GOx hybrids [GOx-(PT-PEO-NH2)] are prepared by covalently bonding phenothiazine (PT)-labeled poly(ethylene oxide) (PEO) oligomers having an amino end group, PT-PEO-NH2, to acidic amino acid residues on the enzyme surface, The rate constant for the mediated FADH/FADH(2) oxidation calculated from the catalytic current under substrate-saturated conditions ranges from 1.7 to 388 s(-1), and the largest value is obtained for GOx hybrids with PT-PEO of molecular weight 3000. Surprisingly effective electron transfer from FADH/FADH(2) to PT+ is achieved in the GOx-(PT-PEO-NH2) hybrids due to the PT modification to aspartic or glutamic acid residues, many of which are located close to the FAD center.