Coenzyme B12-dependent-methylmalonyl coenzyme A (CoA) mutase (MCM)-like family; contains proteins similar to MCM, and the large subunit of Streptomyces coenzyme B12-dependent isobutyryl-CoA mutase (ICM). MCM catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA. The reaction proceeds via radical intermediates beginning with a substrate-induced homolytic cleavage of the Co-C bond of coenzyme B12 to produce cob(II)alamin and the deoxyadenosyl radical. MCM plays an important role in the conversion of propionyl-CoA to succinyl-CoA during the degradation of propionate for the Krebs cycle. In higher animals, MCM is involved in the breakdown of odd-chain fatty acids, several amino acids, and cholesterol. Methylobacterium extorquens MCM participates in the glyoxylate regeneration pathway. In M. extorquens, MCM forms a complex with MeaB; MeaB may protect MCM from irreversible inactivation. In some bacteria, MCM is involved in the reverse metabolic reaction, the rearrangement of succinyl-CoA to methylmalonyl-CoA. Examples include Propionbacterium shermanni MCM during propionic acid fermentation, E.coli MCM in a pathway for the conversion of succinate to propionate and Streptomyces MCM in polyketide biosynthesis. P. shermanni and Streptomyces cinnamonensis MCMs are alpha/beta heterodimers, with both subunits being homologous members of this family. It has been shown for P. shermanni MCM that only the alpha subunit binds coenzyme B12 and substrates. Human MCM is a homodimer with two active sites. Mouse and E.coli MCMs are also homodimers. ICM from S. cinnamonensis is comprised of a large and a small subunit. The holoenzyme appears to be an alpha2beta2 heterotetramer with up to 2 molecules of coenzyme B12 bound. The small subunit binds coenzyme B12. ICM catalyzes the reversible rearrangement of n-butyryl-CoA to isobutyryl-CoA (intermediates in fatty acid and valine catabolism, which in S. cinnamonensis can be converted to methylmalonyl-CoA and used in polyketide synthesis). In humans, impaired activity of MCM results in methylmalonic aciduria, a disorder of propionic acid metabolism.
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