Asp-42 located in the active site of E. coli inorganic pyrophosphatase (PPase) has been substituted by Asn by site-directed mutagenesis. This resulted in a 3-fold increase in hydrolytic activity measured under optimal conditions, a 15.5-fold increase in the Km value and retention of the pK values of groups for enzyme and enzyme-substrate complex. The active site of the enzyme contains 4 metal binding centers (I-IV) [Harutyunyan et al. (1996) Eur. J. Biochem., in press]. Asp-42 is located near centers II and IV. The D42N replacement had no effect on Mg2+ binding with center II. At the same time, occupation of center IV eliminates the inhibition of inorganic pyrophosphate hydrolysis by high Mg2+ concentrations typical of wild-type PPase. It is proposed that the increase in activity and decrease in affinity for substrate of the D42N PPase results from changes in Mg2+ binding to center IV. The Mg2+ binding centers of E. coli PPase are lined up in filling order.