THEORETICAL-EXAMINATION OF THE MECHANISM OF ALDOSE KETOSE ISOMERIZATION

Two mechanisms for an aldose-ketose isomerization have been examined u sing high level ab initio and semiempirical molecular orbital methods. The proton transfer pathway via an enediol intermediate is shown to b e favored in the absence of a metal ion, while the hydride transfer pa thway becomes favored in the presence of a metal ion. Our calculations explain why the proton transfer pathway is operative in most aldose-k etose isomerization reactions. These calculations also provide further support for the previously proposed metal ion-mediated hydride transf er mechanism for xylose isomerase.