OmcZ (outer-surface c-type cytochrome Z, locus GSU2076) is an octaheme c-type cytochrome of Geobacter sulfurreducens that polymerizes outside the cell into helical filaments ("nanowires"). These filaments carry electrons from the cell to extracellular acceptors over distances far greater than a cell length. The 473-residue precursor has a signal peptide and is made as a 50 kDa form (OmcZL). The subtilisin-like serine protease OzpA, encoded immediately downstream in the same operon, cuts it outside the cell into a 30 kDa form (OmcZS, about 258 residues). OmcZS keeps all eight hemes, including one attached through a non-canonical CX14CH motif, and loses a C-terminal region of Ig-like beta-sandwich domains. Cleaved OmcZS assembles spontaneously into filaments. In the cryo-EM structures, seven hemes stack into a linear chain along each subunit and an eighth branches off and is exposed to solvent; unlike the OmcS and OmcE filaments, subunits are not linked through coordination of each other's hemes. The purified protein is very thermostable. Its hemes titrate over -420 to -60 mV (midpoint about -220 mV versus SHE), and it delivers electrons in vitro to Fe(III) citrate, U(VI), Cr(VI), Au(III), Mn(IV) oxide and a humic-acid analogue. Export to the cell surface requires the PilA-N/PilA-C pilus, and in the biofilm OmcZ sits in a polysaccharide-anchored extracellular matrix, concentrated at the interface between the biofilm and an electrode anode. OmcZ is needed for high current density on anodes: deletion cuts current about ten-fold and thick, current-producing biofilms are not formed. Growth on fumarate, Fe(III) citrate or Fe(III) oxide is unaffected in the original deletion study. Production of OmcZ filaments rises when biofilms grow on electrodes or under an applied electric field. OmcZ also contributes to electron uptake from metallic iron and to direct interspecies electron transfer in some strain backgrounds. Homologues, usually encoded next to an OzpA-like serine peptidase, occur in other Geobacteraceae and are widespread in anaerobic methanotrophic archaea, where an OmcZ-type cytochrome forms the surface filaments of electrode-grown cells.
Definition: A helical, non-membrane-bounded protein polymer that lies outside the cell and is built from repeating subunits of a secreted multiheme c-type cytochrome. The hemes stack along the filament axis, so electrons can be transferred along its length between the cell and extracellular electron acceptors or donors.
Justification: Cryo-EM structures now exist for extracellular filaments made of the G. sulfurreducens cytochromes OmcS, OmcE and OmcZ, and of archaeal OmcZ-type and ECN-type cytochromes (Pyrobaculum, Archaeoglobus, 'Ca. Methanoperedens'). GO has no component term for them. The available terms describe either the medium they sit in (extracellular region, biofilm matrix) or the unrelated pilus (GO:0009289 pilus, a proteinaceous hair-like appendage on the surface of bacteria), which would wrongly imply a pilin-based appendage. Members of this class differ in heme packing and evidently arose more than once, so the term should be defined by structure and location, not by homology. A suitable placement is under GO:0099080 supramolecular complex, with part_of GO:0005576 extracellular region. The same term is proposed in the OmcS review; the narrative case for it, the naming question (the field says "extracellular cytochrome nanowire"), and the three further process-term gaps are collected in projects/EXTRACELLULAR_CYTOCHROME_NANOWIRES.md.
