A BASE-OFF ANALOG OF COENZYME-B-12 WITH A MODIFIED NUCLEOTIDE LOOP - H-1-NMR STRUCTURE-ANALYSIS AND KINETIC-STUDIES WITH (R)-METHYLMALONYL-COA MUTASE, GLYCEROL DEHYDRATASE, AND DIOL DEHYDRATASE

(Co beta-5'-Deoxyadenosin-5'-yl)-(p-cresolyl)cobamide (Ado-PCC), an an alogue of the base-off form of coenzyme-B-12 (CoB12), was prepared by alkylation of (Co alpha/beta-cyano/aqua)-(p-cresolyl)cobamide (PCC) wi th 5'-chloro-5'-deoxyadenosine. The 500 MHz H-1-NMR spectrum of Ado-PC C in D2O at pH 7.4 was completely analyzed using COSY and NOESY two-di mensional experiments. The coenzyme and inhibitory activities of Ado-P CC were tested with three coenzyme-B-12-dependent enzymes: (R)-methylm alonyl-CoA mutase, glycerol dehydratase, and diol dehydratase. Ado-PCC showed strong coenzyme activity with methylmalonyl-CoA mutase, which is known to bind the base-off form of CoB12. In contrast, Ado-PCC had no coenzyme activity but acted instead as a competitive inhibitor with glycerol dehydratase and diol dehydratase, which are likely to prefer the base-on form of CoB12. These results indicate that Ado-PCC, whose structure is analogous to the base-off form of CoB12, can be used for probing the mode of coenzyme binding by coenzyme-B-12-dependent enzym es.