Binding of Divalent Magnesium by Escherichia coli Phosphoribosyl Diphosphate Synthetase
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- The mechanism of binding of the substrates MgATP and ribose 5-phosphate as well as Mg2+ to the enzyme 5-phospho-d-ribosyl a-1-diphosphate synthetase from Escherichia coli has been analyzed. By use of the competive inhibitors of ATP and ribose 5-phosphate binding, a,ß-methylene ATP and (+)-1-a,2-a,3-a-trihydroxy-4-ß-cyclopentanemethanol 5-phosphate, respectively, the binding of Mg2+ and the substrates were determined to occur via a steady state ordered mechanism in which Mg2+ binds to the enzyme first and ribose 5-phosphate binds last. Mg2+ binding to the enzyme prior to the binding of substrates and products indicated a role of Mg2+ in preparing the active site of phosphoribosyl diphosphate synthetase for binding of the highly phosphorylated ligands MgATP and phosphoribosyl diphosphate, as evaluated by analysis of the effects of the inhibitors adenosine and ribose 1,5-bisphosphate. Calcium ions, which inhibit the enzyme even in the presence of high concentrations of Mg2+, appeared to compete with free Mg2+ for binding to its activator site on the enzyme. Analysis of the inhibition of Mg2+ binding by MgADP indicated that MgADP binding to the allosteric site may occur in competition with enzyme bound Mg2+. Ligand binding studies showed that 1 mol of MgATP was bound per mol of phosphoribosyl diphosphate synthetase subunit, which indicated that the allosteric sites of the multimeric enzyme were not made up by inactive catalytic sites.
| Original language | English |
|---|---|
| Journal | Biochemistry |
| Volume | 36 |
| Issue number | 16 |
| Pages (from-to) | 5078-5083 |
| Number of pages | 8 |
| ISSN | 0006-2960 |
| DOIs | |
| Publication status | Published - 1997 |
ID: 1359444