A MOLECULAR MECHANISM FOR ENZYMATIC DEHYDROGENATIONS INVOLVING PYRIDINE NUCLEOTIDES
The main kinetic results obtained with the dehydrogenases are briefly summarized. It is shown that the mechanism must involve a ternary enzyme–substrate–coenzyme complex, that acidic and basic groups on the enzyme surface must be involved in the reaction, and that there appears to be a transfer of the substrate from one site to another in the rate-determining step. It is suggested that this step is the actual hydrogen-transfer process. When the substrate is undergoing oxidation, this transfer is brought about by a nucleophilic attack by a basic group B−at the enzyme's active center. This attack may be on a hydroxyl hydrogen atom, and is considered to lead to the transfer of H−from the substrate to the coenzyme. The product is held to the enzyme by hydrogen bonding between the group BH (formed by the addition of a proton to the basic group B−) and the carbonyl group on the substrate.