Phosphodiesterase-phosphomonoesterases from <i>Fusarium Moniliforme</i>:IV. Kinetic Studies

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A new mechanism for the action of Fusarium phosphodiesterase-phosphomonoesterase was proposed, in which an "Ordered Uni Ter" mechanism for the hydrolysis of phosphodiester and an "Ordered Uni Bi" reaction for that of phosphomonoester are combined. Rate equations were derived from this mechanism. Comparision of the equations with the previously reported results showed that inorganic phosphate should be the last product released by the enzyme. The hydrolysis of bis-p-nitrophenyl phosphate was analyzed by means of these equations and some kinetic constants were determined. p-Nitrophenol was a linear noncompetitive inhibitor with both bis-p-nitrophenyl phosphate and p-nitrophenyl phosphate as substrates. The rate of liberation of inorganic phosphate from mixtures of the two phosphate esters agreed well with the value calculated from the equations. These results are consistent with the proposed mechanism.<br> Quantitative analyses of the effects of adenosine, guanosine, cytidine, and uridine on the hydrolysis of p-nitrophenyl phosphate revealed that these ribonucleosides are linear noncompetitive inhibitors. The orders of magnitude of their slope and intercept inhibition constants are in the sequence U>>A>C>G and U>>G>A>>C, respectively.

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