Structure of HIV-1 protease in complex with potent inhibitor KNI-272 determined by high-resolution X-ray and neutron crystallography

HIV-1 protease is a dimeric aspartic protease that plays an essential role in viral replication. To further understand the catalytic mechanism and inhibitor recognition of HIV-1 protease, we need to determine the locations of key hydrogen atoms in the catalytic aspartates Asp-25 and Asp-125. The structure of HIV-1 protease in complex with transition-state analog KNI-272 was determined by combined neutron crystallography at 1.9-angstrom resolution and X-ray crystallography at 1.4-angstrom resolution. The resulting structural data show that the catalytic residue Asp-25 is protonated and that Asp-125 (the catalytic residue from the corresponding diad-related molecule) is deprotonated. The proton on Asp-25 makes a hydrogen bond with the carbonyl group of the allophenylnorstatine (Apns) group in KNI-272. The deprotonated Asp-125 bonds to the hydroxyl proton of Apns. The results provide direct experimental evidence for proposed aspects of the catalytic mechanism of HIV-1 protease and can therefore contribute substantially to the development of specific inhibitors for therapeutic application.