The Ala103 to Gly mutation, introduced within the glucose isomerase from Streptomyces sp. SK (SKGI) decreased its catalytic efficiency (k(cat)/K(m)) toward D-glucose from 7.1 to 3 mM(-1) min(-1). The reverse counterpart replacement Gly103Ala introduced into the glucose isomerase of Streptomyces olivochromogenes (SOGI) considerably improved its catalytic efficiency to be 6.7 instead of 3.2 mM(-1) min(-1). This later mutation also increased the half-life time of the enzyme from 70 to 95 min at 80 degrees C and mainly modified its pH profile. These results provide evidence that the residue Ala103 plays an essential role in the kinetic and physicochemical properties of glucose isomerases from Streptomyces species.