Carbon Skeleton Mutases

FIGURE 21. Mechanisms for the rearrangement of substrate radicals in the reactions catalyzed by carbon skeleton mutases. For 2-methyleneglutarate mutase and the acyl-CoA mutases both associative (upper pathway) and dissociative (lower pathway) mechanisms have been proposed (Halpem, 1985 Bucket Golding, 1996), whereas for glutamate only a dissociative mechanism appears feasible. 

About 10 coenzyme B -dependent enzymes are now known (reviewed in References 13,14, and 76 see Table 1) four carbon skeleton mutases (methylmalonyl-CoA mutase (MMCM), glutamate mutase (GM), methylene glu-tarate mutase (MGM), isobutyryl-CoA mutase (ICM) ), diol dehydratase (DD), glycerol dehydratase, ethanol-amine anunonia lyase (EAL), two amino mutases, and Bi2-dependent ribonucleotide reductase. The coenzyme Bi2-dependent enzymes are unevenly distributed in the living world, and MMCM is the only enzyme that is also indispensable in human metabolism. ... 

Reversible reactions that rearrange molecular skeletons (X is a carbon-based group in the carbon skeleton mutases and NH2 in the amino mutases X is shown in red below)... 

COMPARISON OF THE MODELS FOR B12-DEPENDENT CARBON-SKELETON MUTASES... 

To obtain an overview of the reactions catalyzed by B 12-dependent carbon-skeleton mutases, we present a comparison of the G3(MP2)-RAD(p) barrier heights for the different pathways considered in the present work as a function of the migrating group (Figure 4). 

Figure 4. Comparison of the G3(MP2)-RAD(p) energy requirements for the fragmentation-recombination, addition-elimination and protonated pathways for the model systems of B 12-dependent carbon-skeleton mutases with migrating groups CH=CH2, CH=NH, CH=0 and CH2-NH2.

Although we have only discussed the partial-proton-transfer model for one of the B 12-dependent carbon-skeleton mutases [69], we can expect that there also exists a continuum between no protonation and full protonation of the substrate in the other reactions. Analogously, partial hydride removal from the substrate of glutamate mutase may serve to facilitate this rearrangement. 

Aside from the Bi2-binding and transporting proteins, three major classes of corrinoid enzymes are known methyltransferases, coenzyme B12 utilizing enzymes, and corrinoid dehalogenases. The coenzyme B12 utilizing enzymes can be classified further as carbon skeleton mutases, dehydratases, deaminases, and ribonucleotide reductase. Members of all classes of the Bi2-enzymes are important in microorganisms. In human and animal metabolism B12-binding proteins, the methyl transferase methionine synthase and methylmalonyl-CoA-mutase (MMCM) play essential roles. 

Besides MMCM and GM, two other coenzyme B -dependent carbon skeleton mutases are known. These are (1) methylene glutarate mutase (MGM) from the anaerobe Eubacterium (Clostridium) barkeri, which catalyzes the equilibration of 2-methylene-glutarate with (R)-3-methylitaconate as part of a degradative path of nicotinic acid [175,199] and (2) isobutyryl-CoA mutase (ICM), which is observed in species of gram-positive bacteria Strep-tomyces and catalyzes the reversible rearrangement of iso-butyryl-CoA and n-butyryl-CoA [177]. The isomerization of iso-butyryl-CoA and n-butyryl-CoA in ICM is relevant in the biosynthesis of polyketide antibiotics [177]. 

Thus, an intriguing feature of the coenzyme Bi2-dependent enzymes is the dramatic (> lO -fold) labihzation of the bound organometallic cofactor towards homolysis of the Co-C bond [119,123,173]. The mechanism of the enzyme (and substrate-induced) labihzation of this Co-C bond is stiU a key problem, and much discussed, in Bn-chemistry. Evidence for covalent restructuring of the bound cofactor (except for the formation of the base-off/His-on form in the carbon skeleton mutases) is not available [75,119,123, 173,194]. In addition protein and solvent molecules can only weakly stabihze a radical center [240]. Steric distortions of the protein-bound cofactor were discussed as means for the enhanced rate of Co - C bond homolysis [51,119, 163,217]. Halpem s theory of an upwards conformational distortion of the... 

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