Serine proteases of the chymotrypsin family show a dichotomous amino acid distribution for residue 225. Enzymes carrying Tyr at position 225 are activated by Na(+), whereas those carrying Pro are devoid of Na(+) binding and activation. Previous studies have demonstrated that the Y225P conversion is sufficient to abrogate Na(+) activation in several enzymes. However, the reverse substitution P225Y is necessary but not sufficient to introduce Na(+) binding and activation. Here we report that Streptomyces griseus trypsin, carrying Pro-225, can be engineered into a Na(+)-activated enzyme by replacing residues in the 170, 186, and 220 loops to those of coagulation factor Xa. The findings represent the first instance of an engineered Na(+)-activated enzyme and a proof of principle that should enable the design of other proteases with enhanced catalytic activity and allosteric regulation mediated by monovalent cation binding.