Sulfide:quinone oxidoreductase in membranes of the hyperthermophilic bacterium Aquifex aeolicus (VF5)
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O R I G I N A L PA P E R
Tobias Nübel · Christof Klughammer · Robert Huber · Günter Hauska · Michael Schütz
Sulfide:quinone oxidoreductase in membranes of the hyperthermophilic bacterium Aquifex aeolicus (VF5)
Received: 13 October 1999 / Revised: 10 December 1999 / Accepted: 20 December 1999 / Published online: 14 February 2000 © Springer-Verlag 2000
Abstract The sulfide-dependent reduction of exogenous ubiquinone by membranes of the hyperthermophilic chemotrophic bacterium Aquifex aeolicus (VF5), the sulfide-dependent consumption of oxygen and the reduction of cytochromes by sulfide in membranes were studied. Sulfide reduced decyl-ubiquinone with a maximal rate of up to 3.5 µmol (mg protein)–1 min–1 at 20 °C. Rates of 220 nmol (mg protein)–1 min–1 for the sulfide-dependent consumption of oxygen and 480 nmol (mg protein)–1 min–1 for the oxidation of sulfide at 20 °C were estimated. The reactions were sensitive towards 2-n-nonyl-4-hydroxyquinoline-N-oxide, but insensitive towards cyanide. Both reduction of decyl-ubiquinone and consumption of oxygen by sulfide rapidly increased with increasing temperature. For the sulfide-dependent respiratory activity, a sulfideto-oxygen ratio of 2.3±0.2 was measured. This indicates that sulfide was oxidized to the level of zero-valent sulfur. Reduction of cytochromes by sulfide was monitored with an LED-array spectrophotometer. Reduction of cytochrome b was stimulated by 2-n-nonyl-4-hydroxyquinoline-N-oxide in the presence of excess sulfide under oxic conditions. This “oxidant-induced reduction” of cytochrome b suggests that electron transport from sulfide to oxygen in A. aeolicus employs the cytochrome bc complex via the quinone pool. Comparison of the amino acid sequence with the sequence of the sulfide:quinone oxidoreductase from Rhodobacter capsulatus and of the flav-
T. Nübel · G. Hauska · M. Schütz (✉) Lehrstuhl für Zellbiologie und Pflanzenphysiologie, Universität Regensburg, Universitätsstrasse 31, 93040 Regensburg, Germany e-mail: [email protected], Fax: +49-941-9433352 C. Klughammer Julius-von-Sachs-Institut für Biowissenschaften mit Botanischem Garten, Universität Würzburg, Würzburg, Germany R. Huber Lehrstuhl für Mikrobiologie und Archaeenzentrum, Universität Regensburg, Regensburg, Germany
ocytochrome c from Allochromatium vinosum revealed that the sulfide:quinone oxidoreductase from A. aeolicus belongs to the glutathione reductase family of flavoproteins. Key words Aquifex aeolicus · Sulfide oxidation · Sulfide:quinone oxidoreductase · Hyperthermophiles · Cytochrome bc complex Abbreviations SQR Sulfide:quinone oxidoreductase · UQ Ubiquinone · HQNO 2-Heptyl-4-hyroxyquinolineN-oxide · NQNO 2-n-Nonyl-4-hydroxyquinoline-N-oxide · FCC Flavocytochrome c
Introduction Utilization of inorganic reduced sulfur compounds as electron donors for phototrophic and chemolithotrophic growth of prokaryotes has been known for almost a century, and the mechanisms and enzymes involved have been investigated in numerous efforts (for reviews see Brune 1995; Kell
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