The structure and the mechanism of action of coenzyme B-12-dependent diol dehydratases

Adenosylcobalamin (AdoCbl) (coenzyme B-12) serves as a cofactor for enzymat ic radical reactions. The recently solved X-ray structure of diol dehydrata se in complex with cyanocobalamin (CN-Cbl) revealed the base-on mode of cob alamin binding. The active-site cavity inside the (beta/alpha)(8) barrel se ems to be a common molecular apparatus for coenzyme B-12-dependent enzymes to spatially isolate highly reactive radical intermediates. Based on the di rect ion-dipolar interactions between two hydroxyl groups of substrate and potassium ion in the active site and theoretical calculations, a new mechan ism for diol dehydratase is proposed here in which a potassium ion particip ates directly in the catalysis. The mechanism of activation of the coenzyme 's cobalt-carbon bond by which a catalytic radical is generated in the acti ve site of diol dehydratase is also discussed on the basis of the conformat ion of the enzyme-bound cobalamin. It was highly suggested that the reactiv ity of the cobalt atom is controlled by the length of the Co-N bond. Theref ore, the role of the 5,6-dimethylbenzimidazole (DBI) moiety of cobalamin co enzyme in the enzyme catalysis is most likely to prevent the enzyme from me chanism-based inactivation by keeping the Co-N bond distance long through s teric repulsion between the flattened corrin ring and the base. (C) 2000 El sevier Science B.V. All rights reserved.