Parent term: supramolecular complex
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009055 electron transfer activity | IDA PMID:20400562 Purification and characterization of OmcZ, an outer-surface,... | NEW | Summary: Purified OmcZS, the mature eight-heme form, passes electrons to a range of extracellular acceptors in vitro (Fe(III) citrate, U(VI), Cr(VI), Au(III), Mn(IV) oxide, AQDS). Its hemes cover a broad potential window. The cryo-EM structures place seven of the hemes in a stacked chain that runs through each subunit of the filament. Reason: GOA has no molecular-function annotation for OmcZ, yet carrying electrons is what the protein is for. The activity is shown on the purified protein (PMID:20400562). It is consistent with the heme chain seen in two independent filament structures (PMID:36732469, PMID:36062910), and deleting the solvent-exposed heme II loop slows electron transfer to extracellular acceptors (PMID:36732469). The term is deliberately the generic carrier term. GO:0016722 oxidoreductase activity, acting on metal ions was considered and declined: the in vitro metal reductions are single-turnover electron deliveries by the reduced cytochrome, and the omcZ deletion does not affect growth on Fe(III) citrate or Fe(III) oxide (PMID:19461962), so reducing metals is not shown to be what the protein evolved to do. The term also makes no claim about conduction mechanism. The reported metallic-like conductivity of OmcZ filaments (PMID:32807967) is disputed (PMID:37552847, PMID:38633696), but that dispute concerns how fast and by what physics electrons move, not whether OmcZ transfers them. Supporting Evidence: PMID:20400562 OmcZ(S) transferred electrons in vitro to a diversity of potential extracellular electron acceptors, such as Fe(III) citrate, U(VI), Cr(VI), Au(III), Mn(IV) oxide, and the humic substance analogue anthraquinone-2,6-disulfonate, but not Fe(III) oxide. PMID:36732469 In OmcZ, haems IβV and VII form a linear, central, electron transport chain whereas haem VI is surface exposed PMID:36732469 Using reconstituted OmcZ, we find that deletion of the haem II loop reduces the rate of electron transfer by OmcZ nanowires to extracellular electron acceptors compared with the WT |
| GO:0020037 heme binding | IDA PMID:36732469 Structure of Geobacter cytochrome OmcZ identifies mechanism ... | NEW | Summary: OmcZS carries eight covalently bound c-type hemes. All eight are resolved in the 3.5 Γ
cryo-EM structure of native filaments and were counted in the purified protein. One heme is attached through a non-canonical CX14CH motif. Reason: Heme binding is the cofactor basis of the electron-transfer function and is shown directly for this protein. Two independent cryo-EM structures resolve the heme chain (PMID:36732469; PMID:36062910), and purified OmcZS retains all eight hemes (PMID:20400562). UniProt Q74BG5 records the heme c binding sites from PDB 7LQ5 and 8D9M. GOA has no row for it. As with other multiheme cytochromes in this repository (OmcA, ImcH, CbcL), heme binding is recorded alongside the electron-transfer row and is not treated as a separate core function. Supporting Evidence: PMID:36732469 At this resolution, the backbone density along with many bulky sidechain densities, such as aromatic residues and all eight haem densities, are visible PMID:20400562 OmcZ(S) is a cleaved product of OmcZ(L) retaining all 8 hemes, including 1 heme with the unusual c-type heme-binding motif CX(14)CH DOI:10.1038/s41467-026-78976-x In contrast, the nanowire-forming part of OmcZGs comprises 258 residues and includes one heme bound by a non-canonical motif (CX14CH). |
| GO:0097311 bacterial biofilm matrix | IDA PMID:23761253 Specific localization of the c-type cytochrome OmcZ at the a... | NEW | Summary: Immunogold labelling places OmcZ throughout the extracellular matrix around aggregated cells, and highly concentrated at the biofilm-anode interface in current-producing biofilms. Matrix sheared from cells contains OmcZ bound to polysaccharide, and a mutant that makes less matrix polysaccharide loses it. Reason: GOA has no cellular-component row for OmcZ. The matrix location is where the function is exercised, because the filaments bridge cells and the electrode through the biofilm. It is shown directly for the endogenous protein by immunolabelling (PMID:23761253) and independently by matrix fractionation, where the xap polysaccharide mutant's matrix is nearly free of OmcZ (PMID:21169487). Subcellular fractionation agrees that the mature form is extracellular (PMID:20400562, PMID:34471289). GO:0097311 is a descendant of GO:0005576 extracellular region, so a separate extracellular-region row would be redundant. GO:0009986 cell surface was not used: the mature form is cut off outside the cell and dispersed in the matrix, and no anchoring of OmcZ filaments to the outer