Carnitine Palmitoyltransferase I - Structure

Structure

CPT1 is an integral membrane protein that associates with the mitochondrial outer membrane through transmembrane regions in the peptide chain. Both the N- and C-terminal domains are exposed to the cytosolic side of the membrane.

Three isoforms of CPT1 exist in mammalian tissues. The liver isoform (CPT1A or CPTI-L) is found throughout the body on the mitochondria of all cells except for skeletal muscle cells and white and brown adipose cells. The muscle isoform (CPT1B or CPTI-M) is highly expressed in heart and skeletal muscle cells and white and brown adipose cells. A third isoform, the brain isoform (CPT1C), was isolated in 2002. It is expressed predominantly in the brain and testes. Little is known about CPT1C.

The exact structure of any of the CPT1 isoforms has not yet been determined, although a variety of in silico models for CPT1 have been created based on closely related carnitine acyltransferases, such as carnitine acetyltransferase (CRAT).

An important structural difference between CPT1 and CPT2, CRAT and carnitine octanoyltransferase (COT) is that CPT1 contains an additional domain at its N-terminal consisting of about 160 amino acids. It has been determined that this additional N-terminal domain is important for the key inhibitory molecule of CPT1, malonyl-CoA.

Two distinct binding sites have been proposed to exist in CPT1A and CPT1B. The “A site” or “CoA site” appears to bind both malonyl-CoA and palmitoyl-CoA, as well as other molecules containing coenzyme A, suggesting that the enzyme binds these molecules via interaction with the coenzyme A moiety. It has been suggested that malonyl-CoA may behave as a competitive inhibitor of CPT1A at this site. A second “O site” has been proposed to bind malonyl-CoA more tightly than the A site. Unlike the A site, the O site binds to malonyl-CoA via the dicarbonyl group of the malonate moiety of malonyl-CoA. The binding of malonyl-CoA to either the A and O sites inhibits the action of CPT1A by excluding the binding of carnitine to CPT1A. Since a crystal structure of CTP1A has yet to be isolated and imaged, its exact structure remains to be elucidated.