Voltammetry and Single‐molecule in Situ Scanning Tunnelling Microscopy of the Redox Metalloenzyme Human Sulfite Oxidase

Human sulfite oxidase (hSO) is a homodimeric two‐domain enzyme central in the biological sulfur cycle. A pyranopterin molybdenum cofactor (Moco) is the catalytic site and a heme b5 group located in the N‐terminal domain. The two domains are connected by a flexible linker region. Electrons produced at the Moco in sulfite oxidation, are relayed via heme b5 to electron acceptors or an electrode surface. Inter‐domain conformational changes between an open and a closed enzyme conformation, allowing “gated” electron transfer has been suggested. We first recorded cyclic voltammetry (CV) of hSO on single‐crystal Au(111)‐electrode surfaces modified by self‐assembled monolayers (SAMs) both of a short rigid thiol, cysteamine and of a longer structurally flexible thiol, w‐amino‐octanethiol (AOT). hSO on cysteamine SAMs displays a well‐defined pair of voltammetric peaks around ‐0.207 V vs. SCE in the absence of sulfite substrate, but no electrocatalysis. hSO on AOT SAMs displays well‐defined electrocatalysis, but only “fair” quality voltammetry in the absence of sulfite. We recorded next in situ scanning tunnelling spectroscopy (STS) of hSO on AOT modified Au(111)‐electrodes, disclosing, a 2‐5 % surface coverage of strong molecular scale contrasts, assigned to single hSO molecules, notably with no contrast difference in the absence and presence of sulfite. In situ STS corroborated this observation with a sigmoidal tunnelling current/overpotential correlation.