CcpA (PA4587) is the di-heme cytochrome c551 peroxidase of Pseudomonas aeruginosa, a soluble periplasmic enzyme made with a cleaved 23-residue signal peptide. It reduces hydrogen peroxide to water using electrons from the periplasmic electron shuttles cytochrome c551 and azurin. The mature 323-residue chain folds into two class-I cytochrome c-like domains, each holding a covalently bound heme c. The low-potential heme (about -330 mV, bis-His ligated) is the peroxidatic site where H2O2 is reduced, and the high-potential heme (about +320 mV, His-Met ligated) accepts electrons from the shuttle proteins and passes them to the peroxidatic heme. A calcium site at the domain interface stabilises the fold and inter-heme communication. The enzyme probably protects the cell against toxic peroxides in the periplasm.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004130 cytochrome-c peroxidase activity | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN000779264 (2 seeds, 44 leaf IBAs). Seeds are P. aeruginosa CcpA itself (PseudoCAP IDA, PMID:1657179) and E. coli Ccp (P37197, EcoCyc IDA, PMID:28696311). Reason: CcpA is the type member of the bacterial di-heme cytochrome c peroxidase family, and the crystal structure shows the peroxidatic and electron-transferring hemes. The self-seed is the expected marker of experimental grounding on the target. The IBD would also stand on the E. coli Ccp seed alone. Supporting Evidence: PMID:8591033 one of them is a low-potential (-330 mV) centre where hydrogen peroxide is reduced (the peroxidatic site); the other is a high-potential (+320 mV) centre which feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c and azurin. |
| GO:0004130 cytochrome-c peroxidase activity | IDA PMID:1657179 Structural and functional features of Pseudomonas cytochrome... | ACCEPT | Summary: PseudoCAP IDA citing Ellfolk et al. 1991, a structure-function study of the purified Pseudomonas enzyme (abstract only) that compares the native enzyme with an elastase-cleaved two-fragment complex. Reason: The term is correct for this protein, and the paper works with the purified enzyme; UniProt cites the same paper for the EC number and catalytic activity. The abstract does not quote activity values, so whether the activity itself was assayed here cannot be checked without the full text, but the curator's call is plausible and the function is not in doubt. This is the PTN000779264 seed row; verdict SOUND. Supporting Evidence: PMID:1657179 The cytochromal methionine has been replaced by a lysine with a concomitant lowering of redox-potential thus making peroxidatic activity possible. file:PSEAE/ccpA/ccpA-uniprot.txt Reaction=2 Fe(II)-[cytochrome c] + H2O2 + 2 H(+) = 2 Fe(III)- |
| GO:0004130 cytochrome-c peroxidase activity | IEA GO_REF:0000120 | ACCEPT | Summary: EC 1.11.1.5 / RHEA:16581 mapping of the curated catalytic activity. Reason: Consistent with the structural and biochemical evidence. Supporting Evidence: PMID:8591033 Cytochrome c peroxidase from Pseudomonas aeruginosa (PsCCP) represents a new class of peroxidases which work without the need to create a semi-stable free radical for catalysis. |
| GO:0009055 electron transfer activity | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro2GO electron transfer activity from the cytochrome c-like domain signatures. Reason: The mapping treats each cytochrome c-like domain as an electron carrier. In CcpA the high-potential heme does relay electrons, but only within the enzyme, from cytochrome c551 or azurin to its own peroxidatic heme, as part of the peroxidase reaction captured by GO:0004130. CcpA is the electron acceptor of the shuttle proteins, not a carrier between two other partners. Supporting Evidence: PMID:8591033 the other is a high-potential (+320 mV) centre which feeds electrons to the peroxidatic site from soluble electron-shuttle proteins such as cytochrome c and azurin. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | TOO GENERAL | Summary: InterPro2GO oxidoreductase activity from the CCP/MauG family signatures. Reason: True but uninformative; GO:0004130 cytochrome-c peroxidase activity, accepted above, is an is_a descendant of oxidoreductase activity. |
