Inner mitochondrial membrane carnitine palmitoyl transferase

Fig. 23.5. Transport of long-chain fatty acids into mitochondria. The fatty acyl CoA crosses the outer mitochondrial membrane. Carnitine pahnitoyl transferase I in the outer mitochondrial membrane transfers the fatty acyl group to carnitine and releases Co ASH. The fatty acyl carnitine is translocated into the mitochondrial matrix as carnitine moves out. Carnitine palmitoyl transferase II on the inner mitochondrial membrane transfers the fatty acyl group back to CoASH, to form fatty acyl CoA in the matrix.

Figure 7.11 Mechanism of transport of long-chain fatty adds across the inner mitochondrial membrane as fatty acyl carnitine. CRT is the abbreviation for carnitine palmitoyl transferase. CPT-I resides on the outer surface of the inner membrane, whereas CPT-II resides on the inner side of the inner membrane of the mitochondria. Transport across the inner membrane is achieved by a carrier protein known as a translocase. FACN - fatty acyl carnitine, CN - carnitine. Despite the name, CRT reacts with long-chain fatty acids other than palmitate. CN is transported out of the mitochondria by the same translocase.

Fatty acids are activated on the outer mitochondrial membrane, whereas they are oxidized in the mitochondrial matrix. A special transport mechanism is needed to carry long-chain acyl CoA molecules across the inner mitochondrial membrane. Activated long-chain fatty acids are transported across the membrane by conjugating them to carnitine, a zwitterionic alcohol. The acyl group is transferred from the sulfur atom of CoA to the hydroxyl group of carnitine to form acyl carnitine. This reaction is catalyzed by carnitine acyltransferase I (also called carnitine palmitoyl transferase I), which is bound to the outer mitochondrial membrane. 

Carnitine palmitoyltransferase I (CPTI also called carnitine acyltransferase I, CATI), the enzyme that transfers long-chain fatty acyl groups from CoA to carnitine, is located on the outer mitochondrial membrane (Fig. 23.5). Fatty acylcamitine crosses the inner mitochondrial membrane with the aid of a translocase. The fatty acyl group is transferred back to CoA by a second enzyme, carnitine palmitoyl-transferase II (CPTII or CATII). The carnitine released in this reaction returns to the cytosolic side of the mitochondrial membrane by the same translocase that brings fatty acylcamitine to the matrix side. Long-chain fatty acyl CoA, now located within the mitochondrial matrix, is a substrate for (3-oxidation. 

Mitochondrial P-oxidation of long-chain fatty acids is the major source of energy production in man. The mitochondrial inner membrane is impermeable to long chain fatty acids or their CoA esters whereas acylcamitines are transported. Three different gene products are involved in this carnitine dependent transport shuttle carnitine palmi-toyl transferase I (CPT I), carnitine acyl-camitine carrier (CAC) and carnitine palmitoyl transferase II (CPT II). The first enzyme (CPT I) converts fatty acyl-CoA esters to their carnitine esters which are subsequently translocated across the mitochondrial inner membrane in exchange for free carnitine by the action of the carnitine acyl-camitine carrier (CAC). Once inside the mitochondrion, CPT II reconverts the carnitine ester back to the CoA ester which can then serve as a substrate for the P-oxidation spiral. 

Fatty acid oxidation occurs in mitochondria, and therefore the fatty acid substrate (in the form of fatty acyl CoA) needs to be transported across the mitochondrial membranes. Short- and medium-chain fatty acids can readily penetrate mitochondria. Long-chain acyl-CoA are able to cross the mitochondrial outer membrane, but cannot penetrate the inner membrane. Translocation of these is a carnitine-dependent process involving the coordinate action of isoforms of carnitine palmitoyl transferase on the mitochondrial outer and inner membranes (see Gurr et aL, 2002). Fatty acid oxidation itself involves the progressive removal of 2-carbon units, as acetyl-CoA, from the carboxyl end of the acyl-CoA (Gurr et aL, 2002). It is often termed the p-oxidation spiral since... 

Acylcarnitines are essential compounds for the metabolism of fatty acids and represent intermediates of mitochondrial fatty acid p-oxidation. In this process, fatty acids are first activated to form acyl CoAs in the cytosol of cells (see Section 11.3), then the acyl moieties are transferred to carnitine by carnitine palmitoyl transferase I (CPT-I), which is located at the outer mitochondrial membrane. The formed acylcarnitines are largely and selectively transported into the mitochondria for fatty acid p-oxidation to generate ATP through coordinating activities of CPT-I and CPT-II. The latter is located at the inner mitochondrial membrane and converts acylcarnitines back to acyl CoAs. 

Acyl-CoA synthetase activity towards long-chain fatty-acid substrates is present in the outer mitochondrial membrane. However, fatty acyl-CoAs do not readily traverse biological membranes such as the inner mitochondrial membrane. A highly sophisticated transport system has evolved to allow tight regulation of fatty-acid entry into the mitochondrion (Figure 2). Carnitine palmitoyl transferase 1 (CPTl), located on the inner aspect of the... 

James Chase and 1 tried to explore the mechanism of how carnitine permitted mitochondria to oxidize palmitate and we made some good progress in this. Using the 2-bro-mopalmitoyl derivatives of CoA and carnitine as potent and specific inhibitors, we foimd that the oxidation of palmitoyl-CoA required the sequential action of distinct outer and inner carnitine palmitoyltransferases (now known as CPTI and CPT II) separated functionally by the mitochondrial inner membrane. How the palmitoyl moieties crossed the membrane was unclear because isolated liver mitochondria contained no carnitine we believed that the membrane was impermeable to it and that the inner transferase was vectorially mounted within the membrane, accepting only carnitine substrates on the outer face and CoA ones on the matrix side. 

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