Azurin (azu, PA4922) is a small periplasmic type 1 (blue) copper protein of the cupredoxin family. Its single copper ion is held in a beta-barrel by Gly45, His46, Cys112, His117 and Met121 (mature numbering) and cycles between Cu(II) and Cu(I), making azurin a soluble redox shuttle that exchanges electrons with cytochrome c551 and respiratory oxidoreductases. In vivo it is not required for denitrification; its expression rises in stationary phase and it has been linked to oxidative stress tolerance. Azurin is also secreted by the H2 type VI secretion system and, outside the cell, binds copper and delivers it to the outer-membrane receptor OprC, aiding copper acquisition. It is one of the most studied model proteins for biological electron transfer.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005507 copper ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: IBA from PTN002212874, seeded by azurin itself and Chloroflexus auracyanin A (P27197, UniProt IDA). The clade is type 1 copper proteins (azurins, Laz, auracyanins). Reason: Core; the self-seed reflects azurin's own experimental evidence and is expected. Supporting Evidence: PMID:14758526 Pseudomonas aeruginosa azurin binds copper so tightly that it remains bound even upon polypeptide unfolding. |
| GO:0005507 copper ion binding | IEA GO_REF:0000120 | ACCEPT | Summary: Combined automated annotation from the blue copper (BlueCu_1) domain. Reason: Consistent with the direct binding data. Supporting Evidence: PMID:14758526 Pseudomonas aeruginosa azurin binds copper so tightly that it remains bound even upon polypeptide unfolding. |
| GO:0005507 copper ion binding | IMP PMID:14758526 Methionine-121 coordination determines metal specificity in ... | ACCEPT | Summary: PseudoCAP row and a seed of the PTN002212874 IBD. ITC and spectroscopy on WT and Met121Ala azurin measure Cu(II) and Cu(I) affinities. Reason: Core. The paper measures copper binding directly (closer to IDA than IMP), but the term is right. Seed verdict SOUND. Supporting Evidence: PMID:14758526 Pseudomonas aeruginosa azurin binds copper so tightly that it remains bound even upon polypeptide unfolding. |
| GO:0005515 protein binding | IPI PMID:31790504 A Pseudomonas aeruginosa type VI secretion system regulated ... | REMOVE | Summary: IPI with OprC (G3XD89), the outer-membrane copper receptor to which secreted azurin delivers copper. Reason: Bare protein binding is uninformative. The interaction is real and functionally meaningful (copper delivery), but no MF term describes azurin's side of it better than the copper ion binding already accepted. Supporting Evidence: PMID:31790504 Secreted Azu acquires extracellular Cu2+ and delivers its load via direct interaction with OprC |
| GO:0005886 plasma membrane | IBA GO_REF:0000033 | REMOVE | Summary: IBA from PTN002212868, a sole-seed IBD on Chloroflexus auracyanin A, a membrane-associated cupredoxin. Azurin is a soluble periplasmic protein with a cleaved Sec signal peptide and no membrane anchor. Reason: Over-propagation of a lineage-specific membrane attachment to soluble azurins. Azurin acts in the periplasm (and, after T6SS secretion, outside the cell). Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN002212868 · PTN002212868 SUPPORTS SOURCE BUT NOT TARGET Sole seed auracyanin A; the 11 leaves include soluble periplasmic azurins, CopI and the outer-membrane lipoprotein Laz (GO_CENTRAL defect on this row). UniProtKB:P27197 · auracyanin A (Chloroflexus aurantiacus) SUPPORTS SOURCE BUT NOT TARGET Membrane-associated in a monoderm bacterium without a periplasm; location does not transfer. Supporting Evidence: PMID:31790504 Secreted Azu acquires extracellular Cu2+ and delivers its load via direct interaction with OprC |
| GO:0008270 zinc ion binding | IMP PMID:14758526 Methionine-121 coordination determines metal specificity in ... | MARK AS OVER ANNOTATED | Summary: Zinc substitutes for copper in vitro without structural change, and the paper measures zinc affinities of folded and unfolded azurin. Reason: Real in vitro binding at the copper site, but azurin is a copper protein; zinc occupancy is a substitution, not an evolved function. Supporting Evidence: PMID:14758526 Copper can be substituted with zinc without change in protein structure |
| GO:0009055 electron transfer activity | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO from the blue copper domain. Azurin is a redox-shuttle cupredoxin whose copper site is reduced by external donors. Reason: Core molecular function of azurin. Supporting Evidence: PMID:19299503 Cupredoxins function as redox shuttles and are required to be monomeric PMID:21352498 The reactivity of the H117G variant in the absence of external ligands appears to be 2.5-fold lower than the WT reactivity |
| GO:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt location mapping (periplasm). Azurin has a cleaved Sec signal peptide (1-20) and is a soluble protein; T6SS-mediated secretion and a cytoplasmic pool are also reported. Reason: The main location where azurin acts as an electron carrier. Supporting Evidence: PMID:31790504 Interestingly, our data showed that the distribution of azurin was observed both in the periplasmic and cytoplasmic compartments, which is different from the previous reports that proteins with the Sec signal peptide should be located in the periplasm (Fig 1C) [33–35]. |
| GO:0042802 identical protein binding | IPI PMID:19299503 Metal-binding loop length and not sequence dictates structur... | MARK AS OVER ANNOTATED | Summary: IntAct self-interaction. The abstract reports a strand-swapped dimer only for an engineered metal-binding-loop mutant, and states that cupredoxins work as monomers. Reason: The dimer is an artefact of an engineered loop; native azurin is a monomeric redox shuttle. Supporting Evidence: PMID:19299503 The mutant with 3 alanine residues between the ligands forms a strand-swapped dimer in the crystal structure PMID:19299503 Cupredoxins function as redox shuttles and are required to be monomeric |
| GO:0042802 identical protein binding | IPI PMID:21352498 Probing the reactivity of different forms of azurin by flavi... | UNDECIDED | Summary: IntAct self-interaction from a study of the H117G variant (crystal structure 3N2J and photoreduction kinetics). The abstract does not describe self-association. Reason: Abstract only; the basis of the IntAct curation (likely crystal packing of the variant) cannot be checked. Native azurin is generally a monomer. Supporting Evidence: PMID:19299503 Cupredoxins function as redox shuttles and are required to be monomeric |
| GO:0046914 transition metal ion binding | IDA PMID:23911566 Isomorphic deactivation of a Pseudomonas aeruginosa oxidored... | TOO GENERAL | Summary: PseudoCAP IDA from a crystal structure of silver-metallated apo-azurin; silver binds at the copper site and the adduct is redox inert. Reason: True but uninformative: copper ion binding (is_a transition metal ion binding), accepted in this review, entails it. Silver binding itself is a non-physiological substitution. Supporting Evidence: PMID:23911566 X-ray crystallography of the silver-modified protein reveals that silver binds to azurin at the traditional copper mediated active site with nearly identical geometry. |
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Download this section (compressed HTML)Q: Which periplasmic partners does azurin exchange electrons with in vivo, given that cytochrome c551 rather than azurin feeds nitrite reductase?
Q: Should the T6SS-dependent copper delivery to OprC be captured as a process annotation (e.g. copper ion import), given that azurin supplies the copper but OprC transports it?
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