Inhibition of family II pyrophosphatases by analogs of pyrophosphate and phosphate.

Imidodiphosphate (the pyrophosphate analog containing a nitrogen atom in the bridge position instead of oxygen) is a potent inhibitor of family II pyrophosphatases from Streptococcus mutans and Streptococcus gordonii (inhibition constant Ki approximately 10 microM), which is slowly hydrolyzed by these enzymes with a catalytic constant of approximately 1 min(-1). Diphosphonates with different substituents at the bridge carbon atom are much less effective (Ki = 1-6 mM). The value of Ki for sulfate (a phosphate analog) is only 12 mM. The inhibitory effect of the pyrophosphate analogs exhibits only a weak dependence on the nature of the metal ion (Mn, Mg, or Co) bound in the active site.

Determination of Mn(II) and Co(II) with Arsenazo III.

Complex formation between Arsenazo III and Mn2+ and Co2+ at equilibrium has been investigated at pH 7.2, and the stoichiometry and stability of the complexes have been determined. The data indicate that Arsenazo III is suitable for determination of Mn2+ and Co2+ on the micromolar scale. The dissociation constants of the phosphate complexes of Mn2+ and Co2+ at pH 7.2 were estimated with Arsenazo III as 3.6 and 10 mM, respectively.

The electrophilic and leaving group phosphates in the catalytic mechanism of yeast pyrophosphatase.

Binding of pyrophosphate or two phosphate molecules to the pyrophosphatase (PPase) active site occurs at two subsites, P1 and P2. Mutations at P2 subsite residues (Y93F and K56R) caused a much greater decrease in phosphate binding affinity of yeast PPase in the presence of Mn(2+) or Co(2+) than mutations at P1 subsite residues (R78K and K193R). Phosphate binding was estimated in these experiments from the inhibition of ATP hydrolysis at a sub-K(m) concentration of ATP. Tight phosphate binding required four Mn(2+) ions/active site. These data identify P2 as the high affinity subsite and P1 as the low affinity subsite, the difference in the affinities being at least 250-fold. The time course of five "isotopomers" of phosphate that have from zero to four (18)O during [(18)O]P(i)-[(16)O]H(2)O oxygen exchange indicated that the phosphate containing added water is released after the leaving group phosphate during pyrophosphate hydrolysis. These findings provide support for the structure-based mechanism in which pyrophosphate hydrolysis involves water attack on the phosphorus atom located at the P2 subsite of PPase.

Fluoride effects along the reaction pathway of pyrophosphatase: evidence for a second enzyme.pyrophosphate intermediate.

The fluoride ion is a potent and specific inhibitor of cytoplasmic pyrophosphatase (PPase). Fluoride action on yeast PPase during PP(i) hydrolysis involves rapid and slow phases, the latter being only slowly reversible [Smirnova, I. N., and Baykov, A. A. (1983) Biokhimiya 48, 1643-1653]. A similar behavior is observed during yeast PPase catalyzed PP(i) synthesis. The amount of enzyme.PP(i) complex formed from solution P(i) exhibits a rapid drop upon addition of fluoride, followed, at pH 7.2, by a slow increase to nearly 100% of the total enzyme. The slow reaction results in enzyme inactivation, which is not immediately reversed by dilution. These data show that fluoride binds to an enzyme.PP(i) intermediate during the slow phase and to an enzyme.P(i) intermediate during the rapid phase of the inhibition. In Escherichia coli PPase, the enzyme.PP(i) intermediate binds F(-) rapidly, explaining the lack of time dependence in the inhibition of this enzyme. The enzyme.PP(i) intermediate formed during PP(i) hydrolysis binds fluoride much faster (yeast PPase) or tighter (E. coli PPase) than the similar complex existing at equilibrium with P(i). It is concluded that PPase catalysis involves two enzyme.PP(i) intermediates, of which only one (immediately following PP(i) addition and predominating at acidic pH) can bind fluoride. Simulation experiments have indicated that interconversion of the enzyme.PP(i) intermediates is a partially rate-limiting step in the direction of hydrolysis and an exclusively rate-limiting step in the direction of synthesis.