Pyrrolo-quinoline quinone (PQQ) is a redox coenzyme, which serves as a cofactor for a number of enzymes (quinoproteins) and particularly for some bacterial dehydrogenases [<cite idref="PUB00000066"/>, <cite idref="PUB00005344"/>]. A number of bacterial quinoproteins belong to this family. Quinoprotein dehydrogenase, conserved site