Biphasic transient kinetics are a property of a single site in horse liver alcohol dehydrogenase
Anderson, D.C.; Dahlquist, F.W.
Biochemistry 21(15): 3578-3584
The transient kinetics under single turnover conditions of the dimeric enzyme horse liver alcohol dehydrogenase (LADH) are often biphasic with nearly equal amplitudes in the fast and slow components of the reaction. These observations have been the basis for the suggestion that this enzyme displays half of the sites reactivity or kinetic site-site interactions in its catalytic cycle. Alternatively, the biphasic kinetics could be the result of a complex mechanistic path at a single site with no kinetic interactions between the active sites. To resolve this question, LADH was prepared in which 1 of the 2 active sites is occupied by the strongly bound, slowly exchanging ternary complex inhibitors NAD with trifluoroethanol or NAD with pyrazole. These half-inhibited dimers show essentially the same biphasic transient kinetics as the native enzyme. These results strongly support mechanisms in which the observed kinetics are a property of each independently functioning site of the dimer. The possibility of half of the sites reactivity or kinetic site-site interaction appears to be ruled out.