site only when the protein is reaching full reduction. From these observations a
molecular mechanism for regulating the contribution of FccA in the EET vs
fumarate reduction in Shewanella was proposed [114].
FccA can also transfer electrons to MtrA, the outer-membrane associated decaheme cytochrome implicated in EET [97, 108, 115]. Binding studies performed
in vitro demonstrated that FccA interacts with its redox partners, CymA and MtrA,
through a single heme (heme II), avoiding the establishment of stable redox
complex capable of spanning the periplasmic space [97]. Gene knock-out experiments of FccA showed defective phenotypes in several anaerobic cell growth
conditions involving extracellular electron acceptors [106], which corroborates the
previous studies implicating FccA as a periplasmic electron shuttle involved in the
EET pathway of Shewanella [97, 108, 115]. Furthermore, in vitro and in vivo
studies have also shown the occurrence of electron transfer between CymA and
FccA [92, 97, 108].The high abundance in the periplasm, the interactions with
CymA and MtrA, and the phenotypes of the deletion mutants make FccA a major
player in electron shuttling in the periplasm during EET.
4.2.2 STC
STC is a highly abundant small tetraheme cytochrome c from the periplasm of
Shewanella with a molecular weight of 12 kDa [116]. Based on gene knock-out
experiments, STC is recognized as a key component in the EET pathway of
S. frigidimarina NCIMB400 and S. oneidensis MR-1 [106, 109, 117, 118]. This
was rationalized by studies with double STC and FccA knock-out experiments
showing that at least one of these two cytochromes must be present in the periplasm
to allow reduction of DMSO, ferric citrate or nitrate [106]. Overexpression of STC
showed that it is a major component in the EET pathway of Shewanella [119].
High-resolution crystal structures of STC from S. oneidensis MR-1 and S. algae
are available [120, 121] and a nuclear magnetic resonance (NMR) solution structure
exists for STC from S. frigidimarina NCIMB400 [122]. Comparison of the structures from these three proteins showed that the general fold is very similar, and the
relative positions of the heme groups are well conserved [121]. All four hemes are
low-spin and have a bis–histidine axial ligation to the polypeptide chain [120–122].
The arrangement of the pairs of hemes in perpendicular and parallel geometries
allows a short distance between the cofactors that enables a rapid intramolecular
transfer of the electrons [123].
Microscopic redox properties measured for STC from S. frigidimarina
NCIMB400, S. oneidensis MR-1 and S. algae DSM 9167 revealed similarities
between these three ortholog proteins [121, 124, 125]. The microscopic reduction
potentials for the four hemes of all three STCs cover similar, although not entirely
overlapping reduction potential ranges: −190 to −229 mV; −171 to −243 mV; and
−153 to −207 mV for S. frigidimarina NCIMB400, S. oneidensis MR-1 and
S. algae DSM 9167, respectively. The redox potentials are in the range expected for
bis-histidinyl-ligated heme groups with substantial exposure to the solvent [126].
Also, the results showed that electrostatic effects dominate the heme-heme interactions (covering a range of 8–56 mV for S. frigidimarina; 11–72 mV for
226
B. M. Fonseca et al.
molecular mechanism for regulating the contribution of FccA in the EET vs
fumarate reduction in Shewanella was proposed [114].
FccA can also transfer electrons to MtrA, the outer-membrane associated decaheme cytochrome implicated in EET [97, 108, 115]. Binding studies performed
in vitro demonstrated that FccA interacts with its redox partners, CymA and MtrA,
through a single heme (heme II), avoiding the establishment of stable redox
complex capable of spanning the periplasmic space [97]. Gene knock-out experiments of FccA showed defective phenotypes in several anaerobic cell growth
conditions involving extracellular electron acceptors [106], which corroborates the
previous studies implicating FccA as a periplasmic electron shuttle involved in the
EET pathway of Shewanella [97, 108, 115]. Furthermore, in vitro and in vivo
studies have also shown the occurrence of electron transfer between CymA and
FccA [92, 97, 108].The high abundance in the periplasm, the interactions with
CymA and MtrA, and the phenotypes of the deletion mutants make FccA a major
player in electron shuttling in the periplasm during EET.
4.2.2 STC
STC is a highly abundant small tetraheme cytochrome c from the periplasm of
Shewanella with a molecular weight of 12 kDa [116]. Based on gene knock-out
experiments, STC is recognized as a key component in the EET pathway of
S. frigidimarina NCIMB400 and S. oneidensis MR-1 [106, 109, 117, 118]. This
was rationalized by studies with double STC and FccA knock-out experiments
showing that at least one of these two cytochromes must be present in the periplasm
to allow reduction of DMSO, ferric citrate or nitrate [106]. Overexpression of STC
showed that it is a major component in the EET pathway of Shewanella [119].
High-resolution crystal structures of STC from S. oneidensis MR-1 and S. algae
are available [120, 121] and a nuclear magnetic resonance (NMR) solution structure
exists for STC from S. frigidimarina NCIMB400 [122]. Comparison of the structures from these three proteins showed that the general fold is very similar, and the
relative positions of the heme groups are well conserved [121]. All four hemes are
low-spin and have a bis–histidine axial ligation to the polypeptide chain [120–122].
The arrangement of the pairs of hemes in perpendicular and parallel geometries
allows a short distance between the cofactors that enables a rapid intramolecular
transfer of the electrons [123].
Microscopic redox properties measured for STC from S. frigidimarina
NCIMB400, S. oneidensis MR-1 and S. algae DSM 9167 revealed similarities
between these three ortholog proteins [121, 124, 125]. The microscopic reduction
potentials for the four hemes of all three STCs cover similar, although not entirely
overlapping reduction potential ranges: −190 to −229 mV; −171 to −243 mV; and
−153 to −207 mV for S. frigidimarina NCIMB400, S. oneidensis MR-1 and
S. algae DSM 9167, respectively. The redox potentials are in the range expected for
bis-histidinyl-ligated heme groups with substantial exposure to the solvent [126].
Also, the results showed that electrostatic effects dominate the heme-heme interactions (covering a range of 8–56 mV for S. frigidimarina; 11–72 mV for
226
B. M. Fonseca et al.