title: Comprehensive model for allosteric regulation of mammalian ribonucleotide reductase: refinements and consequences.
abstract: Reduction of NDPs by murine ribonucleotide reductase (mRR) requires catalytic (mR1) and free radical-containing (mR2) subunits and is regulated by nucleoside triphosphate allosteric effectors. Here we present the results of several studies that refine the recently presented comprehensive model for the allosteric control of mRR enzymatic activity [Kashlan, O. B., et al. (2002) Biochemistry 41, 462-474], in which nucleotide binding to the specificity site (s-site) drives formation of an active R1(2)R2(2) dimer, ATP or dATP binding to the adenine site (a-site) drives formation of a tetramer, mR1(4a), which isomerizes to an inactive form, mR1(4b), and ATP binding to the hexamerization site (h-site) drives formation of an active R1(6)R2(6) hexamer. Analysis of the a-site D57N variant of mR1, which differs from wild-type mR1 (wt-mR1) in that its RR activity is activated by both ATP and dATP, demonstrates that dATP activation of the D57N variant RR arises from a blockage in the formation of mR1(4b) from mR1(4a), and provides strong evidence that mR1(4a) forms active complexes with mR2(2). We further demonstrate that (a) differences in the effects of ATP versus dATP binding to the a-site of wt-mR1 provide specific mechanisms by which the dATP/ATP ratio in mammalian cells could modulate in vivo RR enzymatic activity, (b) the comprehensive model is valid over a range of Mg(2+) concentrations that include in vivo concentrations, and (c) equilibrium constants derived for the comprehensive model can be used to simulate the distribution of R1 among dimer, tetramer, and hexamer forms in vivo. Such simulations indicate that mR1(6) predominates over mR1(2) in the cytoplasm of normal mammalian cells, where the great majority of RR activity is located, but that mR1(2) may be important for nuclear RR activity and for RR activity in cells in which the level of ATP is depleted.
authors.affiliation: Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, USA.
authors.forename: Ossama B
authors.initials: OB
authors.lastname: Kashlan
authors.forename: Barry S
authors.initials: BS
authors.lastname: Cooperman
mesh_terms: D000255:Adenosine Triphosphate / Q000737:chemistry; D000494:Allosteric Regulation / Q000235:genetics; D000818:Animals; D001216:Asparagine / Q000235:genetics; D001224:Aspartic Acid / Q000235:genetics; D003565:Cytidine Diphosphate / Q000737:chemistry; D003838:Deoxyadenine Nucleotides / Q000737:chemistry; D019281:Dimerization; D004789:Enzyme Activation / Q000235:genetics; D006153:Guanosine Diphosphate / Q000737:chemistry; D007700:Kinetics; D008027:Light; D008274:Magnesium / Q000737:chemistry; D051379:Mice; D008956:Models, Chemical; D016297:Mutagenesis, Site-Directed; D021122:Protein Subunits / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000235:genetics; D012262:Ribonucleoside Diphosphate Reductase; D012264:Ribonucleotide Reductases / Q000037:antagonists & inhibitors / Q000737:chemistry* / Q000235:genetics; D012542:Scattering, Radiation; D013379:Substrate Specificity / Q000235:genetics; D013942:Thymine Nucleotides / Q000737:chemistry; D014407:Tumor Cells, Cultured
chemical_list: D003838:Deoxyadenine Nucleotides; D021122:Protein Subunits; D011994:Recombinant Proteins; D013942:Thymine Nucleotides; D006153:Guanosine Diphosphate; D001224:Aspartic Acid; D003565:Cytidine Diphosphate; D001216:Asparagine; D000255:Adenosine Triphosphate; D012264:Ribonucleotide Reductases; C108427:ribonucleotide reductase R2 subunit; D012262:Ribonucleoside Diphosphate Reductase; C498690:Rrm1 protein, mouse; D008274:Magnesium; C026600:2'-deoxyadenosine triphosphate; C024157:thymidine 5'-triphosphate