5.1.99.1 (R)-methylmalonyl-CoA = (S)-methylmalonyl-CoA - - 5.1.99.1 (R)-methylmalonyl-CoA = (S)-methylmalonyl-CoA enzyme acts to labilize the proton on the tertiary carbon atom of methylmalonyl-CoA, and thus causes a rapid exchange of this proton with those of water 2440 5.1.99.1 (R)-methylmalonyl-CoA = (S)-methylmalonyl-CoA mechanism involving loss of the alpha-hydrogen atom with subsequent incorporation of a proton from the medium 2439 5.1.99.1 (R)-methylmalonyl-CoA = (S)-methylmalonyl-CoA two enzyme bases are involved in catalysis. One base removes the proton from the substrate, the second provides the new proton. There is no fast isotopic exchange between enzyme-bound intermediates and solvent protons 2436 5.1.99.1 (R)-methylmalonyl-CoA = (S)-methylmalonyl-CoA with (2R)-methylmalonyl-CoA as substrate: two enzyme bases act respectively as proton donor and acceptor, the release of product is slow, and the reprotonation step on the enzyme is reversible, without leading to isotopic exchange with the solvent 2435