CRYSTAL-STRUCTURE OF HOLE INORGANIC PYROPHOSPHATASE FROM ESCHERICHIA-COLI AT 1.9 ANGSTROM RESOLUTION - MECHANISM OF HYDROLYSIS

Crystalline hole inorganic pyrophosphatase from Escherichia coli was g rown in the presence of 250 mM MgCl2. The crystal structure has been s olved by Patterson search techniques and refined to an R-factor of 17. 6% at 1.9 Angstrom resolution. The upper estimate of the root-mean-squ are error in atomic positions is 0.26 Angstrom. These crystals belong to space group P3(2)21 with unit cell dimensions a = b = 110.27 Angstr om and c = 78.17 Angstrom. The asymmetric unit contains a trimer of su bunits, i.e., half of the hexameric molecule. In the central cavity of the enzyme molecule, three Mg2+ ions, each shared by two subunits of the hexamer, are found. In the active sites of two crystallographicall y independent subunits, two Mg2+ ions are bound, The second active sit e Mg2+ ion is missing in the third subunit. A mechanism of catalysis i s proposed whereby a water molecule activated by a Mg2+ ion and Tyr 55 play essential roles.