Stereochemistry of the Methyl Group in (R)-3-Methylitaconate Derived by Rearrangement of 2-Methylideneglutarate Catalysed by a Coenzyme B12-Dependent Mutase

Abstract

2-Methylideneglutarate mutase is an adenosylcobalamin (coenzyme B12)-dependent enzyme that catalyses the equilibration of 2-methylideneglutarate with (R)-3-methylitaconate. This reaction is believed to occur via protein-bound free radicals derived from substrate and product. The stereochemistry of the formation of the methyl group of 3-methylitaconate has been probed using a `chiral methyl group'. The methyl group in 3-([2H1,3H]methyl)itaconate derived from either (R)- or (S)-2-methylidene[3-2H1,3-3H1]glutarate was a 50 : 50 mixture of (R)- and (S)-forms. It is concluded that the barrier to rotation about the C−C bond between the methylene radical centre and adjacent C-atom in the product-related radical [.CH2CH(−O2CC=CH2)CO2−] is relatively low, and that the interaction of the radical with cob(II)alamin is minimal. Hence, cob(II)alamin is a spectator of the molecular rearrangement of the substrate radical to product radical.