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.