AbstractDeoxythymidine triphosphate analogs with various 3’ groups (-OH (dTTP), -H, -N3, -NH2, -F, -O- CH3, and no group (2′,3′-didehydro-2′,3′-dideoxythymidine triphosphate (d4TTP)), and those retaining the 3’-OH but with 4’ additions (4’-C-methyl, 4’-C-ethyl) or sugar ring modifications (D-carba dTTP) were evaluated using pre-steady-state kinetics in low (0.5 mM) and high (6 mM) Mg2+ with HIV reverse transcriptase (RT). All analogs showed diminished incorporation compared to dTTP ranging from about 2-fold (3’-H, -N3, and d4TTP with 6 mM Mg2+) to >10-fold (3’-NH2 and 3’-F with 0.5 mM Mg2+). The exception was 3’-O-CH3 dTTP which was incorporate profoundly more slowly than other analogs. The incorporation rate (k) using 5 µM dTTP and 0.5 mM (free) Mg2+ was modestly slower (1.6-fold) than with 6 mM Mg2+, while analogs with 3’ modifications were incorporated more slowly (2.8-5.1-fold) in 0.5 mM Mg2+. In contrast, 4’-C-methyl and D-carb, which retain the 3’-OH, were not significantly affected by Mg2+. Consistent with the above results, analogs with 3’ modifications were better inhibitors with 6 mM vs. 0.5 mM Mg2+, in primer extension reactions on a long template. Equilibrium dissociation constant (Kd) and kpol determinations for dTTP and analogs lacking a 3’-OH indicated that low Mg2+ caused a several-fold greater reduction in kpol with the analogs but had little effect on Kd. Overall, results emphasize the importance of as yet undefined interactions between Mg2+ and the 3’-OH and indicate that inhibitors with 3’-OH groups may have an advantage in a physiological setting where the concentration of free Mg2+ is low.
Chorismate synthase. Pre-steady-state kinetics of phosphate release from 5-enolpyruvylshikimate 3-phosphate
