Outer membrane c-type cytochromes required for Fe(III) and Mn(IV) oxide reduction in Geobacter sulfurreducens.

OmcE was recovered by shearing intact cells, runs at about 30 kDa, and was predicted to be a tetraheme outer-membrane cytochrome.

"The other abundant c-type cytochrome sheared from the outer surface of G. sulfurreducens, designated OmcE, has a molecular mass of ca. 30 kDa and is predicted to be an outer membrane tetraheme c-type cytochrome."
Deleting omcE abolishes Fe(III) oxide reduction while leaving soluble acceptors unaffected.

"When either omcS or omcE was deleted, G. sulfurreducens could no longer reduce Fe(III) oxide but could still reduce soluble electron acceptors, including Fe(III) citrate."
Expression in trans restores Fe(III) oxide reduction.

"Expressing omcS or omcE in trans restored the capacity for Fe(III) oxide reduction."

Cryo-EM structure of an extracellular Geobacter OmcE cytochrome filament reveals tetrahaem packing.

OmcE forms a second, thinner extracellular filament, solved at about 4.3 angstroms.

"Here we report that G. sulfurreducens also produces a second, thinner appendage comprised of cytochrome OmcE subunits and solve its structure using cryo-electron microscopy at ~4.3 Å resolution."
OmcE and OmcS share a heme packing arrangement, including cross-subunit histidine coordination, despite having no sequence or backbone similarity.

"Although OmcE and OmcS subunits have no overall sequence or structural similarities, upon polymerization both form filaments that share a conserved haem packing arrangement in which haems are coordinated by histidines in adjacent subunits."
The authors propose a class of conductive appendages defined by heme packing rather than homology.

"We propose that there is a broad class of conductive bacterial appendages with conserved haem packing (rather than sequence homology) that enable long-distance electron transport to chemicals or other microbial cells."
The filaments studied came from omcS-mutant strains, in which OmcE filaments were enriched, and the authors interpret this as compensation.

"a serendipitous outcome was that these omcS mutations resulted in an enrichment of extracellular OmcE filaments"
Cross-subunit heme-histidine ligation appears unique among solved c-type cytochrome structures.

"To the best of our knowledge, the coordination of a heme in one subunit by a histidine in an adjacent subunit in both the OmcE and OmcS filaments seems to be unique among all c-type cytochrome structures solved to date."
Only OmcE among the candidate cytochromes has the histidine complement needed to bis-coordinate the observed hemes.

"Of these four cytochromes, only OmcE has eight histidine residues near the Fe densities to assemble bis-coordinated hemes"
The surface densities are hypothesised, not demonstrated, to be glycans.

"which we hypothesize are glycosylations"
The authors caution that conductivity attributed to a single protein is confounded by multiple filament species and extracellular DNA.

"means that measures of conductivity attributed to a specific protein need to be treated with some caution"

PMID:19461962

Anode biofilm transcriptomics reveals outer surface components essential for high density current production in Geobacter sulfurreducens fuel cells.

Deleting omcE has no effect on maximum current production.

"Deletion of omcS, omcB, or omcE had no impact on maximum current production (Table 4)."

PMID:37881251

The varied roles of pilA-N, omcE, omcS, omcT, and omcZ in extracellular electron transfer by Geobacter sulfurreducens.

Deleting omcE impairs ferrihydrite reduction but has minimal effect on anode reduction or on coculture with G. metallireducens.

"Deletion of omcE also impaired bacterial ability to reduce ferrihydrite, but its impacts on the ability for anode reduction and the co-culture of G. metallireducens-G. sulfurreducens were minimal."

PMID:39278116

Specific interaction of resorufin to outer-membrane cytochrome OmcE of Geobacter sulfurreducens: A new insight on artificial electron mediators in promoting extracellular electron transfer.

The biofilm redox shift of resorufin is lost in an omcE mutant but not in omcS or omcZ mutants, indicating a specific interaction with OmcE.

"this shift was attenuated by the mutation of outer-membrane cytochrome gene omcE but not by omcS and omcZ mutation, indicating that RR specifically interacted with OmcE"
Heterologously expressed OmcE binds resorufin with a dissociation constant of 720 nM.

"By using heterologously expressed OmcE monomers in Escherichia coli, it was found that RR bonded with OmcE monomers with a moderate intensity (dissociation constant of 720 nM)"