Dead-End Inhibition in Steady-State Bisubstrate Systems

6 DEAD-END INHIBITION IN STEADY-STATE BISUBSTRATE SYSTEMS 

The kinetics of substrates, products, and alternate products define the form of the rate equation, and are certainly necessary to deduce the kinetic mechanism. However, they are often not sufficient to do this unequivocally and other kinetic approaches are necessary, especially when reaction can be studied in only one direction. One of the most useful approaches in such cases involves the use of dead-end inhibitors (Cleland, 1970, 1979, 1990). 

Dead-end inhibitor is a compound that reacts with one or more enzyme forms to yield a complex that cannot participate in the reaction. These nonreacting 

The velocity equation in the presence of a dead-end inhibitor can be derived in the usual manner by the King-Altman method. However, if we know the velocity equation for the uninhibited reaction, then we can easily write the new velocity equation as modified by the inhibitor, without going through an entire derivation. The effect of a dead-end inhibitor is to multiply certain terms in the denominator of the uninhibited velocity equation by the factor F (F = i -t-//Xj), ths fractional concentration of an inhibitor. The terms multiplied by F, are those representing the enzyme form, or enzyme forms, combining with the inhibitor. Then, the Kj represents the dissociation constant of the specific enzyme form-inhibitor complex. 

In rapid equilibrium systems, the relative concentration of any particular enzyme form is given by a single denominator term in the velocity equation. For example, in the rapid equilibrium ordered bireactant system (Section 8.2), the velocity Eq. (8.2) is 

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