期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2011
卷号：108
期号：24
页码：9815-9820
DOI：10.1073/pnas.1013274108
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Human ribonucleotide reductases (hRNRs) catalyze the conversion of nucleotides to deoxynucleotides and are composed of - and {beta}-subunits that form active n{beta}m (n, m = 2 or 6) complexes. binds NDP substrates (CDP, UDP, ADP, and GDP, C site) as well as ATP and dNTPs (dATP, dGTP, TTP) allosteric effectors that control enzyme activity (A site) and substrate specificity (S site). Clofarabine (ClF), an adenosine analog, is used in the treatment of refractory leukemias. Its mode of cytotoxicity is thought to be associated in part with the triphosphate functioning as an allosteric inhibitor of hRNR. Studies on the mechanism of inhibition of hRNR by ClF di- and triphosphates (ClFDP and ClFTP) are presented. ClFTP is a reversible inhibitor (Ki = 40 nM) that rapidly inactivates hRNR. However, with time, 50% of the activity is recovered. D57N-, a mutant with an altered A site, prevents inhibition by ClFTP, suggesting its A site binding. ClFDP is a slow-binding, reversible inhibitor ([IMG]/medium/pnas.1013274108eq1.gif" ALT="Formula ">; t1/2 = 23 min). CDP protects from its inhibition. The altered off-rate of ClFDP from E[bullet]ClFDP* by ClFTP (A site) or dGTP (S site) and its inhibition of D57N- together implicate its C site binding. Size exclusion chromatography of hRNR or alone with ClFDP or ClFTP, {+/-} ATP or dGTP, reveals in each case that forms a kinetically stable hexameric state. This is the first example of hexamerization of induced by an NDP analog that reversibly binds at the active site.
