Cation Coordination Alters the Conformation of a Thrombin-Binding G-Quadruplex DNA Aptamer That Affects Inhibition of Thrombin

Thrombin-binding aptamers are promising anticoagulants. HD1 is a monomolecular antiparallel G-quadruplex with two G-quartets linked by three loops. Aptamer-thrombin interactions are mediated with two TT-loops that bind thrombin exosite I. Several cations were shown to be coordinated inside the G-quadruplex, including K+, Na+, NH4+, Ba2+, and Sr2+; on the contrary, Mn2+ was coordinated in the grooves, outside the G-quadruplex. K+ or Na+ coordination provides aptamer functional activity. The effect of other cations on aptamer functional activity has not yet been described, because of a lack of relevant tests. Interactions between aptamer HD1 and a series of cations were studied. A previously developed enzymatic method was applied to evaluate aptamer inhibitory activity. The structure-function correlation was studied using the characterization of G-quadruplex conformation by circular dichroism spectroscopy. K+ coordination provided the well-known high inhibitory activity of the aptamer, whereas Na+ coordination supported low activity. Although NH4+ coordination yielded a typical antiparallel G-quadruplex, no inhibitory activity was shown; a similar effect was observed for Ba2+ and Sr2+ coordination. Mn2+ coordination destabilized the G-quadruplex that drastically diminished aptamer inhibitory activity. Therefore, G-quadruplex existence per se is insufficient for aptamer inhibitory activity. To elicit the nature of these effects, we thoroughly analyzed nuclear magnetic resonance (NMR) and X-ray data on the structure of the HD1 G-quadruplex with various cations. The most reasonable explanation is that cation coordination changes the conformation of TT-loops, affecting thrombin binding and inhibition. HD1 counterparts, aptamers 31-TBA and NU172, behaved similarly with some distinctions. In 31-TBA, an additional duplex module stabilized antiparallel G-quadruplex conformation at high concentrations of divalent cations; whereas in NU172, a different sequence of loops in the G-quadruplex module provided an equilibrium of antiparallel and parallel G-quadruplexes that shifted with cation binding. In conclusion, structures of G-quadruplex aptamers are flexible enough and are fine-tuned with different cation coordination.