Stereospecificity and mechanism of adenosylcobalamin-dependent diol dehydratase. Catalysis and inactivation with meso- and dl-2,3-butanediols as substrates

A comparative study of the reaction of meso-, d-, and dl-2,3-butanediols with adenosylcobalamin-dependent dioldehydratase was carried out. While the meso isomer is both a substrate and inactivator of holoenzyme, the d and dl compounds act as purely competitive inhibitors, neither undergoing catalysis nor inactivating holoenzyme. Furthermore, d- and dl-2,3-butanediols protect holoenzyme from oxygen inactivation and enzyme-bound cofactor from photolysis, and do not induce detectable cleavage of the carbon-cobalt bond of cofactor. These results show that the stereospecificity of the inactivation reaction is the same as that of catalysis, suggest that hydrogen abstraction from C-1 of substrate may be concerted with cleavage of the carbon-cobalt bond of adenosylcobalamin, and further suggest that formation of a carbon-cobalt bond between coenzyme and substrate is not obligatory for catalysis