Competition plots for ATP and a second nucleotide.

<p>Initial rates of phosphate release (<i>vi</i>) were measured in reaction mixtures containing two nucleotides at a time. The value of abscissa <i>x</i> is the proportion of [ATP] relative to the reference concentration [ATP]0 (see text). All experiments were performed using 51 nM NS3f and 100 mM KCl while other components were as indicated in Materials and Methods. <i>k_cat</i> and <i>K_M</i> values obtained from substrate curves in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0058508#pone-0058508-g001" target="_blank">Figure 1</a> were used to simulate the prediction of the single-active-site model (continuous line) and the two-independent-active-sites model (dashed line) according to the following equations: and . The former model shows the better correspondence with these results.