membrane has been shown. The precursor (OmcZL) is found in the periplasm on its way out, and that transit is not annotated. Supporting Evidence: PMID:23761253 OmcZ was dispersed throughout the extracellular matrix surrounding the cells that accumulated at the bottom of the culture tubes of cells grown under standard conditions with fumarate as the electron acceptor. PMID:23761253 When G. sulfurreducens grew as a biofilm on a graphite electrode that served as an anode and the sole electron acceptor for growth, OmcZ was highly concentrated at the biofilm-electrode interface. PMID:21169487 The matrix from the mutant contained 60% less sugar and was nearly devoid of c-type cytochromes such as OmcZ. PMID:34471289 Our subcellular localization experiments revealed that both OmcS and OmcZ are present in the extracellular fraction of wild-type cells but remain in the periplasm for ΞpilA-N cells |
| GO:0019645 anaerobic electron transport chain | IMP PMID:19461962 Anode biofilm transcriptomics reveals outer surface componen... | NEW | Summary: OmcZ is the extracellular end of the respiratory chain that G. sulfurreducens uses when an anode is the terminal electron acceptor. Deleting omcZ cuts maximum current about ten-fold, and the mutant does not adapt even after more than 50 days. Expressing omcZ from a plasmid restores current to wild-type levels. Growth with fumarate, Fe(III) citrate or Fe(III) oxide is unaffected. Reason: OmcZ does part of the work of this process, not merely being required for it: the protein is itself an electron carrier whose hemes take electrons on and pass them on toward the acceptor (GO:0009055 row). The deletion phenotype is therefore read together with direct activity rather than on its own. Complementation rules out a polar effect on ozpA, and the specificity of the phenotype places OmcZ in the electrode-specific, extracellular branch of the chain rather than in the shared upstream segment. Comparator check: other terminal extracellular or envelope cytochromes of anaerobic chains reviewed in this repository carry the same term (S. oneidensis OmcA; G. sulfurreducens ImcH and CbcL). No narrower GO process term exists for respiration on an electrode or on another extracellular solid acceptor. OmcZ's roles in electron uptake from Fe(0) (PMID:31088920) and in direct interspecies electron transfer (PMID:37881251) are strain-background dependent and have no fitting GO term, so they are recorded in the notes and the core-function description rather than as rows. Supporting Evidence: PMID:19461962 only deletion of pilA or omcZ severely inhibited current production and biofilm formation in current-harvesting mode PMID:19461962 When these mutations were complemented by expressing pilA or omcZ on a plasmid, wild type levels of current production were restored (Table 4). PMID:19461962 Deletion of omcZ had no impact on growth with any of these electron acceptors (data not shown). PMID:33837010 the ozpA mutant accumulated OmcZL and did not produce OmcZS, which is thought to be a mature form that is essential for the extracellular electron transfer to the electrode |
| GO:0051258 protein polymerization | IDA PMID:36732469 Structure of Geobacter cytochrome OmcZ identifies mechanism ... | NEW | Summary: Once OzpA removes the C-terminal region, OmcZ30 self-assembles in vitro into helical filaments that are indistinguishable from native filaments. The native filament is a homopolymer of OmcZ30 held together by heme and aromatic stacking and hydrophobic contacts at the interface between subunits. Reason: The protein assembles its own filament: polymerization is driven by interfaces on OmcZ itself, with no inter-subunit metal coordination and no other protein in the polymer. So the gene product does the work of the process, not just undergoes it. In vitro reconstitution from purified OzpA and OmcZ50 (native and recombinant) shows it directly (PMID:36732469). The protease is the switch, not a participant in the polymer: uncleaved OmcZ50 does not polymerize. The row is kept at the generic process level. GO has no term for assembly of an extracellular cytochrome filament, and the corresponding component term is proposed below. Supporting Evidence: PMID:36732469 We found that purified OzpA cleaves both native and recombinant OmcZ50 into OmcZ30 which then self-assembled into nanowires PMID:36732469 Notably, reconstituted OmcZ nanowires after proteolytic cleavage had structures that were indistinguishable from native nanowires |
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Download this section (compressed HTML)Q: What is OmcZ for outside the laboratory? It is strongly and specifically induced on poised anodes, but no natural acceptor or growth condition with a comparable induction has been identified, and the omcZ deletion grows normally on Fe(III) oxide in the original study.
