Enzymes and Kinetic Mechanisms

Most enzymes catalyze reactions between two or more substrates to yield two or more products. Yet, most introductory biochemistry texts restrict their discussion to unireactant systems. As a result, it is not always appreciated that the Km for a particular substrate at one fixed set of cosubstrate concentrations may not be the real K , but, instead, an apparent value that changes as the cosubstrate concentrations vary. Similarly, the observed of a preparation at a saturating concentration of one substrate may not be the same Vroax observed when another substrate is saturating. The true Km. for a particular substrate is that observed when all other substrates are saturating. The true is observed when all substrates are present at 

In this section the subject of multireactant enzymes will be introduced by examining some common bireactant systems. We will indicate the ligands as A and B, where B is a substrate and A can be a cosubstrate or coenzyme or an essential activator. 

Theoretically, the reaction might proceed by a number of kinetic mechanisms. These are described below. 

The two substrates (shown as A and B) might add randomly to the enzyme (Fig. 4-41) exactly as S and I do in a classical noncompetitive or mixed-type 

A binds to free E with a dissociation constant Ka (also called Ku, in the Cleland nomenclature). B binds to free E with a dissociation constant -Kb (or Kn). The binding of one substrate may alter the affinity of the enzyme for the other. Thus, A binds to EB with a dissociation constant ctKa. Since the overall equilibrium constant between A and E must be the same regardless of the path taken, B binds to EA with a dissociation constant aKs. o Ka is the same as Km (the K for A at saturating [B]). ocKb is the same as (the for B at saturating [A]). If the rate-limidng step is the slow conversion of EAB to EPQ, we can derive the velocity equation for the forward reaction in the absence of P and Q in the usual manner. In fact, the only difference between the rapid equilibrium random bireactant system and noncompetitive or linear mixed-type inhibition is that now the ternary complex (EAB) is catalyticaUy active, while ESI was not. 

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