Two-Substrate Enzyme Kinetics

The kinetic behavior of many enzymes cannot be described by the simple Michae-lis-Menten equation, because more than one substrate is involved in the reaction. Such is the case for the oxidoreductases, the transferases, and the ligases, three of the six major divisions of enzymes. The kinetic behavior of these systems is necessarily more complicated, and depend on whether or not a ternary enzyme-primary substrate-secondary substrate complex is formed. 

If a ternary complex is formed, the mechanism may be considered ordered or random. The ordered mechanism requires that the first substrate (Si) must bind to the enzyme before the second substrate (S2) will bind, and is represented by Eq. 2.26  

Dehydrogenase enzymes that use NAD+ as a cofactor follow this mechanism, since an enzyme-NAD+ complex forms initially, and changes the local structure at the enzyme s active site to allow substrate binding. 

If no particular order is required for substrate binding, the model shown in Eq. 2.27 is used. 

In this case, either substrate will bind to the enzyme initially, and a ternary complex is then formed that will decompose to products. By assuming that all three complexes, E S1, E S2, and E Si S2 form rapidly, and that the transformation of E Si S2 into products is relatively slow (and rate determining), we obtain Eq. 2.28 for the initial reaction rate  

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