Pyrroloquinoline quinone (PQQ) is the organic cofactor in soybean lipoxygenase‐1

Treatment of soybean lipoxygenase‐1 (SLO) with phenylhydrazine (PH) induced inactivation and a maximum (350 nm) in the absorption spectrum of the enzyme. To detach the product having this absorption maximum, proteolysis was required. The product appeared to be the C(5) phenylhydrazone of pyrroloquinoline quinone (PQQ). Quantification showed that one covalently bound PQQ is present per enzyme molecule. Inspection of published spectroscopic data leads to the conclusion that the established cofactor of SLO, Fe, present as one ion per enzyme molecule, interacts with PQQ, the latter functioning as a terdentate ligand for the Fe ion (the N atom and COOH group of the quinoline ring together with the hydrated C(5) carbonyl group). If it is assumed that the substrate has two sites of interaction with this two‐cofactor complex, namely with the Fe ion and the C(5) carbonyl group of PQQ, the electron‐relay system generated very well explains the reported mechanistic features as well as inhibition effects of substrates with extended conjugation systems. The finding of PQQ in SLO is an additional indication that this cofactor is very versatile with respect to involvement in different types of redox reaction. A further implication might be that PQQ has been overlooked in other well‐known enzymes with an established cofactor but where the presence of this cofactor is unable to explain the mechanistic and spectroscopic features.