Inhibition by IMP.

IC50 of IMP was determined by fitting for each substrate the normalized velocity to the dose-response model provided in GraphPad Prism version 5.01 for Windows, GraphPad Software, San Diego California USA, www.graphpad.com. As an example, a graph showing the influence of IMP on the activity of NUDT16 towards GDP is shown in S5 Fig. Ki was determined using the formula which links IC50 to Ki in the case of a competitive inhibitor, Ki = IC50/(1+[S]/Km) using either Km values reported by Iyama et al. [14] (when the substrates used were IDP or GDP) or the ones we presented on Table 3 (for ITP, XDP and ADP).