Sulfur-cutting enzyme up close

The primordial linkage between geology and life meets headon in sulfur-oxidizing enzymes within ancient microbes such as Acidianus ambivalens, which thrive in volcanic hot springs at 80 °C and pH 2. With a new X-ray crystallographic structure of this archaeon's sulfur oxygenase reductase, Carlos Frazão of the New University of Lisbon, in Portugal, and his colleagues have uncovered key details about how the enzyme processes elemental sulfur into bisulfite ion and hydrogen sulfide under aerobic conditions (Science 2006, 311, 996). "This enzyme bridges the geo- and biological global sulfur cycles," Frazão notes. When the enzyme's 24 protein monomers assemble, they form a sphere with a hollow starlike compartment (cross section shown), which hosts two dozen catalytic pockets. The key features of each are a cysteine molecule with an extra sulfur atom bound to it and a nearby iron center. Besides helping to explain the global sulfur cycle, the structural data should help scientists discern how certain microbes extract metals from sulfidic ores and produce environmentally troublesome acidic effluent in mines.