Probing the stability of the disulfide radical intermediate of thioredoxin using direct electrochemistry

Thioredoxin, a redox active disulfide protein, has been specifically immobilized at a modified gold electrode. The thioredoxin is uniquely oriented relative to the electrode surface via a histidine tag thereby enabling the redox mechanism of protein to be examined. When scanning the applied potential in the negative direction (cathodic), two one-electron reduction waves can be observed. The first of these redox waves occurs at -90 mV and is electrochemically reversible at all scan rates whereas the second wave occurs at -433 mV is irreversible. These two processes are interpreted as the initial reduction of the disulfide form of the protein to a stable (reversible) semi-reduced radical anion intermediate, followed by an electrochemically irreversible process to form a fully reduced thioredoxin. These electron transfer characteristics suggest that a radical intermediate retaining the sulfur-sulfur bond is thermodynamically stable but the addition of a second electron results in bond scission.