A voltammetric study of interdomain electron transfer within sulfite oxidase

Protein film voltammetry of chicken liver sulfite oxidase (SO) bound at the pyrolytic graphite "edge" or modified gold electrodes shows that catalytic electron transport is controlled by the inherent electrochemical characteristics of the heme b domain and conformational changes that allow intramolecular electron transfer with the molybdenum active site. In the absence of sulfite, a single nonturnover electrochemical signal is observed at +90 mV (vs SHE) that is assigned to heme b. In the presence of sulfite, this signal transforms into a catalytic wave at similar potential. The shape and negligible pH dependence of this wave indicate that catalytic turnover is controlled by the one-electron transfers through heme b. The smaller turnover numbers obtained in this experiment (k(cat) approximately 2-4 s(-1), as compared to 100 s(-1) in solution) suggest that only a small fraction of SO is bound at the electrode in a manner that permits the conformational change necessary for fast interdomain electron transfer.