Acyl carnitine transport system

Defects of mitochondrial transport interfere with the movement of molecules across the inner mitochondrial membrane, which is tightly regulated by specific translocation systems. The carnitine cycle is shown in Figure 42-2 and is responsible for the translocation of acyl-CoA thioesters from the cytosol into the mitochondrial matrix. The carnitine cycle involves four elements the plasma membrane carnitine transporter system, CPT I, the carnitine-acyl carnitine translocase system in the inner mitochondrial membrane and CPT II. Genetic defects have been described for each of these four steps, as discussed previously [4,8,9]. 

The most important process in the degradation of fatty acids is p-oxidation—a metabolic pathway in the mitochondrial matrix (see p. 164). initially, the fatty acids in the cytoplasm are activated by binding to coenzyme A into acyl CoA [3]. Then, with the help of a transport system (the carnitine shuttle [4] see p. 164), the activated fatty acids enter the mitochondrial matrix, where they are broken down into acetyl CoA. The resulting acetyl residues can be oxidized to CO2 in the tricarboxylic acid cycle, producing reduced... 

The inner mitochondrial membrane has a group-specific transport system for fatty acids. In the cytoplasm, the acyl groups of activated fatty acids are transferred to carnitine by carnitine acyltransferase [1 ]. They are then channeled into the matrix by an acylcar-nitine/carnitine antiport as acyl carnitine, in exchange for free carnitine. In the matrix, the mitochondrial enzyme carnitine acyltransferase catalyzes the return transfer of the acyl residue to CoA. 

Answer The transport of fatty acid molecules into mitochondria requires a shuttle system involving a fatty acyl-carnitine intermediate. Fatty acids are first converted to fatty acyl-CoA molecules in the cytosol (by the action of acyl-CoA synthetases) then, at the outer mitochondrial membrane, the fatty acyl group is transferred to carnitine (by the action of carnitine acyl-transferase I). After transport of fatty acyl-carnitine through the inner membrane, the fatty acyl group is transferred to mitochondrial CoA. The cytosolic and mitochondrial pools of CoA are thus kept separate, and no labeled CoA from the cytosolic pool enters the mitochondrion. 

A schematic depiction of the intracellular pathway for (5-oxidation of long-chain fatty acids and, ultimately, ketogenesis is shown in Figure 32-5. Long-chain fatty acids are activated prior to entering the mitochondria by converting them to acyl-CoA derivatives. Because the mitochondrial membrane is impermeable to CoA and its derivatives, a specific transport system is required. As shown in Figure 32-5, this system has three components (1) the enzyme carnitine acyltransferase I (CAT I), which transfers the activated acyl unit from fatty acyl-CoA... 

Carnitine is used mainly for facilitating the transport of long-chain fatty adds into the mitochondria. As shown in Figure4.53, this transport system requires the participation of two different carnitine acyl transferases. One is located on the outside of the mitochondrial membrane, the other on the inner side. Once fatty acyl-camitine is inside the organelle, its carnitine is released. A separate transport system is used to transport this carnitine from the interior of the mitochondrion back to the cytoplasm for reuse. 

Prior to oxidation, long-chain fatty acids are activated, forming fatty acyl CoA, which is transported into mitochondria by a carnitine carrier system. 

The fact that the mitochondrial inner membrane is virtually impermeable to long-chain fatty acyl-CoA, while the fatty acid oxidative machinery is located inside the mitochondrial matrix, a space enclosed by the inner membrane, might create a serious problem for cellular energy production. The problem is solved by the development of a transmembrane carnitine-dependent transport system for the long-chain acyl residue of acyl-CoA. Catalyzed by carnitine acyltransferase I (CAT-I), which is attached to the inner surface of the mitochondrial outer membrane, fatty acyl-CoA is converted to fatty acyl-carnitine by replacing the CoA residue with carnitine (Figure 3). Fatty acyl-carnitine is transported across the mitochondrial inner membrane in exchange for a molecule of free carnitine by carnitine-acylcarnitine translocase. After arrival in the mitochondrial matrix, fatty acyl-carnitine is converted back to acyl-CoA by carnitine acyltransferase II (CAT-II), an enzyme located on the inner surface of the mitochondrial inner membrane. 

