THE STRUCTURE OF ESCHERICHIA-COLI SOLUBLE INORGANIC PYROPHOSPHATASE AT 2.7-ANGSTROM RESOLUTION

The structure of E. coli soluble inorganic pyrophosphatase has been re fined at 2.7 Angstrom resolution to an R-factor of 20.9%. The overall fold of the molecule is essentially the same as yeast pyrophosphatase, except that yeast pyrophosphatase is longer at both the N- and C-term ini. Escherichia coli pyrophosphatase is a mixed alpha+beta protein wi th a complicated topology. The active site cavity, which is also very similar to the yeast enzyme, is formed by seven beta-strands and an al pha-helix and has a rather asymmetric distribution of charged residues . Our structure-based alignment extends and improves upon earlier sequ ence alignment studies; it shows that probably no more than 14, not 15 -17 charged and polar residues are part of the conserved enzyme mechan ism of pyrophosphatases. Six of these conserved residues, at the botto m of the active site cavity, form a tight group centred on Asp70 and p robably bind the two essential Mg2+ ions. The others, more spread-out and more positively charged, presumably bind substrate. Escherichia co li pyrophosphatase has an extra aspartate residue in the active site c avity, which may explain why the two enzymes bind divalent cation diff erently. Based on the structure, we have identified a sequence moth th at seems to occur only in soluble inorganic pyrophosphatases.