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 (k_cat) 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.

中文翻译：

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壳聚糖促进人亚硫酸氧化酶的直接电化学

在促进剂壳聚糖存在的情况下，在流延于Au工作电极上的羧酸封端的自组装单层膜上已实现了人类亚硫酸盐氧化酶（HSO）的直接电化学。修饰的电极有助于在750 mM Tris，MOPS和Bicine缓冲溶液中血红素辅助因子（Fe ^{III / II}）产生明确的非转换氧化还原响应。非翻转响应的形式氧化还原电势根据Au电极上硫醇单层的性质而略有不同。将亚硫酸盐添加到电池中后，观察到明显的HSO促进的亚硫酸盐氧化催化电流。测得的催化速率常数（k _cat）约为0.2 s ^–1（相比之下，26 s ^–1（通过溶液测定获得），这表明酶与电极的相互作用降低了整体催化作用，尽管在底物浓度依赖性，pH依赖性和抑制作用方面保留了天然行为。相反，尽管仍观察到来自血红素辅因子的明确的非催化反应，但当HSO限于胺封端的硫醇单分子层时，未观察到催化活性。这些差异与HSO的灵活性有关，后者可以在活性和非活性构象之间切换，还可以在涉及酶和底物的电极表面进行竞争性离子交换过程。