Pyrroloquinoline quinone

Pyrroloquinoline quinone, or PQQ, is a redox-active cofactor with a fused-ring quinone structure and three carboxylic acid groups. Its neutral-acid formula is ${\mathrm{C}}_{14}{\mathrm{H}}_6{\mathrm{N}}_2{\mathrm{O}}_8$. Protonation and counterions must be specified when comparing a solution species or salt with that neutral formula.^[ref]

One established role is in the cytochrome-linked class of methanol dehydrogenase. There, PQQ is associated with calcium at the active site; oxidation of an alcohol supplies reducing equivalents that can pass onward to a cytochrome c acceptor. This is a different cofactor system from NAD-dependent methanol oxidation.^[ref]

A schematic redox balance isThis represents a two-electron reduced state and suppresses the separate ionization states of the carboxylic groups. It is an electron-accounting model, not a complete enzyme mechanism.

The cofactor's regeneration explains why cofactor amount and substrate conversion need not match one-to-one. In a stipulated example, ten cofactor molecules each completing ten cycles could support 100 substrate turnovers, provided the rest of the system restores the active cofactor state each time. Measuring substrate conversion therefore cannot by itself count PQQ molecules. Its role as a bacterial enzyme cofactor also does not establish a human dietary requirement.