Within the peroxisome, the acetyl groups can be transferred from CoA to carnitine by an acetylcarnitine transferase, or they can enter the cytosol. A similar reaction converts medium-chain-length acyl CoAs and the short-chain butyryl CoA to acyl carnitine derivatives. These acylcarnitines diffuse from the peroxisome to the mitochondria, pass through the outer mitochondrial membrane, and are transported through the inner mitochondrial membrane via the carnitine translocase system. 

Although the balance between glucose and fatty acid oxidation is described in Chap. 13, it is relevant to note here that malonyl-CoA inhibits carnitine acyl transferase I (CAT-1), the enzyme that catalyzes the exchange of fatty acids for carnitine as part of the cytosol-to-matrix fatty acid transport system. Inhibition of CAT-I occurs when acetyl-CoA carboxylase is activated by insulin. By inhibiting the uptake of fatty acids into mitochondria, malonyl CoA favors the oxidation of glucose and prevents fatty acids from being oxidized at the same time as they are being synthesized. 

Further desaturation and acyl chain elongation of fatty acids are generally accepted to involve cytosolic membranous systems associated with the endoplasmic reticulum (Stumpf, 1989). A critical importance, therefore, would appear to exist for a detailed understanding of the mechanics by which the fatty acids are exported from the plastid. At present, there is little available information on the precise transport system or its regulation. Although fatty acids are presumed to be released on hydrolysis of acyl-ACPs by specific thioesterases, it is not known whether an initial formation of acyl-CoA is required prior to transport across the plastidic membrane in association with carnitine or some other system. A direct transacylation between ACP and CoA or some carrier system would be less expensive energetically than a process involving hydrolysis and synthesis. 

Acylcarnitine can cross only the inner mitochondrial membrane on a countertransport system that takes in acylcarnitine in exchange for free carnitine being returned to the inter-membrane space. Once inside the mitochondrial inner membrane, acylcarnitine transfers the acyl group onto CoA ready to undergo -oxidation. This counter-transport system provides regulation of the uptake of fatty acids into the mitochondrion for oxidation. As long as there is free CoA available in the mitochondrial matrix, fatty acids can be taken up and the carnitine returned to the outer membrane for uptake of more fatty acids. However, if most of the CoA in the mitochondrion is acylated, then there is no need for further fatty uptake immediately and, indeed, it is not possible. 

CoASH and thus reforming the true immediate substrate for the oxidatioa According to this hypothesis, acyl carnitine, which is not a substrate for the enzymes of the oxidative process, is an admirable substrate for fatty acid degradation in intact mitochondrial systems. In addition, mitochondria depleted of their endogenous energy donors (ATP, GTP) are unable to oxidize add fatty acids unless ATP and carnitine are both present in the system. This observation clearly proves that acyl-CoA is formed outside of the inner mitochondrial membrane—i.e. outside of the oxidation compartment—and is transported to the inner compartment via the carnitine-linked transport mechanism. In agreement with these results an ATP-dependent thiokinase has been identified in the outer mitochondrial membrane, and an acyl-camitine transferase has been found in the inner mitochondrial membrane. 

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... 

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

Acylcarnitines play an important role in fatty acyl transport in and out mitochondria. Two transport machineries are associated with this process, that is, carnitine palmitoyltransferase (CPT) I and II. In the context of inborn mitochondrial diseases, acylcarnitine production has been viewed as a detoxifying system that permits mitochondrial efflux of excess of acyl groups [54]. Unlike long-chain acylcarnitines, medium-chain species do not depend on the CPT system for transfer to the mitochondrial matrix [55]. 

The second site may include two possibilities one is on the carnitine acyl transferase system and the other involves formation of palmitoyl-S-pantetheine and its non-enzymatic transport through the mitochondrial membranes followed by oxidation in -oxidation system after conversion to palmitoyl CoA. The former possibility may be supported by the difference in effectiveness of pantethine between the palmitate and octanoate oxidation reactions ( Fig.3, 4 and 6 ) and between the ketogenic reactions from these two substrates ( Fig.7 ), because octanoic acid is freely permeable through the mitochondrial membranes. The latter possibility was based on the findings on the formation of acyl pantetheine in rat liver micro-somes [ 10 ] and on the enzymatic interconversion between acyl CoA and acyl pantetheine [ 14 ] and on the much lower susceptibility of acyl pantetheine to3-oxidation than acyl CoA [ 14 ]. But this has been ruled out by the finding that palmitoyl-S-pantetheine did not serve as the substrate of the ketogenic reaction ( fig.7 ). 

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