Comparison of catalytic electrochemistry of glucose oxidase between covalently modified and freely diffusing phenothiazine-labeled poly(ethylene oxide) mediator systems

The enzyme catalytic reaction is electrochemically compared under a substrate-saturated and diffusion-limited condition between native glucose oxidase (GOx) mixed with phenothiazine-labeled poly(ethylene oxide) (PT-PEO) and GOx-(PT-PEO) hybrid with PT-PEO covalently bonded to lysine residues on the enzyme surface. Although the catalytic current (i_cat) increases with the ratio of [PT-PEO]/[GOx] in both systems, the dependence of the i_cat on the molecular weight of PT-PEO is totally different. In the mixed systems, the i_cat decreases with increasing the molecular weight of PT-PEO, which reflects the decreases in the diffusion coefficient (D) of PT-PEO and the intermolecular electron transfer (ET) rate from FADH₂/FADH to PT⁺-PEO. In the hybrids, on the other hand, the i_cat reveals a maximum at the molecular weight of 3000 (PT-PEO3000), which originates from the dependence of the intramolecular ET rate from FADH₂/FADH to PT⁺ on the molecular weight of PT-PEO. The greater ET rate is enough to make up for the smaller D of PT groups attached on GOx in the hybrid with PT-PEO3000, yielding to the slightly larger i_cat than that of the corresponding mixed system. The active local motion of the long and hydrophilic PEO chain, the higher local concentration of PT groups due to their immobilization on the GOx surface, and the simultaneous oxidation of multiple attached PT groups at the electrode are possible reasons for the large ET rate constant of the hybrid systems.