Binding of substrate at the effector site of pyrophosphatase increases the rate of its hydrolysis at the active site

It is shown that in addition to the active site, each subunit of Escherichia coli inorganic pyrophosphatase (E-PPase) contains an extra binding site for the substrate magnesium pyrophosphate or its non-hydrolyzable analog magnesium methylenediphosphonate. The occupancy of the extra site stimulates the substrate conversion. Binding affinity of this site decreased or disappeared upon the conversion of E-PPase into a trimeric form or introduction of point mutations. However, when the slowly hydrolyzed substrate, lanthanum pyrophosphate (LaPPi), is used, the extra site was revealed in all enzyme forms of E-PPase and of Y-PPase (Saccharomyces cerevisiae PPase), resulting in about 100-fold activation of hydrolysis. A hypothesis on the localization of the extra site and the mechanism of its effect in E-PPase is presented.