Thermodynamic basis of resistance to HIV-1 protease inhibition: Calorimetric analysis of the V82F/I84V active site resistant mutant

One of the most serious side effects associated with the therapy of HIV-1 i nfection is the appearance of viral strains that exhibit resistance to prot ease inhibitors. The active site mutant V82F/ I84V has been shown to lower the binding affinity of protease inhibitors in clinical use. To identify th e origin of this effect, we have investigated the binding thermodynamics of the protease inhibitors indinavir, ritonavir, saquinavir, and nelfinavir t o the wild-type HIV-1 protease and to the V82F/I84V resistant mutant. The m ain driving force for the binding of all four inhibitors is a large positiv e entropy change originating from the burial of a significant hydrophobic s urface upon binding. At 25 degrees C, the binding enthalpy is unfavorable f or all inhibitors except ritonavir, for which it is slightly favorable (-2. 3 kcal/mol). Since the inhibitors are preshaped to the geometry of the bind ing site, their conformational entropy loss upon binding is small, a proper ty that contributes to their high binding affinity. The V82F/I84V active si te mutation lowers the affinity of the inhibitors by making the binding ent halpy more positive and making the entropy change slightly less favorable. The effect on the enthalpy change is, however, the major one. The predomina ntly enthalpic effect of the V82F/I84V mutation is consistent with the idea that the introduction of the bulkier Phe side chain at position 82 and the Val side chain at position 84 distort the binding site and weaken van der Waals and other favorable interactions with inhibitors preshaped to the wil d-type binding site. Another contribution of the V82F/I84V to binding affin ity originates from an increase in the energy penalty associated with the c onformational change of the protease upon binding. The V82F/I84V mutant is structurally more stable than the wild-type protease by about 1.4 kcal/mol. This effect. however, affects equally the binding affinity of substrate an d inhibitors.