Electron Transfer and Binding of the c-Type Cytochrome TorC to the Trimethylamine N-Oxide Reductase in Escherichia coli *

Reduction of trimethylamine N-oxide (E′0(TMAO/TMA) = +130 mV) in Escherichia coli is carried out by the Tor system, an electron transfer chain encoded by the torCAD operon and made up of the periplasmic terminal reductase TorA and the membrane-anchored pentahemicc-type cytochrome TorC. Although the role of TorA in the reduction of trimethylamine N-oxide (TMAO) has been clearly established, no direct evidence for TorC involvement has been presented. TorC belongs to the NirT/NapC c-type cytochrome family based on homologies of its N-terminal tetrahemic domain (TorCN) to the cytochromes of this family, but TorC contains a C-terminal extension (TorCC) with an additional heme-binding site. In this study, we show that both domains are required for the anaerobic bacterial growth with TMAO. The intact TorC protein and its two domains, TorCN and TorCC, were produced independently and purified for a biochemical characterization. The reduced form of TorC exhibited visible absorption maxima at 552, 523, and 417 nm. Mediated redox potentiometry of the heme centers of the purified components identified two negative midpoint potentials (−177 and −98 mV) localized in the tetrahemic TorCN and one positive midpoint potential (+120 mV) in the monohemic TorCC. In agreement with these values, thein vitro reconstitution of electron transfer between TorC, TorCN, or TorCC and TorA showed that only TorC and TorCC were capable of electron transfer to TorA. Surprisingly, interaction studies revealed that only TorC and TorCN strongly bind TorA. Therefore, TorCCdirectly transfers electrons to TorA, whereas TorCN, which probably receives electrons from the menaquinone pool, is involved in both the electron transfer to TorCC and the binding to TorA.