文摘
Family II inorganic pyrophosphatases (PPases) constitute a new evolutionary group of PPases,with a different fold and mechanism than the common family I enzyme; they are related to the "DHH"family of phosphoesterases. Biochemical studies have shown that Mn2+ and Co2+ preferentially activatefamily II PPases; Mg2+ partially activates; and Zn2+ can either activate or inhibit (Zyryanov et al.,Biochemistry, 43, 14395-14402, accompanying paper in this issue). The three solved family II PPasestructures did not explain the differences between the PPase families nor the metal ion differences describedabove. We therefore solved three new family II PPase structures: Bacillus subtilis PPase (Bs-PPase)dimer core bound to Mn2+ at 1.3 Å resolution, and, at 2.05 Å resolution, metal-free Bs-PPase andStreptococcus gordonii (Sg-PPase) containing sulfate and Zn2+. Comparison of the new and old structuresof various family II PPases demonstrates why the family II enzyme prefers Mn2+ or Co2+, as an activatorrather than Mg2+. Both M1 and M2 undergo significant changes upon substrate binding, changing fromfive-coordinate to octahedral geometry. Mn2+ and Co2+, which readily adopt different coordination statesand geometries, are thus favored. Combining our structures with biochemical data, we identified M2 asthe high-affinity metal site. Zn2+ activates in the M1 site, where octahedral geometry is not essential forcatalysis, but inhibits in the M2 site, because it is unable to assume octahedral geometry but remainstrigonal bipyramidal. Finally, we propose that Lys205-Gln81-Gln80 form a hydrophilic channel to speedproduct release from the active site.