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Midpoint potentials of hemes a and a(3) in the quinol oxidase from Acidianus ambivalens are inverted
Zitatschlüssel ISI:000232257100044
Autor Todorovic, S and Pereira, M M and Bandeiras, T M and Teixeira, M and Hildebrandt, P and Murgida, D H
Seiten 13561-13566
Jahr 2005
ISSN 0002-7863
DOI 10.1021/ja052921l
Adresse 1155 16TH ST, NW, WASHINGTON, DC 20036 USA
Journal J. Am. Chem. Soc
Jahrgang 127
Nummer 39
Monat OCT 5
Verlag AMER CHEMICAL SOC
Zusammenfassung The aa(3) type B oxygen reductase from the thermophilic archaeon Acidianus ambivalens (QO) was immobilized on silver electrodes and studied by potential-dependent surface-enhanced resonance Raman (SERR) spectroscopy. The immobilized enzyme retains the native structure at the level of the heme pockets and exhibits reversible electrochemistry. From the potential dependence of specific spectral marker bands, the midpoint potentials of hemes a and a(3) were unambiguously determined for the first time, being 320 +/- 20 mV for the former and 390 +/- 20 mV for the latter. Both hemes could be treated as independent one-electron Nernstian redox couples, indicating that the interaction potential is smaller than 50 mV. The reversed order of the midpoint potentials compared to those of type A (mitochondrial-like) oxidases, as well as the lack of substantial Coulombic interactions, suggests a different mechanism of electroprotonic energy transduction. In contrast to type A enzymes, a-a(3) intraprotein electron transfer in QO is already guaranteed by the order of the midpoint potentials at the onset of enzyme reduction and, therefore, does not require a complex network of cooperativities to ensure exergonicity. In the immobilized state, conformational transitions of the QO a(3)-Cu-B active site, which are believed to be essential for proton translocation, are drastically slowed compared to those in solution. We ascribe this finding to the effect of the interfacial electric field, which is of the same order of magnitude as in biological membranes. These results suggest that the membrane potential may play an active role in the regulation of the enzymatic activity of QO.
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