Characterisation of the PQQ cofactor radical in quinoprotein ethanol dehydrogenase of Pseudomonas aeruginosa by electron paramagnetic resonance spectroscopy.

The binding pocket of the pyrroloquinoline quinone (PQQ) cofactor in quinoprotein alcohol dehydrogenases contains a characteristic disulphide ring formed by two adjacent cysteine residues. To analyse the function of this unusual structural motif we have investigated the wild-type and a double cysteine:alanine mutant of the quinoprotein ethanol dehydrogenase from Pseudomonas aeruginosa by electron paramagnetic resonance (EPR) spectroscopy. Thus, we have obtained the principal values for the full rhombic g-tensor of the PQQ semiquinone radical by high-field (94 GHz) EPR necessary for a discrimination of radical species in dehydrogenases containing PQQ together with other redox-active cofactors. Our results show that the characteristic disulphide ring is no prerequisite for the formation of the functionally important semiquinone form of PQQ.