Kinetics analysis of consecutive HIV proteolytic cleavages of the Gag-Pol polyprotein.

The ordered, sequential cleavages of the Gag-Pol polyprotein by human immunodeficiency virus (HIV) protease present the virus with severe limitations on viable mutations of the enzyme. An extension of the method of Kuchel et al. (Kuchel, P. W., Nichol, L. W., and Jeffrey, P. D. (1974) J. Theor. Biol. 48, 39-49) for the analysis of consecutive enzyme reactions leads to a simple description of the catalytic efficiency of mutant and wild type HIV protease in the presence or absence of inhibitors. The overall catalytic efficiency of a mutant HIV protease relative to the wild type enzyme is given by the product of the ratios of their respective efficiencies for the 8 obligatory cleavages. Under no conditions is HIV viable when the geometric mean efficiency of a mutant HIV protease is less than 61% of the wild type activity for each cleavage. The lower catalytic efficiencies of the mutant enzymes coupled with the exponential dependence on 1/(1 + [I]/Ki) more than offset the inhibitor resistance acquired by HIV protease. The conclusion of this analysis is that inhibitor-resistant mutant HIV proteases are very unlikely to contribute to viral viability in vivo. The results strongly suggest that future protease inhibitor clinical trials should measure the infectivity of the virions in blood plasma instead of relying on viral RNA levels.