Ara-CTP does not fit into the A2 site of SAMHD1.

A) Evaluating ara-CTP hydrolysis in the presence of dGTP, using as A1site activator. When dGTP was present, ara-CTP and dCTP were significantly hydrolyzed (p < 0.001) by SAMHD1. Data are presented as the percent compound remaining (y-axis). B) Determining if ara-CTP is hydrolysis by SAMHD1 in the presence of GTP. GTP will only fit into the A1 site of SAMHD1, thus requiring ara-CTP to occupy the A2 and catalytic sites for ara-CTP hydrolysis to occur. The percentage of ara-CTP remained constant with and without SAMHD1, indicating that ara-CTP cannot occupy the A2 site. Reactions containing dCTP was conducted and led to hydrolysis of dCTP in the presence of SAMHD1. Mean and SEM are plotted with significant or no significant (n.s.) differences determined using T test analysis.