Role of induced fit in limiting discrimination against AZT by HIV reverse transcriptase

Single turnover kinetic studies were conducted using fluorescently labeled HIV reverse transcriptase (RT) to evaluate the role of nucleotide-induced changes in enzyme structure in the selectivity against AZT in order to explore why AZT-resistant forms of the enzyme fail to significantly discriminate against AZT. Fluorescent labeling of HIV RT provided a signal to monitor the isomerization from "open" to "closed" states following nucleotide binding. We measured the rate constants governing nucleotide binding and enzyme isomerization for TTP and AZT-triphosphate by the wild-type and AZT-resistant forms of the enzyme containing the thymidine analogue mutations (TAMs). We show that the TAMs alter the kinetics of AZT incorporation by weakening ground-state nucleotide binding and decreasing the rate of chemistry relative to the wild-type enzyme. However, the slower rate of incorporation of AZT by the TAMs HIV RT is counterbalanced a lower K(m), resulting from the equilibration of the conformational change step. In contrast, the K(m) for the wild-type enzyme reflects the balance between rates of binding and incorporation so the conformational change step does not come to equilibrium. These data once again demonstrate that the rate of substrate release, limited by the reverse of the substrate-induced conformational change, is the key determinant of the role of induced fit in enzyme specificity. Mutations leading to slower rates of incorporation have the unfortunate consequence of lowering the K(m) value by allowing the conformational change step to come to equilibrium.