Figure 1
From: The pyrroloquinoline quinone biosynthesis pathway revisited: A structural approach
Chemical structure of PQQ. Chemical structure of PQQ (4,5-dihydro-4,5-dioxo-1H-pyrrolo-[2,3-f]quinoline-2,7,9-tricarboxylic acid) with atom nomenclature. All carbon and nitrogen atoms of PQQ are derived from conserved tyrosine and glutamate residues of the PqqA peptide. R1 and R3 represent the N- and C-terminal portions of PqqA, respectively. R2 represent a three-amino-acid linker between Glu and Tyr [26, 37, 38].