Editing Nucleoside triphosphate (section)

=== Ribonucleotide reductase ===
[[Ribonucleotide reductase]] (RNR) is the enzyme responsible for converting NTPs to dNTPs. Given that dNTPs are used in DNA replication, the activity of RNR is tightly regulated.<ref name="Wyngaarden_1976"/> RNR can only process NDPs, so NTPs are first dephosphorylated to NDPs before conversion to dNDPs.<ref name="auto">{{cite journal | vauthors = Kolberg M, Strand KR, Graff P, Andersson KK | title = Structure, function, and mechanism of ribonucleotide reductases | journal = Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics | volume = 1699 | issue = 1–2 | pages = 1–34 | date = June 2004 | pmid = 15158709 | doi = 10.1016/j.bbapap.2004.02.007 }}</ref> dNDPs are then typically re-phosphorylated. RNR has 2 subunits and 3 sites: the catalytic site, activity (A) site, and specificity (S) site.<ref name="auto"/> The catalytic site is where the NDP to dNDP reaction takes place, the activity site determines whether or not the enzyme is active, and the specificity site determines which reaction takes place in the catalytic site.

The activity site can bind either ATP or dATP.<ref name=":7">{{cite book |doi=10.1016/B978-0-12-386931-9.00014-3 |chapter=The Structural Basis for the Allosteric Regulation of Ribonucleotide Reductase |title=Oligomerization in Health and Disease |series=Progress in Molecular Biology and Translational Science |year=2013 | vauthors = Ahmad MF, Dealwis CG |volume=117 |pages=389–410 |pmid=23663976 |pmc=4059395 |isbn=9780123869319 }}</ref> When bound to ATP, RNR is active. When ATP or dATP is bound to the S site, RNR will catalyze synthesis of dCDP and dUDP from CDP and UDP. dCDP and dUDP can go on to indirectly make dTTP. dTTP bound to the S site will catalyze synthesis of dGDP from GDP, and binding of dGDP to the S site will promote synthesis of dADP from ADP.<ref>{{cite journal | vauthors = Fairman JW, Wijerathna SR, Ahmad MF, Xu H, Nakano R, Jha S, Prendergast J, Welin RM, Flodin S, Roos A, Nordlund P, Li Z, Walz T, Dealwis CG | title = Structural basis for allosteric regulation of human ribonucleotide reductase by nucleotide-induced oligomerization | journal = Nature Structural & Molecular Biology | volume = 18 | issue = 3 | pages = 316–22 | date = March 2011 | pmid = 21336276 | doi = 10.1038/nsmb.2007 | pmc=3101628}}</ref> dADP is then phosphorylated to give dATP, which can bind to the A site and turn RNR off.<ref name=":7" />