| GO:0019645 anaerobic electron transport chain | IBA GO_REF:0000033 | MARK AS OVER ANNOTATED | Summary: IBA from PTN002023662, seeded only by E. coli Ccp (P37197, EcoCyc IDA acts_upstream_of_or_within, PMID:28696311). Reason: E. coli Ccp is a membrane-associated quinol peroxidase that lets E. coli use H2O2 as a terminal electron acceptor during anaerobic growth. P. aeruginosa CcpA is a soluble periplasmic enzyme fed by cytochrome c551 and azurin, and its proposed role is peroxide protection. No study shows that CcpA serves as a terminal reductase of an anaerobic respiratory chain in P. aeruginosa. The transfer may be right (CcpA does accept electrons from respiratory-chain shuttles), but it is not established for this gene. Propagation Review Root cause: SOURCE WEAK OR INFERRED Failure modes: FUNCTIONAL DIVERGENCE CONTEXT OR TISSUE MISMATCH Sources checked: PANTHER:PTN002023662 · PTN002023662 UNRESOLVED Sole seed is E. coli Ccp, a quinol peroxidase with a different electron donor and membrane association; whether the respiratory role is ancestral to the clade that includes the soluble cytochrome c551 peroxidases is not shown. UniProtKB:P37197 · E. coli Ccp SUPPORTS SOURCE BUT NOT TARGET IDA acts_upstream_of_or_within GO:0019645 (PMID:28696311) for E. coli only. Supporting Evidence: PMID:8591033 The enzyme is located in the bacterial periplasm where its likely function is to provide protection against toxic peroxides. |
| GO:0020037 heme binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO heme binding from the cytochrome c domain signatures. Reason: Correct: the crystal structure shows two covalently bound c-type hemes. Heme is the cofactor of the core peroxidase activity rather than a function in its own right. Supporting Evidence: PMID:8591033 The soluble 323-residue single polypeptide chain contains two covalent c-type haems with very different properties |
| GO:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: Periplasm from the UniProt subcellular-location mapping. Reason: The enzyme is made with an N-terminal signal peptide removed in the mature protein (N-terminal sequencing) and is described as periplasmic. Supporting Evidence: PMID:8591033 The enzyme is located in the bacterial periplasm where its likely function is to provide protection against toxic peroxides. |
| GO:0042743 hydrogen peroxide metabolic process | IBA GO_REF:0000033 | MODIFY | Summary: IBA hydrogen peroxide metabolic process from PTN002023662 (E. coli Ccp seed). Reason: Correct but general. CcpA reduces H2O2 to water, so the process it performs is hydrogen peroxide catabolism (GO:0042744, an is_a child of GO:0042743). Propagation Review Root cause: NO FAILURE CORE Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN002023662 · PTN002023662 SUPPORTS TRANSFER Node seeded by E. coli Ccp only. The metabolic-process term is sound for a peroxidase clade; the catabolic child would be more informative. UniProtKB:P37197 · E. coli Ccp SUPPORTS TRANSFER IMP acts_upstream_of_or_within GO:0042743 (PMID:28696311). Proposed replacements: hydrogen peroxide catabolic process Supporting Evidence: PMID:8591033 one of them is a low-potential (-330 mV) centre where hydrogen peroxide is reduced (the peroxidatic site) |
| GO:0098869 cellular oxidant detoxification | IEA GO_REF:0000108 | ACCEPT | Summary: Inter-ontology inference from cytochrome-c peroxidase activity. Reason: Follows from the peroxidase activity, and the proposed physiological role of the periplasmic enzyme is protection against toxic peroxides. Supporting Evidence: PMID:8591033 The enzyme is located in the bacterial periplasm where its likely function is to provide protection against toxic peroxides. |
Loading supporting content…
Download this section (compressed HTML)Q: Does CcpA act as a terminal H2O2 reductase supporting growth or energy conservation in P. aeruginosa under low oxygen (as E. coli Ccp does), or only as a peroxide-protection enzyme?
Experiment: Compare growth and H2O2 consumption of a PAO1 ccpA deletion with wild type under anaerobic nitrate-respiring and microaerobic conditions with added H2O2, and measure H2O2-dependent oxidation of reduced cytochrome c551 in periplasmic extracts.
Loading supporting content…
Download this section (compressed HTML)Loading supporting content…
Download this section (compressed HTML)