Electroactivity of avidin and streptavidin. Avidin signals at mercury and carbon electrodes respond to biotin binding
Avidin and streptavidin were studied by phase-sensitive AC and cyclic voltammetry as well as by constant current chronopotentiometry at mercury (in alkaline media) and carbon electrodes (in acid medium). In contrast to the generally accepted belief that these proteins are electroinactive, we observed various electrochemical responses at these electrodes. Both proteins produced peaks due to oxidation of tyrosine and tryptophan residues at carbon electrodes and a catalytic peak H at a hanging mercury drop electrode. At the latter electrode avidin produced phase- in AC voltammetric and cyclic voltammetric peaks close to - 0.6 V (peak S) which were assigned to Hg-S interactions, involving cystine/cysteine residues. In cobalt containing solution avidin produced a characteristic catalytic double wave requiring presence of cystine/cysteine residues in the protein molecule. Streptavidin, which does not contain these residues, yielded neither the catalytic double wave nor peak S. All the above avidin signals responded to biotin binding; peak S increased (up to 4 biotin molecules bound) while other avidin signals decreased as a result of biotin binding. A tentative scheme of interfacial behavior of avidin and avidin-biotin complex, depending on the electrode charge, was suggested.