pqqE encodes PqqA peptide cyclase (EC 1.21.98.4), a radical S-adenosylmethionine (SAM) enzyme that catalyzes the critical cross-linking of glutamate and tyrosine residues in the PqqA precursor protein during pyrroloquinoline quinone (PQQ) biosynthesis. PQQ is the essential cofactor for both calcium-dependent (MxaFI) and lanthanide-dependent (XoxF) methanol dehydrogenases, making PqqE absolutely required for methylotrophic growth. The enzyme contains a [4Fe-4S] cluster coordinated by three cysteines and an exchangeable S-adenosyl-L-methionine, characteristic of the radical SAM superfamily. PqqE forms a ternary complex with the peptide chaperone PqqD and the substrate PqqA; this interaction with PqqD is necessary for PqqE activity. Crystal structure has been solved at 3.20 Γ resolution (PDB: 6C8V). The enzyme catalyzes de novo carbon-carbon cross-linking within the PqqA peptide substrate, forming the E-Y cross-linked intermediate that is further processed to PQQ.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Root-level catalytic activity. PqqE has a specific, well-characterized radical SAM cross-linking activity captured by GO:0009975 (cyclase activity); this generic root term is uninformative on its own. Reason: Subsumed by the more specific MF GO:0009975 (cyclase activity); retained as non-core because it does not convey the actual reaction. |
| GO:0005506 iron ion binding | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: PqqE binds iron exclusively as part of iron-sulfur clusters, not as a mononuclear iron ion. The radical SAM [4Fe-4S] cluster plus two auxiliary SPASM-domain clusters are better captured by the iron-sulfur cluster terms (GO:0051536, GO:0051539). This generic mononuclear-iron term is a less precise UniRule transfer. Reason: The iron in PqqE is organized into [4Fe-4S]/auxiliary Fe-S clusters; the iron-sulfur cluster binding terms are more accurate, so this term is retained as non-core rather than as a core function. |
| GO:0009975 cyclase activity | IEA GO_REF:0000104 | ACCEPT | Summary: Correct and represents the core molecular function. PqqE is a radical SAM peptide cyclase (EC 1.21.98.4) that catalyzes intramolecular C-C ring closure by cross-linking a glutamate and a tyrosine side chain in the PqqA precursor peptide. No more specific GO MF term exists for this PqqA peptide cyclase reaction. Reason: Best available MF term for the experimentally demonstrated Glu-Tyr C-C cross-linking (ring-closure) reaction on PqqA; this is the core function. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md installation of a CβC bond (crosslink) between the side chains of a conserved glutamate and tyrosine on the ribosomally produced precursor peptide PqqA |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: PqqE is a radical SAM enzyme that reductively cleaves SAM via its [4Fe-4S] cluster to generate a 5'-deoxyadenosyl radical; this redox chemistry makes oxidoreductase activity correct, though the cyclase term (GO:0009975) better captures the net catalytic outcome. Reason: Accurate at the superfamily level (radical SAM redox chemistry) but less informative than the specific cyclase activity term; kept as non-core. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md Radical SAM enzymes use a **[4Feβ4S] cluster** to reductively cleave **S-adenosyl-L-methionine (SAM)** to generate a highly reactive **5β²-deoxyadenosyl radical (5β²-dAdoβ’)** |
| GO:0018189 pyrroloquinoline quinone biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Correct and core. PqqE catalyzes the first committed chemical step of PQQ biosynthesis - the radical SAM Glu-Tyr cross-linking of the PqqA precursor peptide. PQQ is the redox cofactor required by the periplasmic methanol/alcohol dehydrogenases central to methylotrophy in M. extorquens. Reason: Directly supported by experimental characterization in M. extorquens AM1; this is the biological process the core MF serves. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md catalyzes the **first committed chemical step** in pyrroloquinoline quinone (**PQQ**) biosynthesis |
| GO:0046872 metal ion binding | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Generic metal ion binding, redundant with the more specific iron-sulfur cluster binding terms (GO:0051536, GO:0051539) that describe PqqE's actual [4Fe-4S] and auxiliary Fe-S clusters. Reason: Subsumed by the specific iron-sulfur cluster binding terms; uninformative on its own. |
| GO:0051536 iron-sulfur cluster binding | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - PqqE binds the canonical radical SAM [4Fe-4S] cluster plus two auxiliary Fe-S clusters in its C-terminal SPASM domain. Slightly less specific than GO:0051539 for the catalytic cluster, but accurate for the auxiliary clusters as well. Reason: Accurate description of PqqE's multiple iron-sulfur clusters; the auxiliary SPASM-domain clusters make this broader term appropriate alongside GO:0051539. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md PqqE contains the canonical radical SAM cluster and **two auxiliary FeβS clusters (AuxI and AuxII)** in its C-terminal SPASM domain |
| GO:0051539 4 iron, 4 sulfur cluster binding | IEA GO_REF:0000120 | ACCEPT | Summary: Correct - PqqE binds the catalytic radical SAM [4Fe-4S] cluster coordinated by three cysteines and an exchangeable SAM. The crystal structure additionally assigns the AuxII auxiliary cluster as a [4Fe-4S] coordinated by three cysteines and Asp319. Reason: Most specific accurate term for the catalytic radical SAM cluster (and AuxII), supported by UniProt cofactor annotation and structural literature. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md AuxII** coordinates a canonical **[4Feβ4S] cluster** using three cysteines and **Asp319** |
| GO:1904047 S-adenosyl-L-methionine binding | IEA GO_REF:0000104 | ACCEPT | Summary: Correct - As a radical SAM enzyme, PqqE binds SAM, which is reductively cleaved by the [4Fe-4S] cluster to generate the 5'-deoxyadenosyl radical that abstracts a hydrogen from the glutamate side chain to initiate cross-linking. Reason: SAM is the co-substrate of the radical SAM reaction; binding is intrinsic to the catalytic mechanism. Supporting Evidence: file:METEA/pqqE/pqqE-deep-research-falcon.md supporting **Ξ²-H abstraction from glutamate** by the 5β²-dAdo radical and formation of a peptide-centered radical |
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)