Electrochemistry and electrocatalytic activities of superoxide dismutases at gold electrodes modified with a self-assembled monolayer

In this article, the electrochemical properties and electrocatalytic activity of three kinds of superoxide dismutases (SODs), that is, bovine erythrocyte copper-zinc superoxide dismutase (Cu/Zn-SOD), iron superoxide dismutase from Escherichia coli (Fe-SOD), and manganese superoxide dismutase from E. coli (Mn-SOD), in the SOD family were studied. It was revealed that the direct electron transfer of the three kinds of SODs could be efficiently promoted by a self-assembled monolayer (SAM) of 3-mercaptopropionic acid (MPA) confined on a gold electrode. The electrochemical properties of the SODs at the MPA-SAM electrode vary with the sort of SOD with respect to the formal potential, reversibility of electrode reactions, kinetic parameters, and pH dependence, suggesting different mechanisms for the electrode reactions of the individual SODs. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SODs via a redox cycle of their active metals substantially offered a flexible electrochemical route to determination of O-2(.-) where O-2(.-) can be sensed with the SOD-based biosensors in both anodic and cathodic polarizations. Such an intrinsic feature of the SOD-based biosensors successfully enabled a sensitive determination scheme for O-2(.-) free from the interference from some coexisting electroactive species, such as ascorbic acid (AA) and uric acid (UA). Further potential applications for in vivo determination of O-2(.-) is also suggested.