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Enzymatic Synthesis of Deoxyribonucleotides

IX. ALLOSTERIC EFFECTS IN THE REDUCTION OF PYRIMIDINE RIBONUCLEOTIDES BY THE RIBONUCLEOSIDE DIPHOSPHATE REDUCTASE SYSTEM OF ESCHERICHIA COLI

From the ¹ From the Department of Chemistry II, Karolinska Institutet, Stockholm, Sweden

In Escherichia coli B the enzymatic reductions of cytidine diphosphate and uridine diphosphate require four different protein fractions (thioredoxin, thioredoxin reductase, Enzyme B1, and Enzyme B2), reduced triphosphopyridine nucleotide, and magnesium ions. Both reactions are stimulated by different nucleoside triphosphates, in particular adenosine triphosphate and deoxythymidine triphosphate. dTTP (apparent K_m value in CDP reduction, 1.5 x 10^-6 m) is effective at much lower concentrations than ATP (apparent K_m value in CDP reduction, 0.7 x 10^-4 m), but the same maximal velocity is attained with both nucleotides. The simultaneous addition of ATP and dTTP results in a rather complex type of inhibition.

dATP strongly inhibits both reactions. The inhibition is reversed by ATP but not by dTTP. The different nucleoside triphosphates are considered to act as allosteric effectors in the reduction of pyrimidine ribonucleoside diphosphates.

CDP and UDP inhibit the reduction of each other in a competitive manner, an indication that the same enzyme is involved in both reactions.

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