Gram-negative outer membrane is %70 Å [147], the estimated length of MtrA
would be sufficient for transferring electrons heme-to-heme across the outer
membrane even though the Small Angle Neutron Scattering model of the whole
MtrCAB complex suggests that MtrA is only partially inserted in MtrB [143].
Indeed, the 3D-structure determination of an MtrCAB complex revealed that MtrA
has very little secondary structure [132], which confer greater flexibility to the
protein, allowing it to accept electrons from its various physiological partners
[97, 108].
4.3.3 Decaheme Cytochromes MtrC and OmcA
Decaheme cytochromes MtrC and OmcA are two cytochromes with 75 and 85 kDa,
respectively, anchored to the outer membrane via a lipidated cysteine [129, 148,
149]. Treatment by proteinase K significantly degraded MtrC and OmcA by 31 and
71%, respectively [148]. This indicates that both proteins are exposed on the outer
surface of the cells and that MtrC is not as exposed to the extracellular environment
as OmcA, which is coherent with the proposal that MtrC becomes partly buried
upon association with the b-barrel protein MtrB [143]. The Small Angle Neutron
Scattering structural data agree with previous in vivo cross-linking studies that
revealed an interaction of MtrC with the b-barrel protein MtrB forming in combination with the decaheme cytochrome MtrA an outer membrane protein complex
MtrCAB, with a 1:1:1 stoichiometry [144]. Cross-linking assays demonstrated that
MtrC and OmcA physically interact with each other on the bacterial cells [144,
150], and in vitro studies reported a dissociation constant smaller than 0.5 lM for
the MtrC-OmcA complex [151].
Several studies showed that MtrC and OmcA are highly expressed by S. oneidensis MR-1 under ferric iron reducing conditions [129, 130, 152] and are capable
of direct electron transfer to iron oxides [153–155]. Both MtrC and OmcA
polypeptides were shown to contain a putative hematite-binding motif
(Ser/Thr-Pro-Ser/Thr) [156] and the physical interaction between MtrC and OmcA
synergistically boosts the metal reductase activity of these outer-membrane cytochromes [151]. Disruption of the mtrC or omcA genes did not affect the growth of S.
oneidensis MR-1 on soluble terminal electron acceptors, such as fumarate, nitrate
and DMSO [149]. In contrast, reduction of insoluble iron oxides and electron
transfer to MFC anodes was severely diminished [58, 82, 118, 152, 157, 158].
A series of knock-out mutations of all the outer-membrane cytochromes and subsequent expression of each one individually, showed that MtrC is critical for EET
and that mutants containing only the OmcA cytochrome were not capable of
transferring electrons to iron [130]. This fact suggests that while OmcA is an iron
terminal reductase [60, 149], its contact with the periplasmic redox chain is
mediated by MtrC [130]. Additionally, it has been shown that MtrC is responsible
for most of the electron transfer to carbon electrodes, while OmcA is mainly
involved in cellular attachment to solid surfaces, playing a smaller role in electron
transfer [58]. This is coherent with data obtained by antibody functionalized atomic
force microscopy (AFM) tips that showed OmcA in the interface between the cell
and insoluble substrate, while MtrC displays a more uniform distribution across the
230
B. M. Fonseca et al.
would be sufficient for transferring electrons heme-to-heme across the outer
membrane even though the Small Angle Neutron Scattering model of the whole
MtrCAB complex suggests that MtrA is only partially inserted in MtrB [143].
Indeed, the 3D-structure determination of an MtrCAB complex revealed that MtrA
has very little secondary structure [132], which confer greater flexibility to the
protein, allowing it to accept electrons from its various physiological partners
[97, 108].
4.3.3 Decaheme Cytochromes MtrC and OmcA
Decaheme cytochromes MtrC and OmcA are two cytochromes with 75 and 85 kDa,
respectively, anchored to the outer membrane via a lipidated cysteine [129, 148,
149]. Treatment by proteinase K significantly degraded MtrC and OmcA by 31 and
71%, respectively [148]. This indicates that both proteins are exposed on the outer
surface of the cells and that MtrC is not as exposed to the extracellular environment
as OmcA, which is coherent with the proposal that MtrC becomes partly buried
upon association with the b-barrel protein MtrB [143]. The Small Angle Neutron
Scattering structural data agree with previous in vivo cross-linking studies that
revealed an interaction of MtrC with the b-barrel protein MtrB forming in combination with the decaheme cytochrome MtrA an outer membrane protein complex
MtrCAB, with a 1:1:1 stoichiometry [144]. Cross-linking assays demonstrated that
MtrC and OmcA physically interact with each other on the bacterial cells [144,
150], and in vitro studies reported a dissociation constant smaller than 0.5 lM for
the MtrC-OmcA complex [151].
Several studies showed that MtrC and OmcA are highly expressed by S. oneidensis MR-1 under ferric iron reducing conditions [129, 130, 152] and are capable
of direct electron transfer to iron oxides [153–155]. Both MtrC and OmcA
polypeptides were shown to contain a putative hematite-binding motif
(Ser/Thr-Pro-Ser/Thr) [156] and the physical interaction between MtrC and OmcA
synergistically boosts the metal reductase activity of these outer-membrane cytochromes [151]. Disruption of the mtrC or omcA genes did not affect the growth of S.
oneidensis MR-1 on soluble terminal electron acceptors, such as fumarate, nitrate
and DMSO [149]. In contrast, reduction of insoluble iron oxides and electron
transfer to MFC anodes was severely diminished [58, 82, 118, 152, 157, 158].
A series of knock-out mutations of all the outer-membrane cytochromes and subsequent expression of each one individually, showed that MtrC is critical for EET
and that mutants containing only the OmcA cytochrome were not capable of
transferring electrons to iron [130]. This fact suggests that while OmcA is an iron
terminal reductase [60, 149], its contact with the periplasmic redox chain is
mediated by MtrC [130]. Additionally, it has been shown that MtrC is responsible
for most of the electron transfer to carbon electrodes, while OmcA is mainly
involved in cellular attachment to solid surfaces, playing a smaller role in electron
transfer [58]. This is coherent with data obtained by antibody functionalized atomic
force microscopy (AFM) tips that showed OmcA in the interface between the cell
and insoluble substrate, while MtrC displays a more uniform distribution across the
230
B. M. Fonseca et al.