Direct catalytic electrochemistry of sulfite dehydrogenase: Mechanistic insights and contrasts with related Mo enzymes

Direct electrochemistry of human sulfite oxidase (HSO) has been achieved on carboxylate-terminated self-assembled monolayers cast on a Au working electrode in the presence of the promoter chitosan. The modified electrode facilitates a well-defined nonturnover redox response from the heme cofactor (Fe(III/II)) in 750 mM Tris, MOPS, and bicine buffer solutions. The formal redox potential of the nonturnover response varies slightly depending on the nature of the thiol monolayer on the Au electrode. Upon addition of sulfite to the cell a pronounced catalytic current from HSO-facilitated sulfite oxidation is observed. The measured catalytic rate constant (kcat) is around 0.2 s(-1) (compared with 26 s(-1) obtained from solution assays), which indicates that interaction of the enzyme with the electrode lowers overall catalysis although native behavior is retained in terms of substrate concentration dependence, pH dependence, and inhibition effects. In contrast, no catalytic activity is observed when HSO is confined to amine-terminated thiol monolayers although well-defined noncatalytic responses from the heme cofactor are still observed. These differences are linked to flexibility of HSO, which can switch between active and inactive conformations, and also competitive ion exchange processes at the electrode surface involving the enzyme and substrate