Abstract
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 (LaPP(i)), 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.
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