guanosine-diphosphate and 2--deoxyadenosine-triphosphate

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.

Topics: Adenosine Triphosphate; Allosteric Regulation; Animals; Asparagine; Aspartic Acid; Cytidine Diphosphate; Deoxyadenine Nucleotides; Dimerization; Enzyme Activation; Guanosine Diphosphate; Kinetics; Light; Magnesium; Mice; Models, Chemical; Mutagenesis, Site-Directed; Protein Subunits; Recombinant Proteins; Ribonucleoside Diphosphate Reductase; Ribonucleotide Reductases; Scattering, Radiation; Substrate Specificity; Thymine Nucleotides; Tumor Cells, Cultured

Using HPLC methods, we measured the concentrations of nucleosides and nucleotides for a patient with no purine nucleoside phosphorylase (PNP; EC 2.4.2.1) enzymatic activity. Concentrations of inosine and guanosine were abnormally high in urine and plasma, whereas guanosine diphosphate (GDP) and guanosine triphosphate (GTP) concentrations in erythrocytes were depleted. The unusual presence of deoxyribonucleosides (deoxyinosine and deoxyguanosine) and deoxyribonucleotides (dGDP and dGTP) was also notable. Thus, HPLC represents an accurate and useful tool for the study of purine metabolic disorders.

Topics: Chromatography, High Pressure Liquid; Deoxyadenine Nucleotides; Deoxyguanine Nucleotides; Erythrocytes; Guanosine; Guanosine Diphosphate; Guanosine Triphosphate; Humans; Infant; Inosine; Male; Purine-Nucleoside Phosphorylase; Purine-Pyrimidine Metabolism, Inborn Errors

Topics: Deoxyadenine Nucleotides; Deoxyguanine Nucleotides; Escherichia coli; Guanosine Diphosphate; Macromolecular Substances; Photochemistry; Ribonucleotide Reductases; Thymine Nucleotides

The participation of Mg complex of nucleoside diphosphates and nucleoside triphosphates in the reverse and forward reactions catalyzed by purified carbamyl phosphokinase (ATP : carbamate phosphotransferase, EC 2.7.2.2) of Streptococcus faecalis R, ATCC-8043 were studied. The results of initial velocity studies of approx. 1 mM free Mg2+ concentration have indicated that in the reverse reaction MgdADP was as effective a substrate as MgADP. The phosphoryl group transfer from carbamyl phosphate to MgGDP, MgCDP and MgUDP was also observed at relatively higher concentrations of the enzyme and respective magnesium nucleoside diphosphate. In the forward direction MgdATP was found to be as efficient a phosphate donor as MgATP. On the other hand, Mg complexes of GTP, CTP and UTP were ineffective even at higher concentrations of the enzyme and respective magnesium nucleoside triphosphate. Product inhibition studies carried out at non-inhibitory level of approx. 1 mM free Mg2+ concentration have revealed that the enzyme has two distinct sites, one for nucleoside diphosphate or nucleoside triphosphate and the other for carbamyl phosphate or carbamate, and its reaction with the substrates is of the random type. Further tests of numerical values for kinetic constants have indicated that they are partially consistent with the Haldane relationship which is characteristic of rapid equilibrium and random mechanism.

Topics: Adenosine Diphosphate; Adenosine Triphosphate; Cytidine Diphosphate; Deoxyadenine Nucleotides; Enterococcus faecalis; Guanosine Diphosphate; Magnesium; Nucleotides; Phosphotransferases; Phosphotransferases (Carboxyl Group Acceptor); Substrate Specificity; Uridine Diphosphate