A hydrophilic omega-loop (Tyr181 to Tyr188) in the nonsubstrate binding area of HIV-1 reverse transcriptase.

Using the molecular "cloud" of the HIV-1 reverse transcriptase (RT) as starting point, the peptide backbone of the polymerase subunit was visualized by molecular modelling. Then, the two subregions 98-106 and 179-190 of the allosteric area were "isolated". From the latter subregion, the Tyr181 to Tyr188 segment containing two amino acids (Asp185, Asp186) of the catalytic aspartyl triad and two amino acids (Tyr181, Tyr188) of the nonnucleoside RT inhibitor (NNRTI) binding sites, was excised. It was shown that the segment has a omega-like loop configuration which is highly hydrophilic. The two phenolic side chains of Tyr181 and Tyr188 represent the lipophilic "horizontal axes" of the omega-loop shape. The relative rigidity of the omega-loop is mainly based on a hydrogen bond between the peptide CO of Tyr181 and the peptide NH of Tyr188. Solvation in water increases the number of intramolecular hydrogen bonds. Therefore, desolvation is one of the conditions of binding with NNRTIs. Site-directed mutagenesis affects the hydrophilicity of the omega-loop while steric features are less influenced.