Resolving the Fast Equilibrations of Ternary Complexes in Liver Alcohol Dehydrogenase

Liver alcohol dehydrogenase is an enzyme system which has been investigated extensively. In a recent paper by Sekhar & Plapp (1989), possibly as many as eight steps were revealed, most of them interconversions among ternary complexes. Many of the rate constants reported were near or above the reciprocal of the reported dead time of the mixing apparatus used. Thus, many of these rate constants may represent lower limits. However, even with a higher speed mixing apparatus, some of the rate constants still can only be estimated. In particular, there is a proton transfer step to/from the buffer system which is expected to be much faster than adjoining rearrangements. To the extent some steps involve purely electronic changes, they most likely are also faster than adjoining structural rearrangements. It can be shown that chemical relaxation can be used to analyze the kinetics of fast steps between slower ones, not accessible by rapid mixing (also not accessible at overall equilibrium where the required concentrations are too small). Using the best data from the literature, one may simulate two experimental protocols to demonstrate what can be accomplished by combining rapid flow with chemical relaxation. In one protocol, the ternary complexes are formed by mixing the pre-mixed binary complex of enzyme and NADH with acetaldehyde; the rate conditions are such that chemical relaxation has to be initiated 4 msec after mixing to observe the rapid proton transfer reactions. In the other protocol, the ternary complexes are formed by mixing the pre-mixed binary complex of enzyme and NAD+ with ethanol; the rate conditions are such that chemical relaxation has to be initiated 0·2 msec after mixing to observe the rapid proton transfer reactions. The simulations for both protocols reveal clearly the relaxation processes of the fast steps imbedded between slow steps. Thus, this combination of rapid flow with chemical relaxation reveals mechanistic steps not presently accessible any other way.