Electrical communication between glucose oxidase and electrodes mediated by phenothiazine-labeled poly(ethylene oxide) bonded to lysine residues on the enzyme surface

A series of glucose oxidase (GOx) hybrids (GOx-phenothiazine-labeled poly(ethylene oxide) (PT-PEO)) capable of direct electrical communication with electrodes is synthesized by covalently modifying PT-PEO to lysine residues on the enzyme surface. The length of the PEO chain and the number of PT groups are systematically altered. After the PT-PEO modification, all the hybrids maintain more than 50% of enzyme activity relative to that of native GOx, although loss of the activity becomes greater with increasing PEO chain length. The catalytic current, i_cat, is observed at a potential more positive than 0.55 V after the addition of glucose, due to the intramolecular electron transfer (ET) from reduced forms of flavin adenine dinucletide (FADH₂/FADH) to PT⁺ that are electrogenerated at the electrode. The i_cat value increases with the number of PT groups, indicating that most of the modified PT groups act as mediators. The magnitude of the i_cat increase depends on the PEO chain length and reveals a maximum for PT-PEO with the molecular weight of 3000. In contrast, the i_cat is almost constant for GOx-2-(10-phenothiazyl)propionic acid (PT-PA) hybrids with more than two PT groups synthesized by covalently modifying PT-PA to surface lysines, indicating that only a few key PT groups function as mediators. The maximum rate constant (130 s^-1) for the ET from FADH₂/FADH to PT⁺ is obtained for the GOx hybrid modified with five PT-PEO groups with the molecular weight of 3000.