The six-electron reduction of sulfite to sulfide is the pivot point of the biogeochemical cycle of the element sulfur'. The octahaem cytochrome c MccA (also known as SirA) catalyses this reaction for dissimilatory sulfite utilization by various bacteria. It is distinct from known sulfite reductases because it has a substantially higher catalytic activity and a relatively low reactivity towards nitrite. The mechanistic reasons for the increased efficiency of MccA remain to be elucidated. Here we show that anoxically purified MccA exhibited a 2- to 5.5-fold higher specific sulfite reductase activity than the enzyme isolated under oxic conditions2. We determined the threedimensional structure of MccA to 2.2 A resolution by single-wavelength anomalous dispersion. We find a homotrimer with an unprecedented fold and haem arrangement, as well as a haem bound to a CX15CH motif'. The heterobimetallic active-site haem 2 has a Cu(I) ion juxtaposed to a haem cat a Fe-Cu distance of 4.4 A. While the combination of metals is reminiscent of respiratory haem-copper oxidases, the oxidation-labile Cu(I) centre of MccA did not seem to undergo a redox transition during catalysis. Intact MccA tightly bound SO2 at haem 2, a dehydration product of the substrate sulfite that was partially turned over due to photoreduction by X-ray irradiation, yielding the reaction intermediate SO. Our data show the biometal copper in a new context and function and provide a chemical rationale for the comparatively high catalytic activity of MccA.