Carnitine fatty acid acyl transferase

Carnitine fatty acid acyl transferase Chap. 12... 

Carnitine serves as a cofactor for several enzymes, including carnitine translo-case and acyl carnitine transferases I and II, which are essential for the movement of activated long-chain fatty acids from the cytoplasm into the mitochondria (Figure 11.2). The translocation of fatty acids (FAs) is critical for the genaation of adenosine triphosphate (ATP) within skeletal muscle, via 3-oxidation. These activated FAs become esterified to acylcamitines with carnitine via camitine-acyl-transferase I (CAT I) in the outer mitochondrial membrane. Acylcamitines can easily permeate the membrane of the mitochondria and are translocated across the membrane by carnitine translocase. Carnitine s actions are not yet complete because the mitochondrion has two membranes to cross thus, through the action of CAT II, the acylcar-nitines are converted back to acyl-CoA and carnitine. Acyl-CoA can be used to generate ATP via 3-oxidation, Krebs cycle, and the electron transport chain. Carnitine is recycled to the cytoplasm for fumre use. 

The steps in the subsequent utilization of muscle LCFAs may be summarized as follows. The free fatty acids, liberated from triglycerides by a neutral triglyceride lipase, are activated to form acyl CoAs by the mediation of LCFAcyl-CoA synthetase which is situated on the outer mitochondrial membrane. The next step involves carnitine palmitoyl transferase I (CPT I, see Figure 9) which is also located on the outer mitochondrial membrane and catalyzes the transfer of LCFAcyl residues from CoA to carnitine (y-trimethyl-amino-P-hydroxybutyrate). LCFAcyl... 

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.

One enzyme regulated by AMPK is acetyl-CoA carboxylase, which produces malonyl-CoA, the first intermediate committed to fatty acid synthesis. Malonyl-CoA is a powerful inhibitor of the enzyme carnitine acyl-transferase I, which starts the process of ]3 oxidation by transporting fatty acids into the mitochondrion (see Fig. 17-6). By phosphorylating and inactivating acetyl-CoA carboxylase, AMPK inhibits fatty acid synthesis while relieving the inhibition (by malonyl-CoA) of )3 oxidation (Fig. 23-37). 

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. 

Carnitine acyltransferase 1 is strongly inhibited by malonyl CoA, andmuscle has both acetyl CoA carboxylase, which forms malonyl CoA, and malonyl CoA decarboxylase, which acts to remove malonyl CoA and relieve the inhibition of carnitine acyl transferase. The two enzymes are regulated in opposite directions in response to insulin, which stimulates fatty acid synthesis and reduces /S-oxidation, and glucagon that reduces fatty acid synthesis and increases p-oxidation (Kerner and Hoppel, 2000 Louet et al., 2001 Eaton, 2002). 

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. 

A number of diseases have been traced to a deficiency of carnitine, the transferase or the translocase. The symptoms of carnitine deficiency range from mild muscle cramping to severe weakness and even death. The muscle, kidney, and heart are the tissues primarily affected. Muscle weakness during prolonged exercise is an important characteristic of a deficiency of carnitine acyl transferases because muscle relies on fatty acids as a long-term source of energy. Medium-chain (Cg-Cjo) fatty acids, which do not require carnitine to enter the mitochondria, are oxidized normally in these patients. These diseases illustrate that the impaired flow of a metabolite from one compartment of a cell to another can lead to a pathological condition. 

The study described in Figure 4.54 illustrates the role of cytosolic malonyl-CoA in controlling the entry of fatty acids into the mitochondrion. The activity of carnitine acyltransferasc w as measured after adding various concentrations of fatty acyl-CoA (0-250 jiM) to a suspension of mitochondria both w ith and without malonyl-CoA. Transferase activity increased with increasing concentrations of fatty acyl-CoA until a plateau was reached. At this point, the transferase was saturated and unable to operate at a faster rate when presented w ith higher concentrations of its substrate. The activity of the transferase w as impaired in the presence of malonyl-CoA. This indicates that an increase in the rate of fatty acid synthesis, with the resultant increase in concentration of malonyl oA, will impair the oxidation of fatty acids. Therefore, the pathways of fatty acid oxidation and synthesis are coordinated. Figure 4-54 also indicates that malonyl-CoA inhibition can be overcome by higher levels of fatty acy)-CoA. Other studies have revealed... 

