Structure of the Pyrroloquinoline Quinone Radical in Quinoprotein Ethanol Dehydrogenase*
- ‡Institut für Experimentalphysik, Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany and §Fachgebiet Technische Biochemie, Institut für Biotechnologie, Technische Universität Berlin, 13353 Berlin, Germany
- ↵1 To whom correspondence should be addressed: Institut für Experimentalphysik, Fachbereich Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany. Tel.: 49-30-8385-3394; Fax: 49-30-8385-6046; E-mail: chris.kay{at}physik.fu-berlin.de.
Abstract
Quinoprotein alcohol dehydrogenases use the pyrroloquinoline quinone (PQQ) cofactor to catalyze the oxidation of alcohols. The catalytic cycle is thought to involve a hydride transfer from the alcohol to the oxidized PQQ, resulting in the generation of aldehyde and reduced PQQ. Reoxidation of the cofactor by cytochrome proceeds in two sequential steps via the PQQ radical. We have used a combination of electron nuclear double resonance and density functional theory to show that the PQQ radical is not protonated at either O-4 or O-5, a result that is at variance with the general presumption of a singly protonated radical. The quantum mechanical calculations also show that reduced PQQ is unlikely to be protonated at O-5; rather, it is either singly protonated at O-4 or not protonated at either O-4 or O-5, a result that also challenges the common assumption of a reduced PQQ protonated at both O-4 and O-5. The reaction cycle of PQQ-dependent alcohol dehydrogenases is revised in light of these findings.
- Received October 12, 2005.
- The American Society for Biochemistry and Molecular Biology, Inc.
