The name/synonym for ligand PQQ (curated from ChEBI, PubChem, and PDB databases):

1PYRROLOQUINOLINE QUINONE(3-)
2PYRROLOQUINOLINE QUINONE TRIANION
34,5-DIOXO-4,5-DIHYDRO-1H-PYRROLO[2,3-F]QUINOLINE-2,7,9-TRICARBOXYLATE
4PYRROLOQUINOLINE QUINONE