Probes of mechanism and transition-state structure in the alcohol dehydrogenase reaction

Detailed studies of chemical mechanism and transition state structure in enzyme-catalyzed reactions are frequently hampered by rate determining protein isomerization and product desorption steps. Among dehydrogenases, the alcohol dehydrogenases are almost unique in the successful kinetic isolation of the bond rearrangement step(s). Some of the pertinent mechanistic questions are (1) the mode of hydrogen activation (hydride ion vs. hydrogen atom), (2) the existence and nature of chemical intermediates, (3) a role for an active site Zn+2 vs. Zn+2-OH2 in acid-base catalysis, (4) the "concertedness" of such catalysis with heavy atom rearrangements, and (5) the extent to which the structure of the transition state resembles oxidized substrates vs. reduced products. Although definitive answers to each of these questions are not yet available, a wealth of information has been amassed for both yeast and horse liver alcohol dehydrogenase. Importantly, kinetic studies support a conservation of mechanism and transition state structure among dehydrogenases from divergent evolutionary sources.