There can be few elements with a biochemistry as coherent as that of sulfur. This important element is crucial to myriad aspects of metabo- lism, catalysis, and structure. The plurality of functions in which sulfur is involved derives squarely from the numerous oxidation states in which it may exist, some having great stability, some being capable of ready redox interconversions, and yet others having great instability. As a result, the flux of sulfur...