Lakshmanan Govindasamy, Mavis Agbandje-McKenna, Robert McKenna

Crystal structure of CAT

Proposed mechanism of CAT

In eukaryotic cells, the carnitine system plays a vital role in fatty acid b-oxidation and maintenance of acyl-coenzyme A (acyl-CoA) pools. The important physiological functions of carnitine are made possible by the presence of carnitine/acylcarnitine transporters on cellular membranes and a group of enzymes, the carnitine acyltransferases, which catalyze the reversible transfer of acyl groups between coenzyme A (CoA) and carnitine. Acyl-CoA + carnitine = CoA + Acyl-carnitine The three known classes of carnitine acyltransferases differ in their acyl-group specificity, sub-cellular localization, tissue distribution and physiological function (3-5). Carnitine acetyltransferase (CAT) has a substrate preference for short-chain acyl-CoAs and is found in the outer mitochondria membrane, the mitochondrial matrix, the endoplasmic reticulum, and the peroxisomes. Carnitine octanoyltransferase (COT) is predominantly localized in peroxisomes with a substrate preference for medium-length acyl-CoAs. Carnitine palmitoyltransferases (CPT) are found both in the outer mitochondrial membrane (CPT I) and the mitochondrial matrix (CPT II) with a substrate preference for long-chain acyl-CoAs. Arguably the most critical member is CPT I, which is responsible for facilitating the transfer of long chain fatty acids into the mitochondria. There are two different types of CPT I in mammalian tissues, a liver (L-CPT I) and a muscle isoform (M-CPT I). Both isoforms of CPT I are inhibited by malonyl-CoA, which is the committed metabolite for the biosynthesis of fatty acids. Malonyl-CoA inhibition regulates and balances b-oxidation and the biosynthesis of fatty acids depending on the metabolic needs of the cell. The important metabolic roles of CPT I make it a potential drug target for diabetes, coronary heart disease and other disorders involving abnormal fatty-acid metabolism. We are currently studying the structure of wild type, mutants and substrate complexes of human peroxisomal carnitine using X-ray crystallography to understand its catalytic mechanism of action and aid in the rational design of selective new drugs.