Mutase reactions, coenzyme

Biochemical findings are variable. The blood cobala-min and folate levels often are normal. Patients often have homocysteinemia with hypomethioninemia, the latter finding discriminating this group from homocystinuria secondary to cystathionine- P-synthase deficiency. Urinary excretion of methylmalonic acid may be high, reflecting the fact that vitamin B12 serves as a cofactor for the methyl-malonyl-CoA (coenzyme A) mutase reaction. 

A review of the biochemistry of coenzyme B12 reactions, including the methylmalonyl-CoA mutase reaction. 

Leutbecher, U., Albracht, S. P. J., and Buckel, W., 1992, Identification of a paramagnetic species as an early intermediate in the coenzyme B]2 -dependent glutamate mutase reaction a cob(II)amide , FEBS Lett. 307(2) 144nl46. 

Wollowitz, S., and Halpem, J., 1984, Free-radical rearrangement involving the 1,2-migration of a thioester groupoModel for the coenzyme-B12 dependent methylmalonyl-CoA mutase reaction, J. Am. Chem. Soc. 106 831998321. 

Zhao, Y., Such, P., and REtey, J., 1992, Radical intermediates in the coenzyme B12 dependent methylmalonyl-CoA mutase reaction shown by EPR spectroscopy, Angew. Chem. Int. Ed. Engl. 31 2159216. 

Concerning the Mechanism of Coenzyme B12 Dependent Mutase Reactions... 

Coenzyme B12 is involved in wframolecular alkyl group transfer in the methylmalonyl-CoA isomerase and glutamate mutase reactions. (see reference I4), both of which may be schematically represented as ... 

Of possible biological interest is the report, based on a model reaction study with hydroxocobalamine, that the coenzyme-B, 2-dependent methyl-malonyl-CoA mutase reaction could involve intramolecular addition of a primary radical to a thioester group. The cyclopropyloxy radical so formed in a Cy3/Cy4 case would open to the carbethoxy-substituted and hence stabilized radical to give the rearranged product. ... 

A rare inborn error of metabolism in which methylmalonic acid is excreted in the urine. It can be due to a deficiency of the enzyme, methymalonate coenzyme A mutase, which converts methylmalonate CoA to succinyl CoA. Another variant of the disease is due to defective biosynthesis of the vitamin B,2 coenzyme required for the mutase reaction. 

The substrate analogue ethylmalonyl coenzyme A has been used to obtain further information concerning the mechanism of the methylmalonyl-coenzyme A mutase reaction. ... 

The coenzyme acts in concert with a variety of enzymes to catalyze reactions of three main types. In the first, two substituents on adjacent carbon atoms, -X and -H, are permuted in the isomerase or mutase reaction. The generalized process is shown in Eq. 16.1, and specific examples are given in Eq. 16.2 and Eq. 16.4. CoA has nothing to do with cobalt, but is the biochemical symbol for coenzyme A, a thiol that activates carboxylic acids by forming a reactive thioester. 

Adenosylcobalamin (coenzyme B 2) is required in a number of rearrangement reactions that occurring in humans is the methylmalonyl-Co A mutase-mediated conversion of (R)-methylmalonyl-Co A (6) to succinjl-CoA (7) (eq. 1). The mechanism of this reaction is poorly understood, although probably free radical in nature (29). The reaction is involved in the cataboHsm of valine and isoleucine. In bacterial systems, adenosylcobalamin drives many 1,2-migrations of the type exemplified by equation 1 (30). 

Methylmalonyl-CoA mutase 5 -deoxyadenosylco-balamin is part of dimethylbenzimidazolecobamide coenzyme, a constituent of methylmalonyl-CoA mutase. This mutase catalyses the isomerization of methylmalonyl-CoA to succinyl-CoA (anaplerotic reaction of the citric acid cycle). 

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

Various coenzymes are involved in these reactions. The carboxylase [3] requires biotin, and the mutase [4] is dependent on coenzyme Bj2 (5 -deoxyadenosyl cobalamin see p. 108). Succinyl-CoA is an intermediate in the tricar-... 

Propionyl-CoA is first carboxylated to form the d stereoisomer of methylmalonyl-CoA (Pig. 17—11) by propionyl-CoA carboxylase, which contains the cofactor biotin. In this enzymatic reaction, as in the pyruvate carboxylase reaction (see Pig. 16-16), C02 (or its hydrated ion, HCO ) is activated by attachment to biotin before its transfer to the substrate, in this case the propionate moiety. Formation of the carboxybiotin intermediate requires energy, which is provided by the cleavage of ATP to ADP and Pi- The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its l stereoisomer by methylmalonyl-CoA epimerase (Pig. 17-11). The L-methylmal onyl -CoA then undergoes an intramolecular rearrangement to form succinyl-CoA, which can enter the citric acid cycle. This rearrangement is catalyzed by methylmalonyl-CoA mutase, which requires as its coenzyme 5 -deoxyadenosyl-cobalamin, or coenzyme Bi2, which is derived from vitamin B12 (cobalamin). Box 17—2 describes the role of coenzyme B12 in this remarkable exchange reaction. 

Oxidation of unsaturated fatty acids requires two additional enzymes enoyl-CoA isomerase and 2,4-dienoyl-CoA reductase. Odd-number fatty acids are oxidized by the /3-oxidation pathway to yield acetyl-CoA and a molecule of propionyl-CoA This is carboxylated to methylmalonyl-CoA, which is isomerized to succinyl-CoA in a reaction catalyzed by methylmalonyl-CoA mutase, an enzyme requiring coenzyme B12. 

Co corrinoids play central roles in the two classes of enzymic reactions, i.e. methyl transfer mediated by vitamin B,2 and mutase or isomerase reactions catalyzed by coenzyme B. 253 Though there remain many ambiguities, the former is considered to be a combination of Scheme 100, i and its reverse process, and the latter to be represented by Scheme 103. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

Vitamin B12 is a biologically active corrinoid, a group of cobalt-containing compounds with macrocyclic pyrrol rings. Vitamin B12 functions as a cofactor for two enzymes, methionine synthase and L-methylmalonyl coenzyme A (CoA) mutase. Methionine synthase requires methylcobalamin for the methyl transfer from methyltetrahydrofolate to homocysteine to form methionine tetrahy-drofolate. L-methylmalonyl-CoA mutase requires adenosylcobalamin to convert L-methylmalonyl-CoA to succinyl-CoA in an isomerization reaction. An inadequate supply of vitamin B12 results in neuropathy, megaloblastic anemia, and gastrointestinal symptoms (Baik and Russell, 1999). 

Vitamin B12 is required by only two enzymes in human metabolism methionine synthetase and L-methylmalonyl-CoA mutase. Methionine synthetase has an absolute requirement for methylcobalamin and catalyzes the conversion of homocysteine to methionine (Fig. 28-5). 5-Methyltetrahydrofolate is converted to tetrahydrofolate (THF) in this reaction. This vitamin B12-catalyzed reaction is the only means by which THF can be regenerated from 5-methyltetrahydrofolate in humans. Therefore, in vitamin B12 deficiency, folic acid can become trapped in the 5-methyltetrahydrofolate form, and THF is then unavailable for conversion to other coenzyme forms required for purine, pyrimidine, and amino acid synthesis (Fig. 28-6). All folate-dependent reactions are impaired in vitamin B12 deficiency, resulting in indistinguishable hematological abnormalities in both folate and vitamin B12 deficiencies. 

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