pqqA (synonym pqqD; locus MexAM1_META1p1751) encodes the 29-amino acid ribosomally synthesized precursor peptide for pyrroloquinoline quinone (PQQ) biosynthesis in Methylorubrum (Methylobacterium) extorquens AM1. PqqA is not an enzyme; it is the biosynthetic substrate of the pathway. Its conserved glutamate (Glu16) and tyrosine (Tyr20) residues are recognized by the radical-SAM enzyme PqqE (presented via the PqqD chaperone) and cross-linked through a de novo C-C bond (CROSSLNK 16..20, Glu-Tyr); the modified core is then excised by proteolysis and oxidatively matured (PqqB hydroxylase, PqqC oxidase) into the mature PQQ cofactor. The peptide is synthesized and processed in the cytosol, whereas mature PQQ is deployed by periplasmic methanol dehydrogenases (Ca-dependent MxaFI and lanthanide-dependent XoxF) central to AM1 methylotrophy. Notably, deletion and insertion mutants of pqqA in M. extorquens AM1 still grow on methanol and produce PQQ, indicating that this organism has an alternative source for the cross-linked precursor; nonetheless PqqA represents the canonical, conserved PQQ biosynthetic substrate. NOTE on nomenclature - in older AM1 literature the precursor-peptide locus was called pqqD, which collides with the modern usage of "PqqD" for the separate peptide chaperone; UniProt Q49148 is the precursor peptide (PqqA), not the chaperone.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0018189 pyrroloquinoline quinone biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Correct biological-process annotation. PqqA is the precursor peptide substrate for PQQ biosynthesis, contributing the conserved Glu16 and Tyr20 residues that are cross-linked (CROSSLNK 16..20, Glu-Tyr) and excised to build the PQQ core. UniProt states the peptide is required for PQQ biosynthesis, and the falcon deep-research synthesis confirms PqqA is the ribosomally synthesized precursor peptide of the pathway rather than a catalytic enzyme. The IEA annotation is the most appropriate representation of its core role. Reason: PqqA is not a standalone enzyme, but it is the pathway precursor peptide required for pyrroloquinoline quinone biosynthesis; the BP annotation correctly captures its core function. Supporting Evidence: file:METEA/pqqA/pqqA-uniprot.txt Required for coenzyme pyrroloquinoline quinone (PQQ) file:METEA/pqqA/pqqA-uniprot.txt Pyrroloquinoline quinone (Glu-Tyr) file:METEA/pqqA/pqqA-goa.tsv pyrroloquinoline quinone biosynthetic process file:METEA/pqqA/pqqA-deep-research-falcon.md PqqA as a short peptide precursor file:METEA/pqqA/pqqA-deep-research-falcon.md The peptide itself does **not catalyze a reaction**; it is the biosynthetic substrate. |
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Download this section (compressed HTML)Q: What is the alternative source of the cross-linked Glu-Tyr precursor that allows M. extorquens AM1 pqqA deletion mutants to still produce PQQ?
Experiment: Delete pqqA and complement with wild-type and Glu16/Tyr20 substitution variants, then quantify cellular PQQ and methanol dehydrogenase activity; proteomically search for a substitute precursor peptide in the pqqA deletion background.
Hypothesis: PqqA supplies the Glu-Tyr precursor for PQQ production, but a redundant peptide substitutes in AM1.
Type: targeted genetics and metabolite analysis
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