Methylmalonyl-CoA epimerase

D-Methylmalonyl-CoA, the product of this reaction, is converted to the L-isomer by methylmalonyl-CoA epunerase (Figure 24.19). (This enzyme has often and incorrectly been called methylmalonyl-CoA racemase. It is not a racemase because the CoA moiety contains five other asymmetric centers.) The epimerase reaction also appears to involve a carbanion at the a-position (Figure 24.20). The reaction is readily reversible and involves a reversible dissociation of the acidic a-proton. The L-isomer is the substrate for methylmalonyl-CoA mutase. Methylmalonyl-CoA epimerase is an impressive catalyst. The for the proton that must dissociate to initiate this reaction is approximately 21 If binding of a proton to the a-anion is diffusion-limited, with = 10 M sec then the initial proton dissociation must be rate-limiting, and the rate constant must be... 

The turnover number of methylmalonyl-CoA epimerase is 100 sec and thus the enzyme enhances the reaction rate by a factor of 10. ... 

FIGURE 24.20 The methylmalonyl-CoA epimerase mechanism involves a resonance-stabilized carbanion at the oj-position. 

PROPIONYL-CoA CARBOXYLASE METHYLMALONYL-CoA DECARBOXYLASE METHYLMALONYL-CoA EPIMERASE d-METHYLMALONYL-CoA HYDROLASE... 

R) -2-Methyl-3-oxopropanoyl-coenzyme A, METHYLMALONYL-CoA EPIMERASE l-METHYLMALONYL-CoA MUTASE... 

Table 9.2 Normalized structure-factor magnitude statistics for the peak-wavelength data for methylmalonyl-coA epimerase (1JC4)...

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. 

Methylmalonyl-CoA epimerase shifts the CoA thioester from C-l (of the original propionyl group) to the newly added carboxylate, making the product L-methylmalonyl-CoA. 

Sprinson and coworkers [30] conducted the methylmalonyl-CoA mutase reaction in deuterium oxide using a crude mitochondrial preparation. The presence of methylmalonyl-CoA epimerase insured that (1) all substrate molecules incorporated one atom of deuterium into position 2, and (2) in the course of the reaction the (2R)-epimer of methylmalonyl-CoA was continuously supplied by epimerization of the (25)-epimer, which was in turn generated by the enzymic carboxylation of propionyl-CoA. Alkaline hydrolysis of the product and subsequent purification furnished succinic acid which was mainly monodeuterated (70% 2H,-, 15% 2H2-labelled and 13% unlabelled species). A positive ORD curve revealed its (5) configuration indicating stereochemical retention for the AdoCbl-dependent rearrangement (Fig. 22). No plausible explanation could be offered for the formation of doubly deuterated and unlabelled species. Essentially the same results were later obtained with a highly purified mutase preparation from Propionibacterium sher-manii (J. Retey, unpublished). 

In animals, the breakdown of lipids involves conversion of propionyl-CoA to succinyl-CoA. Methylmalonyl-CoA is a metabolic intermediate in this process. In vivo, it is necessary to convert the 2-(S)-form of methylmalonyl-CoA to the 2-(R)-form, for reaction with methylmalonyl-CoA mutase. This reaction is catalyzed by methylmalonyl-CoA epimerase (MMCE) [4, 66-68]. Methylmalonate is also employed in polyketide antibiotic biosynthesis, in the form of methylmalonate units, although less is known about the stereochemical requirements of these processes [69, 70]. 

Methylmalonyl CoA epimerase and some others, in whose substrates a CH3 group is bound to the chiral centers (E. C. class 5.1.99). 

Methylmalonyl-CoA is present biologically in both the D and L isomers, which can be interconverted by the enzyme methylmalonyl-CoA epimerase (Figure 18.19). This reaction is important in the metabolism of propionyl-CoA, which arises from oxidation of odd-numbered chains of fatty acids. L-methylmalonyl-CoA is further acted upon by methylmalonyl-CoA mutase. 

See also Methylmalonyl-CoA Epimerase Propionyl-CoA Carboxylase, Oxidation of Odd-Numbered Fatty Acids,... 

See also Methylmalonyl-CoA Epimerase, Coenzymes in Nitrogen Metabolism, Adenosylmethionine and Biological Methylation, Porphyrin and Heme Metabolism... 

Propionyl-CoA carboxylase Methylmalonyl-CoA epimerase Methylmalonyl-CoA mutase (B12 enzyme)... 

Fig. 4 A, B. Polyketide synthase substrate routes. Potential substrates have been boxed A enzymes performing one enzymatic conversion 1, acetyl-CoA synthetase (alternatively, 1 represents a two enzyme pathway, acetate kinase followed by acetylphosphotransferase) 2, acetyl-CoA carboxylase 3, mal-onyl-CoA decarboxylase 4, malonyl-CoA synthetase B enzymes performing one enzymatic conversion 1, propionyl-CoA synthetase (T, propionate kinase followed by propionylphosphotransferase) 2, propionyl-CoA carboxylase 3, methylmalonyl-CoA decarboxylase 4, methylmalonyl-CoA epimerase 5, methylmalonyl-CoA mutase 6, isobutyryl-CoA mutase...

Methylmalonyl-CoA mutase (EC 5.4.99.2). Failure to convert (/ )-methylmalonyl-CoA into succinyl-CoA. Large quantities of methylmalonic acid appear in plasma and urine. Affected children fail to thrive and show pronounced ketoacidosis. Often fatal in early life. Hyperammonemia and intermittent hyperglycinemia are also typical. Restricted protein intake and synthetic diets are helpful, in particular low intakes of leucine, isoleucine, valine, threonine and methionine. A similar condition may arise from a congenital deficiency of methylmalonyl-CoA epimerase (EC 5.1.99.1). Both conditions unresponsive to vitamin Bj2. Another type of methylmalonyl aciduria is thought to result from an hereditary deficiency of deoxyadenosyl transferase (transfers the 5 -deoxyade-nosyl group in cobalamin synthesis), which provides the coenzyme of methylmalonyl-CoA mutase. This condition responds to injection of B,2. Dietary B12 deficiency also results in methylmalonic aciduria. 

Figure 14 Production of 3-hydroxypropionic add as a nrtetabolic intermediate and an end product via 3-hydroxypropionate pathway in ChlCH ofiexus aurantiacus. Enzymes accACD, acetyl-CoA carboxylase mcr, malonyl-CoA reductase pcs, propionyl-CoA synthase pcc, propionyl-CoA carboxylase mcee, methylmalonyl-CoA epimerase mut, methylmalonyl-CoA mutase smtAB, sucdnyl-CoA (S)-malate-CoA transferase sdh, succinate dehydrogenase fh, fumarate hydratase mcl, (S)-malyl-CoA/-methylmalyl-CoA/(S)-citramalyl-CoA (MMC) lyase mch, methylmalyl-CoA dehydratase met, mesaconyl-CoA-Cl-C4 CoA transferase and meh, mesaconyl-C4-CoA hydratase [10] (Reference Metacyc).

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