Enzymatic and physiological properties of the tungsten‐substituted molybdenum TMAO reductase from Escherichia coli

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Abstract
The trimethylamine N‐oxide (TMAO) reductase of Escherichia coli is a molybdoenzyme that catalyses the reduction of the TMAO to trimethylamine (TMA) with a redox potential of + 130 mV. We have successfully substituted the molybdenum with tungsten and obtained an active tungsto‐TMAO reductase. Kinetic studies revealed that the catalytic efficiency of the tungsto‐substituted TMAO reductase (W‐TorA) was increased significantly (twofold), although a decrease of about 50% in its kcat was found compared with the molybdo‐TMAO reductase (Mo‐TorA). W‐TorA is more sensitive to high pH, is less sensitive to high NaCl concentration and is more heat resistant than Mo‐TorA. Most importantly, the W‐TorA becomes capable of reducing sulphoxides and supports the anaerobic growth of a bacterial host on these substrates. The evolutionary implication and mechanistic significance of the tungsten substitution are discussed.