Site directed mutagenesis study was carried out with Escherichia coli pyrroloquinoline quinone glucose dehydrogenase (PQQGDH) by substitution of His775 with either Asn (H775N) or Asp (H775D). The mutated PQQGDHs had different substrate specificity and catalytic activity from the wild type PQQGDH. The K-m values of H775N for 2-deoxy-D-glucose and for D-allose increased for 10-fold. The K-m values for both D-mannose and D-galactose were estimated much higher than 100 mM. H775D also showed the increase in K-m values toward saccharides. As a result, these mutants possessed narrower substrate specificity than wild type E. coli PQQGDH. H775D showed the increase in K-m value for glucose versus wild type PQQGDH (25-fold), therefore H775D is suitable for the direct measurement of blood glucose. The role of His775 in E. coli. PQQGDH is also discussed.