2-Methyleneglutarate mutase from the anaerobe Eubacterium (Clostridium) barkeri is an adenosylcobalamin (coenzyme B-12)-dependent enzyme that catalyzes the equilibration of 2-methylene-glutarate with (R)-3-methylitaconate. Two possibilities for the mechanism of the carbon skeleton rearrangement of the substrate-derived radical to the product-related radical are considered. In both mechanisms an acrylate group migrates from C-3 of 2-methyleneglutarate to C-4. In the "addition -elimination" mechanism this 1,2-shift occurs via an intermediate, a 1-methylenecyclopropane-1,2-dicarboxylate radical, in which the migrating acrylate is simultaneously attached to both C-3 and C-4. In the 1 ragmentation-recombination" mechanism the migrating group, a 2-acrylyl radical, becomes detached from C-3 before it starts bonding to C-4. In an attempt to distinguish between these two possibilities we have investigated the action of 2-methyleneglutarate mutase on the stereospecifically cleuterated substrates (2)-3-methyl[2'-H-2(1)]itaconate and (Z)-3-[2'-H-2(1),methyl-H-2(3)]methylitaconate. The enzyme catalyzes the equilibration of both compounds with their corresponding E-isomers and with a 1:1 mixture of the corresponding (E)- and (2)-2-methylene[2'-H-2(1)]-glutarates, as shown by monitoring of the reactions with H-1 and H-2 NMR. In the initial phase of the enzyme-catalyzed equilibration a significant excess (8-11 %) of (E)-3-methyl[2'-H-2(1)]itaconate over its equilibrium value was observed ("E-overshoot"). The E-overshoot was only 3-4% with (Z)-3-[2'-H-2(1),methyl-H-2(3)]methylitaconate because the presence of the deuterated methyl group raises the energy barrier from 3-methylitaconate to the corresponding radical. The overshoot is explained by postulating that the migrating acrylate group has to overcome an additional energy barrier from the state leading back to the substrate-derived radical to the state leading forward to the product-related radical. It is concluded that the fragmentation-recombination mechanism can provide an explanation for the results in terms of an additional energy barrier, despite the higher calculated activation energy for this pathway.