Abstract

When two alternative substrates are present simultaneously in an enzyme reaction, and one is a better substrate (higher V_max) than the other, the combined rate of the two reactions may be greater than, equal to, or less than the rate observed with the better substrate alone. At some concentration of the better substrate, the poorer substrate will appear to have no effect. On a plot of the combined velocity against the concentration of the more effective substrate, the family of curves obtained, each representing a different level of the poorer substrate, will have a common point of intersection. This phenomenon can be exploited effectively to determine whether or not a single enzyme catalyzes two similar reactions, or to determine whether or not two drugs act on the same receptor.

In examining the applicability of these conclusions to more complex enzyme mechanisms, six variations of two major bi-bi (two-substrate, two-product) mechanisms have been studied. It was found that in every case, the formal theory based on the simple Michaelis-Menten mechanism holds, provided that apparent Michaelis constants are used.