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J. Biol. Chem., Vol. 276, Issue 15, 11545-11551, April 13, 2001
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From the 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 pentahemic
c-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 (
Electron Transfer and Binding of the c-Type
Cytochrome TorC to the Trimethylamine N-Oxide Reductase
in Escherichia coli*
,, and¶
Laboratoire de Chimie Bactérienne and
§ Laboratoire de Bioénergétique et
Ingénierie des Protéines, Institut de Biologie
Structurale et Microbiologie, Centre National de la Recherche
Scientifique, 31 chemin Joseph Aiguier, BP 71, 13402 Marseille Cedex 20, France
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, the in 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, TorCC
directly 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.
*
This work was supported by grants from the Center National
de la Recherche Scientifique, the Université de la
Méditerranée, and an Ministère de l'Education Nationale,
de la Recherche et de la Technologie fellowship (to S. G.).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc.
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