N domain and a C domain, and is composed of 20 α helices and 16 β strands. The N domain consists of an eight-stranded β sheet flanked on both sides by eight α helices. A six-stranded mixed β sheet and eleven α helices comprise the enzyme’s C domain. When compared, the cores of the two domains reflect significantly similar peptide backbone folding. This occurs despite the fact that only 4% of the amino acids that comprise those peptide backbones corresponds to one another. Active site His343 is the catalytic residue in CRAT. It is located at the interface between the enzyme’s C and N domains towards the heart of CRAT. His343 is accessible via two 15-18 Å channels that approach the residue from opposite ends of the CRAT enzyme. These channels are utilized by the substrates of CRAT, one channel for carnitine, and one for CoA. The side chain of His343 is positioned irregularly, with the δ1 ring nitrogen hydrogen bonded to the carbonyl oxygen