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 Respiratory chains rely on membrane-bound protein complexes to perform a stepwise transduction of the free energy of redox reactions into an electrochemical potential difference across the membrane that drives the synthesis of ATP. In aerobic eukaryotic organisms, a fixed and mostly linear respiratory chain is present, in contrast to bacteria and archaea whose flexible and usually highly branched respiratory chains reflect their ability to use an array of different electrons donors and acceptors (1). This flexibility enables prokaryotes to explore virtually every possible energy source and oxidant pairs that can provide energy to sustain life and, thus, colonize multiple habitats on earth, including the most extreme environments. In addition, the recycling of elements provided by bacterial energy metabolism is a crucial component for the maintenance of life in our planet (2). The study of bacterial respiratory complexes (BRCs) 3 has highlighted the diversity of mechanisms by which energy conservation can be achieved (3, 4). BRCs usually have simpler compositions than their eukaryotic counterparts and are, therefore, useful models to understand the molecular mechanisms involved in those processes. Furthermore, bacteria contain many unique BRCs whose architectures have highly modular characters, with dif-