| Topic | Key points | Evidence (with citation IDs) | Publication date & URL |
|---|---|---|---|
| Identity verification | **Target verified as UniProt Q49148 from *Methylorubrum extorquens* AM1** (syn. *Methylobacterium extorquens* AM1), ordered locus **MexAM1_META1p1751**. Critical caveat: AM1 literature contains a **historical naming swap** in which an older **pqqD** assignment corresponds to what later literature calls **pqqA**; thus Q49148 should be interpreted as the **precursor-peptide locus**, not the standalone PqqD chaperone characterized in many mechanistic studies. | Historical nomenclature mismatch and AM1-specific mapping of old pqqD to later pqqA are explicitly noted in reviews and AM1 pathway papers (pqac-00000001, pqac-00000002, pqac-00000003). | 2019-10-11, https://doi.org/10.1074/jbc.ra119.009684; 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001 |
| Protein type | The gene product corresponding to this AM1 locus is best understood as a **small ribosomally synthesized precursor peptide (RiPP precursor)** for PQQ biosynthesis, not an enzyme. Reported size in the literature is **~22-24 aa**, containing the conserved **Glu** and **Tyr** residues that furnish atoms to the PQQ core. | PqqA described as a short peptide precursor essential for PQQ formation, with conserved Glu/Tyr and mutagenesis support (pqac-00000000, pqac-00000002, pqac-00000003, pqac-00000006). | 2019-10-11, https://doi.org/10.1074/jbc.ra119.009684; 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001; 2021-06, https://doi.org/10.3389/fagro.2021.667339 |
| Pathway step | **Primary function:** substrate peptide for the first committed PQQ-biosynthetic transformation. In the cytosol, the **PqqD/PqqE system** acts on PqqA to install a **de novo C-C bond between Glu and Tyr** (cross-linked **PqqA\*** intermediate), after which proteolysis and downstream PqqB/PqqC chemistry complete PQQ formation. The peptide itself does **not catalyze a reaction**; it is the biosynthetic substrate. | Reviews and AM1 experimental work support PqqA as the precursor and PqqD/PqqE as the machinery for Glu-Tyr cross-linking; pathway schematic confirms this placement (pqac-00000000, pqac-00000003, pqac-00000005, pqac-00000007, pqac-00000011). | 2019-10-11, https://doi.org/10.1074/jbc.ra119.009684; 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001; 2026-02, https://doi.org/10.1039/d5cs00585j |
| Key interactions | **PqqA-PqqD:** high-affinity, specific binding; **PqqD-PqqE:** direct interaction documented by multiple biophysical methods. PqqD functions as a **peptide chaperone/RRE-like factor**, presenting PqqA to radical SAM enzyme PqqE. Thus, for Q49148, the biologically relevant interaction network is **precursor peptide \u2192 chaperone (PqqD) \u2192 maturase (PqqE)**. | Tight PqqD-PqqA complex and mapped PqqD-PqqE contacts shown by native MS/SPR/ITC/NMR/EPR/HDX-type evidence summarized in review literature; AM1 studies support the same pathway logic (pqac-00000000, pqac-00000001, pqac-00000005, pqac-00000007). | 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001; 2026-02, https://doi.org/10.1039/d5cs00585j |
| Localization | **Biosynthesis stage:** expected **cytosolic** localization, because PqqA, PqqD, PqqE, and early tailoring/proteolysis steps operate on the intracellular peptide precursor. **Physiological end use:** mature **PQQ** then serves as a redox cofactor for **periplasmic methanol dehydrogenases** such as MxaFI/XoxF in methylotroph physiology. | Cytosolic pathway logic for precursor processing and direct linkage of PQQ to methanol dehydrogenase in methylotrophs are supported in pathway reviews and AM1/methylotroph literature (pqac-00000003, pqac-00000010). | 2019-10-11, https://doi.org/10.1074/jbc.ra119.009684; 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001 |
| Engineering / application links | **Functional significance:** PQQ biosynthesis underpins **PQQ-dependent alcohol/methanol dehydrogenases**, including **lanthanide-linked methylotrophic systems**. In AM1, PQQ is connected to methanol oxidation and rare-earth-dependent metabolism; AM1 has also been developed for **REE bioleaching/recovery**. More broadly, engineered PQQ production has reached industrially relevant levels in methylotrophs: **1.52 g/L PQQ**, **40.3 mg/g DCW**, after **144 h** in a **5-L fed-batch** *Hyphomicrobium denitrificans* process; heterologous systems cited in the same study yielded **2 mg/L in *E. coli***, **0.56-0.78 mg/L in engineered *Klebsiella pneumoniae***, **~51.3 mg/L in *Gluconobacter***, and a cell-free system converted **~2.5 mg/mL PqqA** to PQQ at **70-80% conversion**. | Quantitative production statistics from recent engineering study; AM1 application to REE leaching/recovery and link between PQQ and methylotrophic/lanthanide systems from recent environmental biotechnology work; PQQ-MDH link from foundational review (pqac-00000009, pqac-00000010). | 2023-01-24, https://doi.org/10.1186/s13068-023-02261-y; 2023-12-19, https://doi.org/10.1021/acs.est.3c06775; 2020-12, https://doi.org/10.1016/j.cbpa.2020.05.001 |


*Table: This table summarizes the identity, biochemical role, naming ambiguity, interaction partners, localization, and application relevance of UniProt Q49148 in *Methylorubrum extorquens* AM1. It is designed to help distinguish the AM1 precursor-peptide locus from the separate PqqD chaperone discussed in broader PQQ literature.*