ALPHA-METHYLENEGLUTARATE MUTASE - AN ADENOSYLCOBALAMIN-DEPENDENT ENZYME

The adenosylcobalamin-dependent enzyme, alpha-methyleneglutarate mutas e, catalyses the interconversion of alpha-methyleneglutarate and beta- methylitaconate in Clostridium barkeri, an anaerobic bacterium. This p aper reviews the assay, separation and purification, and the physical, chemical, thermodynamic and kinetic properties of alpha-methyleneglut arate mutase. Model reactions for alpha-methyleneglutarate mutase are discussed, and a mechanism is proposed. In the isomerization reaction catalysed by alpha-methyleneglutarate mutase, adenosylcobalamin acts a s a carrier for the hydrogen atom that is abstracted from alpha-methyl eneglutarate and replaced by the acrylyl group of alpha-methylenegluta rate.The steric course of the reaction is such that the acrylyl residu e migrates from the beta- to the alpha-carbon of propionate with inver sion of configuration at the alpha-carbon. ESR spectroscopy shows that a cobalt(II) species is formed during catalysis in the presence of su bstrate. It is proposed that homolytic fission of adenosylcobalamin oc curs to give cobalt(II) and the 5'-deoxyadenosine free radical, which then abstracts a hydrogen atom from alpha-methyleneglutarate to give t he corresponding free radical. This rearranges via a substituted cyclo propylmethyl free radical to a free radical corresponding to beta-meth ylitaconate, which abstracts a hydrogen atom from 5'-deoxyadenosine to give the product.