[Detection and characterization of an additional site for binding of substrate and its analogs by inorganic...

Phosphate, pyrophosphate, imidodiphosphate, EDTA and tripolyphosphate increase the rate constant for dissociation of the inorganic pyrophosphatase-substrate intermediate formed after cessation of the reaction by fluoride. The effect is enhanced in the given order 19-fold, the dependence of this effect on ligand concentration being hyperbolic. The values of the dissociation constants of the enzyme-ligand complexes lie within the concentration range of 0.16-1.0 mM. At high concentrations of Na2+ added simultaneously with the ligands this effect is decreased. The value of tau 1/2 for Pi binding to the enzyme-substrate compound is 0.15 min. The data obtained suggest that pyrophosphatase contains an anion ligand binding site, differing from that of the active one. This site does not affect the hydrolytic function of pyrophosphatase, as can be evidenced from the fact that Pi (9.5 mM) does not change the rate of enzymatic cleavage of PPi.