Molecular basis for the enantioselectivity of HIV-1 reverse transcriptase:role of the 3 '-hydroxyl group of the L-(beta)-ribose in chiral discrimination between D- and L-enantiomers of deoxy- and dideoxy-nucleoside triphosphate analogs

In order to identify the basis for the relaxed enantioselectivity of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase (RT) and to eva luate possible cross-resistance patterns between L-nucleoside-, D-nucleosid e- and non-nucleoside RT inhibitors, to be utilised in anti-HIV-1 combinati on therapy, we applied an in vitro approach based on the utilisation of six recombinant HIV-1 RT mutants containing single amino acid substitutions kn own to confer Nevirapine resistance in treated patients, The mutants were c ompared on different RNA/DNA and DNA/DNA substrates to the wild type (wt) e nzyme for their sensitivity towards inhibition by the D- and L-enantiomers of 2'-deoxy- and 2',3'-dideoxynucleoside triphosphate analogs, The results showed that the 3'-hydroxyl group of the L-(beta)-2'-deoxyribose moiety cau sed an unfavourable steric hindrance with critic residues in the HIV-1 RT a ctive site and this steric barrier was increased by the Y1811 mutation. Eli mination of the 3'-hydroxyl group removed this hindrance and significantly improved binding to the HIV-1 RT wt and to the mutants, These results demon strate the critical role of both the tyrosine 181 of RT and the 3'-position of the sugar ring, in chiral discrimination between D- and L-nucleoside tr iphosphates, Moreover, they provide an important rationale for the combinat ion of D- and L-(beta)-dideoxynucleoside analogs with non-nucleoside Ri inh ibitors in anti-HIV chemotherapy, since non-nucleoside inhibitors resistanc e mutations did not confer cross-resistance to dideoxynucleoside analogs.