cell surface [158]. Purified MtrC and OmcA were reported to reduce iron oxides at
much slower rates compared to measurements with intact Shewanella cells and that
with the addition of flavins, rates were increased to values comparable to those
measured with intact cells [60]. These results demonstrate a role of electron shuttle
for flavins during MtrC and OmcA mediated reduction of ferric iron oxides.
Both cytochromes have 10 low-spin c-type hemes [95, 148, 157, 158]. Potentiometric titrations revealed that both MtrC and OmcA titrate over a broad range of
redox potential from +100 mV to −500 mV and −20 mV to −320 mV, respectively [101, 151, 157]. Crystal structures of these proteins have revealed that the
proteins are formed by 4 domains, two multiheme domains that are flanked by two
b-barrels with b-strands arranged in Greek key motifs [159, 160]. Both MtrC and
OmcA contain a conserved decaheme staggered cross cofactor arrangement, where
an octaheme chain formed by hemes V, IV, III, I, VI, VIII, IX, X is crossed by a
tetraheme chain consisting of hemes II, I, VI, VII. All the hemes display bis‐
histidine axial ligand coordination to the heme iron, with each heme within 7 Å of
its nearest neighbor, ensuring rapid intra‐molecular electron transfer. In vitro
experiments revealed that OmcA and MtrC (as well as its homologues MtrF and
UndA) are capable of transferring electrons to chemically varied soluble electron
acceptors typically found in the oxic-anoxic interface habitats where Shewanella is
found, with clear differences in the rates for different acceptors [161]. NMR and
computational docking studies revealed that whereas for negatively charged FMN
and AQDS binding occurs near heme II, neutral riboflavin binds near hemes IX and
X and positively charged phenazine methosulphate binds near heme X in a different
position. For OmcA, which plays a more important role in surface attachment than
MtrC, it was observed that iron oxide particles and graphene oxides do not come
into close proximity to the hemes, in agreement with experimentally observed slow
electron transfer [162]. Altogether these studies reveal that the structure of these
proteins appears to be designed such as the staggered cross provides different exit
points for electron through different exposed hemes [161].
EET Enhancement in Shewanella
To increase EET in MFCs, over-expression of the MHC involved in EET of S.
oneidensis MR-1 has been used to enhance current output. For example, it was
observed that overexpression of mtrC in S. oneidensis MR-1 could generate 35%
more current in MFCs than that of wild-type organism [82]. Furthermore, the
co-expression of the metal-reducing biosynthesis gene cluster mtrC-mtrA-mtrB also
exhibited an increase in maximum current density of approximately 87% [163].
More recently, the genetic manipulation of S. oneidensis MR-1 where the proteins
that may compete with STC for EET processes in the periplasmic space were
replaced by STC, led to the creation of a mutant that presented 23% higher current
generation when compared with the wild-type strain [119]. These studies highlight
the importance of genetic engineering to design and tailor MHC towards enhanced
electron transfer processes to push forward the practical implementation of electroactive organisms in BES [18].
Bacterial Power: An Alternative Energy Source
231
much slower rates compared to measurements with intact Shewanella cells and that
with the addition of flavins, rates were increased to values comparable to those
measured with intact cells [60]. These results demonstrate a role of electron shuttle
for flavins during MtrC and OmcA mediated reduction of ferric iron oxides.
Both cytochromes have 10 low-spin c-type hemes [95, 148, 157, 158]. Potentiometric titrations revealed that both MtrC and OmcA titrate over a broad range of
redox potential from +100 mV to −500 mV and −20 mV to −320 mV, respectively [101, 151, 157]. Crystal structures of these proteins have revealed that the
proteins are formed by 4 domains, two multiheme domains that are flanked by two
b-barrels with b-strands arranged in Greek key motifs [159, 160]. Both MtrC and
OmcA contain a conserved decaheme staggered cross cofactor arrangement, where
an octaheme chain formed by hemes V, IV, III, I, VI, VIII, IX, X is crossed by a
tetraheme chain consisting of hemes II, I, VI, VII. All the hemes display bis‐
histidine axial ligand coordination to the heme iron, with each heme within 7 Å of
its nearest neighbor, ensuring rapid intra‐molecular electron transfer. In vitro
experiments revealed that OmcA and MtrC (as well as its homologues MtrF and
UndA) are capable of transferring electrons to chemically varied soluble electron
acceptors typically found in the oxic-anoxic interface habitats where Shewanella is
found, with clear differences in the rates for different acceptors [161]. NMR and
computational docking studies revealed that whereas for negatively charged FMN
and AQDS binding occurs near heme II, neutral riboflavin binds near hemes IX and
X and positively charged phenazine methosulphate binds near heme X in a different
position. For OmcA, which plays a more important role in surface attachment than
MtrC, it was observed that iron oxide particles and graphene oxides do not come
into close proximity to the hemes, in agreement with experimentally observed slow
electron transfer [162]. Altogether these studies reveal that the structure of these
proteins appears to be designed such as the staggered cross provides different exit
points for electron through different exposed hemes [161].
EET Enhancement in Shewanella
To increase EET in MFCs, over-expression of the MHC involved in EET of S.
oneidensis MR-1 has been used to enhance current output. For example, it was
observed that overexpression of mtrC in S. oneidensis MR-1 could generate 35%
more current in MFCs than that of wild-type organism [82]. Furthermore, the
co-expression of the metal-reducing biosynthesis gene cluster mtrC-mtrA-mtrB also
exhibited an increase in maximum current density of approximately 87% [163].
More recently, the genetic manipulation of S. oneidensis MR-1 where the proteins
that may compete with STC for EET processes in the periplasmic space were
replaced by STC, led to the creation of a mutant that presented 23% higher current
generation when compared with the wild-type strain [119]. These studies highlight
the importance of genetic engineering to design and tailor MHC towards enhanced
electron transfer processes to push forward the practical implementation of electroactive organisms in BES [18].
Bacterial Power: An Alternative Energy Source
231