T. Toraya, Radical catalysis of B-12 enzymes: structure, mechanism, inactivation, andreactivation of diol and glycerol dehydratases, CELL MOL L, 57(1), 2000, pp. 106-127
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.