Direct participation of potassium ion in the catalysis of coenzyme B-12-dependent diol dehydratase

\The direct ion-dipolar interactions between potassium ion (K+) and the two hydroxyl groups of the substrate are the most striking feature of the crys tal structure of coenzyme B-12-dependent diol dehydratase. We carried out d ensity-functional-theory computations to determine whether K+ can assist th e 1,2-shift of the hydroxyl group in the substrate-derived radical. Between a stepwise abstraction/recombination reaction proceeding aia a direct hydr oxide abstraction by K+ and a concerted hydroxyl group migration assisted b y K+, only a transition state for the latter concerted mechanism was found from our computations. The barrier height for the transition state from the complexed radical decreases by only 2.3 kcal/mol upon coordination of the migrating hydroxyl group to Rt, which corresponds to a 42-fold rate acceler ation at 37 degrees C, The nea binding energy upon replacement of the K+-bo und water for substrate was calculated to be 10.7 kcal/mol, It can be consi dered that such a large binding energy is at least partly used for the subs trate-induced conformational changes in the enzyme that trigger the homolyt ic cleavage of the Co-C bond of the coenzyme and the subsequent catalysis b y a radical mechanism. We propose here a new mechanism for diol dehydratase in which K+ plays a direct role in the catalysis.