The Qrc Membrane Complex,Related to the Alternative Complex III,Is a Menaquinone Reductase Involved in Sulfate Respiration |
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Authors: | Sofia S. Venceslau Rita R. Lino Ines A. C. Pereira |
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Affiliation: | From the Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Oeiras 2780-157, Portugal |
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Abstract: | Biological sulfate reduction is a process with high environmental significance due to its major contribution to the carbon and sulfur cycles in anaerobic environments. However, the respiratory chain of sulfate-reducing bacteria is still poorly understood. Here we describe a new respiratory complex that was isolated as a major protein present in the membranes of Desulfovibrio vulgaris Hildenborough. The complex, which was named Qrc, is the first representative of a new family of redox complexes. It has three subunits related to the complex iron-sulfur molybdoenzyme family and a multiheme cytochrome c and binds six hemes c, one [3Fe-4S]+1/0 cluster, and several interacting [4Fe-4S]2+/1+ clusters but no molybdenum. Qrc is related to the alternative complex III, and we show that it has the reverse catalytic activity, acting as a Type I cytochrome c3:menaquinone oxidoreductase. The qrc genes are found in the genomes of deltaproteobacterial sulfate reducers, which have periplasmic hydrogenases and formate dehydrogenases that lack a membrane subunit for reduction of the quinone pool. In these organisms, Qrc acts as a menaquinone reductase with electrons from periplasmic hydrogen or formate oxidation. Binding of a menaquinone analogue affects the EPR spectrum of the [3Fe-4S]+1/0 cluster, indicating the presence of a quinone-binding site close to the periplasmic subunits. Qrc is the first respiratory complex from sulfate reducers to have its physiological function clearly elucidated. |
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Keywords: | Bacterial Metabolism Cytochromes Energy Metabolism Iron-Sulfur protein Membrane Proteins Quinones Respiratory Chain Hydrogen Metabolism Hydrogenase Sulfate Reduction |
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