Mg2+ activation of Escherichia coli inorganic pyrophosphatase

Further refinement of X-ray data on Escherichia coli inorganic pyrophosphatase [Oganessyan et al, (1994) FEES Lett, 348, 301-304] to 2.2 Angstrom reveals a system of noncovalent interactions involving Tyr(55) and Tyr(141) in the active site, The pK(a) for one of the eight Tyr residues in wild-type pyrophosphatase is as low as 9.1 and further decreases to 8.1 upon Mg2+ binding, generating characteristic changes in the absorption spectrum, These effects are lost in a Y55F but not in a Y141F variant, It is suggested that the lower-affinity site for Mg2+ in the enzyme is formed by Tyr(55) and Asp(70), which are in close proximity in the ape-enzyme structure.