Determining nucleotide analog specificity for SAMHD1.

<p>A) Modification of the 2' sugar position of a nucleotide can lead to several different outcomes. First, (2'<i>R</i>)-2'-F and (2'<i>R</i>)-2'-OH sugar moieties have been shown not to be substrates for SAMHD1. Additional analogs with (2'<i>R</i>)-2'-F and (2'<i>R</i>)-2'-OH sugar moieties would be predicted not to be substrates for SAMHD1. Second, canonical dNTPs and the non-canonical dUTP are substrates for SAMHD1. Our data shows that ((2'<i>S</i>)-2'-OH) arabinose nucleoside-5'-triphosphates are also substrates for SAMHD1. Therefore, we also predict several other arabinose nucleoside analogs would be substrates for SAMHD1. Moreover, clofarabine-TP ((2'<i>S</i>)-2'-F) was reported hydrolyzed by SAMHD1 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169052#pone.0169052.ref043" target="_blank">43</a>]. Finally, we found the SMDU-TP, (2'<i>R</i>)-2'-methyl sugar moiety, inhibited the triphosphohydrolase activity of SAMHD1. We postulate that the (2'<i>R</i>)-2'-methyl moiety may prevent the conformational change in the catalytic site of SAMHD1 due to the size of the methyl group clashing with Y374. Therefore, we predicted that nucleotides with a (2'<i>S</i>)-2'-cyano moiety may also inhibit dNTPase activity of SAMHD1. B) A SAMHD1 inhibitor has been reported [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169052#pone.0169052.ref034" target="_blank">34</a>]. The pppCH<sub>2</sub>-dU analog has a 5'-methylene modification, making the analog non-hydrolysable in the catalytic site, but also was shown to block homotetramerization when present in the A2 site [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0169052#pone.0169052.ref034" target="_blank">34</a>]. C) Modification of the 3'-OH sugar moiety is not permissive. NRTIs and ddNTPs lack a 3'-OH moiety, making them chain terminators for DNA polymerases, are not substrates for SAMHD1. D) Base modifications for different nucleoside analogs are permissive substrates for SAMHD1.</p>