Q: Is OmcZ needed for ferrihydrite reduction? Jiang et al. 2023 report an impaired ΞomcZ, whereas Nevin et al. 2009 and Gu et al. 2023 report no requirement. Do strain background (PCA versus CL-1 or KN400) and the compensating up-regulation of other outer-surface cytochromes explain the difference?
Q: How do OmcZ filaments receive electrons from the outer-membrane porin-cytochrome conduits, given that the mature filament is released by cleavage outside the cell and no anchor to the outer membrane is known?
Q: Is the peptidyl-prolyl isomerase GSU2074 in the omcZ operon actually needed for OmcZ folding? Archaeal OmcZ-type cytochromes form filaments without an isomerase in the cluster.
Q: Should GO capture conduction along an extracellular cytochrome filament with a dedicated activity or component term, rather than generic electron transfer activity plus a biofilm-matrix location?
Experiment: In a background that makes only OmcZ filaments (omcS, omcE and omcT deleted), delete each outer-membrane conduit in turn (extABCD, ombB-omaB-omcB, ombC-omaC-omcC) and measure current density and biofilm thickness on a poised anode, with and without plasmid-borne complementation. A conduit whose loss phenocopies omcZ deletion while OmcZ filaments are still present and correctly localized (checked by immunogold TEM) identifies the entry point of electrons into the filament.
Hypothesis: OmcZ filaments receive their electrons from a specific outer-membrane porin-cytochrome conduit at the cell surface.
Type: genetic epistasis with electrochemistry
Experiment: Build omcZ deletions in the same backgrounds used by the conflicting studies (PCA, CL-1, KN400) with a common method, and measure Fe(III) oxide reduction rates side by side under identical conditions. In each mutant, measure transcript and protein levels of omcS, omcT, omcE and the ext clusters, to test whether the strains that tolerate omcZ loss are the ones that up-regulate an alternative.
Hypothesis: The conflicting ferrihydrite phenotypes reflect strain background and compensatory expression of other outer-surface cytochromes rather than a real difference in what OmcZ does.
Type: comparative mutant phenotyping with transcriptomics
Experiment: Delete GSU2074 alone, without disturbing omcZ or ozpA transcription (e.g. by in-frame deletion plus in trans complementation controls), and ask whether OmcZS is still produced, still assembles into filaments of native morphology by negative-stain TEM, and still supports current on an anode. If filaments form normally, the isomerase acts on something else, as the archaeal clusters that lack one would predict.
Hypothesis: The peptidyl-prolyl isomerase GSU2074 is not required for OmcZ folding or filament assembly, despite being co-transcribed with omcZ and ozpA.
Type: targeted deletion with structural readout
Experiment: Compare wild type and an omcZ deletion for growth and reduction rate across a panel of extracellular acceptors that have not been systematically tested, including humic substances and their analogues, Mn(IV) oxides of different crystallinity, structural Fe(III) in clays, and elemental sulfur, under conditions where omcZ expression is first confirmed by RT-qPCR. The point is to find a condition in which omcZ is induced to anode-like levels, which no study has yet reported.
Hypothesis: OmcZ has a natural extracellular acceptor whose reduction has been missed because anodes dominate the assay repertoire.
Type: phenotype screen across acceptors
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