Pyrroloquinoline quinone biosynthesis

Pyrroloquinoline quinone (PQQ) is made from conserved glutamate and tyrosine residues in the short ribosomally synthesized precursor peptide PqqA. A PqqD-family peptide chaperone presents PqqA to the radical-SAM enzyme PqqE, which forms the defining carbon-carbon cross-link. Proteolytic processing and PqqB-dependent oxygenation both contribute to formation of a late small-molecule precursor, although their relative order is unresolved. PqqC completes oxidative ring closure to mature PQQ. PQQ export and use by quinoprotein dehydrogenases are downstream of this module.

This is a reusable pathway module. Pseudomonas putida KT2440 proteins are representative members, not a taxonomic restriction or an exhaustive member list. Two PqqD paralogs from KT2440 illustrate that one family role may have multiple concrete implementations; the module does not require two PqqD proteins in every organism. Proteolytic processing and PqqB oxygenation are maturation operations, but their exact substrate handoff and order are left unresolved. PP_0375 is a strong Pseudomonas pqqG candidate, but its S9 peptidase family differs from the M16B PqqG partner characterized in some other bacteria; neither is forced into the conserved core without a defined reaction. PQQ-dependent dehydrogenases, transport, pathway regulation, and unrelated proteins adjacent to pqq loci are outside the boundary. The OpenScientist module report inconsistently states both that proteolysis must precede small-molecule steps and that proteolysis-versus-PqqB ordering is unresolved; the latter, evidence-limited interpretation is retained here. The species-aware report's proposed GO:0033735 identifier for PqqC and its use of biological-process GO:0018189 as though it captured PqqE molecular function were rejected. Its neighborhood-based PqqD2 specialization remains a hypothesis; PqqD1 and PqqD2 are exemplars of one nonduplicated family role.