Radical catalysis of B-12 enzymes: structure, mechanism, inactivation, andreactivation of diol and glycerol dehydratases

Enzymatic radical catalysis is defined as a mechanism of catalysis by which enzymes catalyze chemically difficult reactions by utilizing the high reac tivity of free radicals. Adenosylcobalamin (coenzyme B-12) serves as a cofa ctor for enzymatic radical reactions. The recent structural analysis of ade nosylcobalamin-dependent diol dehydratase revealed that the substrate 1,2-p ropanediol and an essential potassium ion are located inside a (beta/alpha) (8) barrel. Two hydroxyl groups of the substrate coordinate directly to the potassium ion which binds to the negatively charged inner part of the cavi ty. Cobalamin bound in the base-on mode covers the cavity to isolate the ac tive site from solvent. Based on the three-dimensional structure and theore tical calculations, a new mechanism for diol dehydratase is proposed in whi ch the potassium ion plays a direct role in the catalysis. The mechanisms f or generation of a catalytic radical by homolysis of the coenzyme Co-C bond and for protection of radical intermediates from undesired side reactions during catalysis are discussed based on the structure. The reactivating fac tors for diol and glycerol dehydratases have been identified. These factors are a new type of molecular chaperone which participate in reactivation of the inactivated holoenzymes by mediating ATP-dependent exchange of the mod ified coenzyme for free intact coenzyme.