Pyrroloquinoline Quinone as Cofactor in Galactose Oxidase

Galactose oxidase from Dactylium dendroides was shown to contain one molecule of covalently bound pyrroloquinoline quinone (PQQ)/enzyme molecule. From the spectroscopic characteristics reported for the enzyme forms, a mechanistic role for PQQ could be deduced. In analogy with other quinoproteins, the initial formation of a PQQ-substrate adduct is proposed. Following internal hydrogen transfer, leading to aldehyde product and reduced pyrroloquinoline quinone, reoxidation of the organic cofactor with molecular oxygen could be mediated by the PQQ-liganded copper ion with concomitant formation of hydrogen peroxide. With PQQ as an additional (two-electron) redox center the occurrence of a “superoxidized” enzyme form must be considered. Possible consequences of this view, in relation to a physiological function of the enzyme and interpretation of ESR data, are discussed.