The octahaem MccA is a haem c–copper sulfite reductase

Sulfite-reducing microbes couple the reduction of sulfite to the generation of a proton motive force that sustains organismic growth; here, two X-ray crystal structures are solved of MccA, a c -type cytochrome enzyme with eight haem groups that catalyses the six-electron reduction of sulfite to sulfide at a novel haem–copper active site. A second metal in highly reactive sulfite reductase Sulfate-reducing microbes obtain energy by coupling the reduction of sulfate and sulfite to the generation of a proton motive force. In this manuscript, the authors solve the X-ray crystal structure of MccA, a highly active sulfite reductase from the bacterium Wolinella succinogenes . Surprisingly, the authors find that the enzyme's active site contains a second metal ion beside haem iron — identified as Cu( I ) — bridging two cysteine thiolates. In contrast to the copper in respiratory haem–copper oxidases, the Cu( I ) seems not to undergo redox transition during catalysis, placing the biometal copper in a new context that may contribute to the notably high catalytic activity of MccA. The six-electron reduction of sulfite to sulfide is the pivot point of the biogeochemical cycle of the element sulfur 1 . 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 conditions 2 . We determined the three-dimensional structure of MccA to 2.2 Å 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 CX 15 CH motif 3 . The heterobimetallic active-site haem 2 has a Cu(I) ion juxtaposed to a haem c at a Fe–Cu distance of 4.4 Å. 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 SO 2 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 bio