A comparative study of some mechanisms describing interaction effects in a dimeric, two-substrate enzyme catalysed reaction

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A number of mechanisms are described which, when applied to a dimeric two substrate enzyme, lead to a second degree relationship between the initial velocity of the enzyme catalysed reaction and a variable substrate concentration, the concentration of the second substrate being held constant. The mechanisms fall into three classes. In the first, the subunits of the enzyme are independent of one another, but alternate pathways leading to product formation exist in each subunit. In the second class, a linear mechanism operates within each subunit, but the subunits interact with one another. In the third class, a temporary dissociation of the enzyme into smaller units is an integral part of the substrate binding process. These models are examined in four main ways: (i) the slope of the Hill plots is determined (ii) the form of the relationship between the initial velocity of product formation and substrate concentrations is investigated (iii) a matrix rank analysis of initial rates of substrate binding is described and (iv) the time course of substrate binding is determined. As a result, it appears that characteristic differences between the models exist and may serve to discriminate between them.