AZT Binds RNA at Multiple Sites

Abstract

Azidothymidine (AZT) is a widely used inhibitor of type I human immunodeficiency virus (HIV) reverse transcriptase that act as a DNA chain terminator. Studies have shown primer unblocking and rescue of DNA synthesis AZT-resistant HIV-1 reverse transcriptase on DNA and RNA templates. Our recent study showed AZT bindings to the G-C, A-T base pairs and the backbone phosphate group of DNA duplex resulting in partial DNA conformational changes. This study was designed to examine the interaction of AZT with RNA in aqueous solution at physiological condition, using diferent drug/RNA (phosphate) molar ratios of 1/800 to 1/2 and constant RNA concentration of 1.25 or 12.5 mM (phosphate). Capillary electrophoresis, FTIR, and UV-visible difference spectroscopic methods and molecular modeling were used to determine the drug binding sites, binding constants, and the effects of AZT complexation on RNA conformation. Structural analysis showed that AZT binds RNA through G-C and A-U bases with two binding constants of K ₁ = 7.3 × 10⁵ M^-1 and K ₂ = 1.90 × 10⁵ M^-1. The drug distributions were 54% with G-C, 36% A-U, and 10% with the backbone phosphate group. RNA remains in A-family structure and drug sugar pucker in the C2'-endo/anti conformation in the AZT–RNA complexes. Molecular modeling studies show hydrogen bondings between RNA and AZT donor groups.