Quinoproteins.

As used today, the word quinoprotein defines three distinct groups of enzymes. Before 1979, the structures of the essential, quinonoid oxidation-reduction cofactors were a mystery for all these enzymes. The first proteins proven to harbor this type prosthetic group are those with noncovalently bound pyrroloquinoline quinone (PQQ). PQQ-containing enzymes can be described as alcohol dehydrogenases, with the exception of a single protein, which is an amine dehydrogenase. More recently, it was discovered that copper-containing amine oxidases contain 6-hydroxydopa quinone, also known as topa quinone (TQ), whereas certain bacterial amine dehydrogenases require 2',4-bitryptophan-6,7-dione (tryptophan tryptophylquinone, TTQ) for activity. These latter two quinones are formed, by unknown processes, from a specific tyrosyl residue for the amine oxidases, and from two widely separate tryptophyl residues in the polypeptide of the amine dehydrogenases.