Carnitine acetyl transferase

Rat (Sprague- Dawley) 5 d 1x/d (GO) Hepatic 10 100 (increased liver weight and activity of palmitoyl CoA oxidase and carnitine acetyl transferase) Dostal et al. 1987a ... 

Transport dependent upon carnitine Carnitine participates in the transport of long-chain acyl-CoA into the mitochondria and plays a similar role in the transport of acetyl-CoA out of mitochondria. However, carnitine acetyl transferases have a minor role in acetyl-CoA transport. 

Carnitine acetyl transferase 2 High level in skeletal muscle and heart to facilitate use of acetate as a fuel... 

Chase, J.F.A Tubbs, P.K. (1969) Conditions for the self-catalysed inactivation of carnitine acetyl-transferase. A novel form of enzyme inhibition. Biochem. J. Ill, 225-235. 

Figure 3. Dependence of trifunctional protein activity on [acetyl-CoA]/[CoA]. Incubations were carried out as for Fig. 2, except that lactate dehydrogenase was omitted, carnitine acetyl>transferase was included and [acetyl-CoA]/[CoA] was varied by altering [camitine]/[acetyl-camitine].

Fritz, I. B., S. K. Schultz, and P. A.Srere Properties of partially purified carnitine acetyl-transferase. J. biol. Chem. 238, 2509 (1963). 

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). 

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). 

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. 

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. 

ACC-2 produces malonyl CoA, which inhibits carnitine palmitoyl transferase I, thereby blocking fatty acid entry into the mitochondria. Muscle also contains malonyl CoA decarboxylase, which catalyzes the conversion of malonyl CoA to acetyl CoA and carbon dioxide. Thus, both the synthesis and degradation of malonyl CoA is carefully regulated in muscle cells to balance glucose and fatty acid oxidation. Both allosteric and covalent means of regulation are employed. Citrate activates ACC-2, and phosphorylation of ACC-2 by the adenosine monophosphate (AMP)-activated protein kinase inhibits ACC-2 activity. Phosphorylation of malonyl CoA decarboxylase by the AMP-activated protein kinase activates the enzyme, further enhancing fatty acid oxidation when energy levels are low. 

Fatty acid uptake by muscle requires the participation of fatty acid-binding proteins and the usual enzymes of fatty acid oxidation. Fatty acyl-CoA uptake into the mitochondria is controlled by malonyl-CoA, which is produced by an isozyme of acetyl-coA carboxylase (ACC-2 the ACC-1 isozyme is found in liver and adipose tissue and is used for fatty acid biosynthesis). ACC-2 is inhibited by phosphorylation by the AMP-activated protein kinase (AMP-PK) such that when energy levels are low the levels of malonyl CoA will drop, allowing fatty acid oxidation by the mitochondria. In addition, muscle cells also contain the enzyme malonyl CoA decarboxylase, which is activated by phosphorylation by the AMP-PK. Malonyl CoA decarboxylase converts malonyl CoA to acetyl CoA, thereby relieving the inhibition of carnitine palmitoyl transferase I (CPT-I) and stimulating fatty acid oxidation (Fig. 47.5). Muscle cells do not synthesize fatty acids the presence of acetyl CoA carboxylase in muscle is exclusively for regulatory purposes. 

Fig. 47.5. Regulation of fatly acyl CoA entry into muscle mitochondria. 1. Acetyl CoA carboxylase-2 (ACC-2) converts acetyl CoA to malonyl CoA, which inhibits carnitine pahnitoyl transferase I (CPT-I), thereby blocking fatty acyl CoA entry into the mitochondria. 2. However, as energy levels drop, AMP levels rise because of the activity of the adenylate kinase reaction. 3. The increase in AMP levels activates the AMP-activated protein kinase (AMP-PK), which phosphorylates and inactivates ACC-2, and also phosphorylates and activates malonyl CoA decarboxylase (MCoADC). The decarboxylase converts malonyl CoA to acetyl CoA, thereby relieving the inhibition of CPT-1, and allowing fatty acyl CoA entry into the mitochondria. This allows the muscle to generate ATP via the oxidation of fatty acids.

Acetate is an excellent fuel for skeletal muscle. It is treated by the muscle as a very-short-chain fatty acid. It is activated to acetyl CoA in the cytosol and then transferred into the mitochondria via acetylcamitine transferase, an isozyme of carnitine palmitoyl transferase. Sources of acetate include the diet (vinegar is acetic acid) and acetate produced in the liver from alcohol metabolism. Certain commercial power bars for athletes contain acetate. 

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. 

It may seem logical that a rise in fatty acid availability will cause an increase in the rate of fatty acid oxidation. However, the rate of oxidation of fuel is matched purely to the demand for ATP and if glucose oxidation provides sufficient ATP, the extra supply of fatty acids is not metabolized. Fatty acid oxidation can be regulated by controlling the rate at which the fatty acids enter the mitochondria, and this, in turn, is dependent on the activity of carnitine acyl transferase I. This transferase is inhibited by malonyl CoA, the production of which (by acetyl-CoA carboxylase) is stimulated by insulin. So, under conditions of hypo-insulinemia, malonyl-CoA concentrations fall and carnitine acyl transferase I is activated. This stimulates the uptake of fatty acids into the mitochondrial matrix and promotes P-oxidation. It is not so much the rise in fatty acids in the blood that stimulates P-oxidation, but the fall in insulin concentration. 

Furthermore, ACC iso2ymes are bound to the outer face of the mitochondria and in conjunction with other proteins, in particular carnitine phosphate transferase, a complex that regulates the flux of malonyl-CoA out of the mitochondria [16]. Recently, a specific ACC from yeast was shown to be targeted to mitochondria [17]. Moreover, Focke et al. [18] have presented biochemical evidence for a mitochondrial localized acetyl-coenzyme A carboxylase in barley. 

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... 

As shown in Figure 5.28, the first reaction in the synthesis of fatty acids is carboxylation of acetyl CoA to malonyl CoA. This is a biotin-dependent reaction (section 11.12.2) and, as discussed above (section 5.5.1), the activity of acetyl CoA carboxylase is regulated in response to insulin and glucagon. Malonyl CoA is not only the substrate for fatty acid synthesis, but also a potent inhibitor of carnitine palmitoyl transferase, so inhibiting the uptake of fatty acids into the mitochondrion for P-oxidation. 

Carnitine acyl transferase activity is controlled by malonyl CoA. As discussed in section 10.5.2, in liver and adipose tissue this serves to inhibit mitochondrial uptake and P-oxidation of fatty acids when fatty acids are being synthesized in the cytosol. Muscle also has an active acetyl CoA carboxylase, and synthesizes malonyl CoA, although it does not synthesize fatty acids, and muscle carnitine acyl transferase is more sensitive to inhibition by malonyl CoA than is the enzyme in liver and adipose tissue. 

Production of Malonyl-CoA for the Fatty Acid Biosynthesis. Acetyl-CoA serves as a substrate in the production of malonyl-CoA. There are several routes by which acetyl-CoA is supplied to die cytoplasm. One route is the transfer of acetyl residues from the mitochondrial matrix across the mitochondrial membrane into the cyto-plasm. This process resembles a fatty acid transport and is likewise effected with the participation of carnitine and the enzyme acetyl-CoA-camitine transferase. Another route is the production of acetyl-CoA from citrate. Citrate is delivered from the mitochondria and undergoes cleavage in the cytoplasm by the action of the enzyme ATP-citrate lyase ... 

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