This hepatomegaly is correlated to PPARa activation of acyl-CoA oxidase (AOX), the first enzyme of peroxisomal /3-oxidation of fatty acids and a gene with a PPRE in its promoter region14. In addition, hepatic mitochondrial /3-oxidation and microsomal w-oxidation of fatty acids are increased, as a direct result of PPARa activation of mRNA of specific enzymes associated with these pathways (carnitine palmitoyl transferase I and cytochrome P4504A, respectively). Activation of fatty acid oxidation by these three pathways would lead to enhanced fatty acid oxidation, given the appropriate substrate. PPARa has also been shown to enhance delivery of fatty acids to the oxidizing systems (Fig. 3). 

This transport is accomplished by carnitine (L-jS-hydroxy-y-trimethylammonium butyrate), which is required in catalytic amounts for the oxidation of fatty acids (Figure 18-1). Carnitine also participates in the transport of acetyl-CoA for cytosolic fatty acid synthesis. Two carnitine acyl-transferases are involved in acyl-CoA transport carnitine palmitoyltransferase I (CPTI), located on the outer surface of the inner mitochondrial membrane, and carnitine palmitoyltransferase II (CPTII), located on the inner surface. 

Fatty acid synthesis and degradation are reciprocally regulated so that both are not simultaneously active. Acetyl CoA carboxylase, the essential control site, is phosphorylated and inactivated by AMP-dependent kinase. The phosphorylation is reversed by a protein phosphatase. Citrate, which signals an abundance of building blocks and energy, partly reverses the inhibition by phosphorylation. Carboxylase activity is stimulated by insulin and inhibited by glucagon and epinephrine. In times of plenty, fatty acyl CoAs do not enter the mitochondrial matrix, because malonyl CoA inhibits carnitine acyl transferase I. 

The answer is b. (Murray, pp 505-626. Scriver, pp 4029-4240. Sack, pp 121-138. Wilson, pp 287-320.) A deficiency in carnitine, carnitine acyl-transferase 1, carnitine acyltransferase 11, or acylcarnitine translocase can lead to an inability to oxidize long-chain fatty acids. This occurs because all of these components are needed to translocate activated long-chain (>10 carbons long) fatty acyl CoA across mitochondrial inner membrane into the matrix where P oxidation takes place. Once long-chain fatty acids are coupled to the sulfur atom of CoA on the outer mitochondrial membrane, they can be transferred to carnitine by the enzyme carnitine acyltransferase I, which is located on the cytosolic side of the inner mitochondrial membrane. Acyl carnitine is transferred across the inner membrane to the matrix surface by translocase. At this point the acyl group is reattached to a CoA sulfhydryl by the carnitine acyltransferase 11 located on the matrix face of the inner mitochondrial membrane. 

As noted above, there have been reports that link some cases of APLP with a defect in fatty acid metabolism in the fetus. These include fetal deficiencies of long chain 3-hydroxyacyl-coenzyme A dehydrogenase (LCHAD), carnitine-palmitoyl transferase 1 (CPT 1), and medium chain acyl-coenzyme A dehydrogenase (MCAD). The mechanism by which defective fetal fatty acid oxidation causes maternal illness is not known. However, since the fetus uses primarily glucose metabolism for its energy needs, it is likely that toxic products from the placenta, which does use fatty acid oxidation, cause the maternal liver failure. 

The control of fatty-acid oxidation is related to the availability of circulating fatty acids and the activity of palmitoyl carnitine transferase 1. When circulating fatty acids are elevated, considerable fatty-acyl CoA is formed in a number of tissues, including the liver, which is sufficient to inhibit both acetyl CoA carboxylase in the cytosol and, indirectly, pyruvate dehydrogenase in the mitochondrion. Under this condition, neither malonyl CoA nor citrate would accumulate thus, there would be a diminution of fatty-acid synthesis. When large amounts of fatty... 

When large amounts of exogenous fatty acid enter the liver, how can they be transferred into the mitochondrion, for beta oxidation, with malonyl CoA inhibition This dichotomy is overcome because fatty-acyl CoAs inhibit acetyl CoA carboxylase and the malonyl CoA present proceeds onto fatty acids. When the malonyl CoA is converted to fatty acid, the level of malonyl CoA drops and is not restored. Thus, the inhibition of acyl carnitine transferase 1 is removed and fatty-acid oxidation can proceed. 

FIGURE 23-37 Regulation of fatty acid synthesis and /S oxidation by AMPK action on acetyl-CoA carboxylase. When activated by elevated 5 -AMP, AMPK phosphorylates a Thr residue on acetyl-CoA carboxylase (ACC), inactivating it. This prevents the synthesis of malonyl-CoA, the first intermediate in fatty acid synthesis, and reduction in [malonyl-CoA] relieves the inhibition of carnitine acyl-transferase I, allowing fatty acids to enter the mitochondrial matrix to undergo j8 oxidation. 

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.

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