Covalent Attachment of Ferrocene to Soybean Peroxidase Glycans: Electron Transfer Mediation to Redox Enzymes

Abstract

The electrochemical properties of native soybean peroxidase (SBP) and of SBP modified with covalently attached ferrocene electron-transfer mediators within microcavities etched at the tip of 25 mum radius platinum microelectrodes are reported. The microcavities incorporate approximately 50 fmol of SBP. In the microcavity, native SBP undergoes a relatively fast reduction but a very slow oxidation. Ferrocene mediators have been attached to the SBP glycans (Fc-SBP) (approximately 1.5 ferrocene mediators per SBP molecule). Cyclic voltammetry reveals that these centers are capable of mediating the reduction of oxidized SBP and increase the rate of heterogeneous electron transfer between the enzyme and the electrode by >10-fold. Micromolar concentrations of H2O2 chemically oxidize the SBP and Fc-SBP systems leading to an electrocatalytic reduction at approximately -0.1 V. Successive additions of 2.5 micromol of H2O2 at a constant applied potential of -0.1 V gave a steady-state constant current of approximately 60 nA within 20 s. The steady-state current increased linearly with peroxide concentration for 2.5 <or= [H2O2] <or= 42 microM. Ferrocene-modified SBP shows an approximately 3-fold increase in the sensitivity of the steady-state current response to successive additions of hydrogen peroxide compared to the native enzyme. This observation indicates increased turnover of the redox enzyme per unit time in the presence of the covalently attached ferrocene mediators.