While essential to aerobic life, oxygen poses a great risk to organisms in the form of superoxide, O2-

eliminating the danger of the oxygen radical. The rate of this reaction is diffusion limited, up to 10 Ms. Several types of superoxide dismutase (SOD) exist. Fe and Mn SOD are both single metal active sites coordinated by an Asp, three histidines, and water. The Fe is found mainly in prokaryotes, and the Mn is found in both prokaryotes and eukaryotes. Ni SOD, found in cyanobacteria and some Streptomyces species, was only recently found to contain an active site with three cysteines and a histidine coordinated. While all the SODs are interesting, for our purposes, the Cu/Zn SOD is particularly interesting for several reasons, including its dual metal center. In this active site, the Cu(II) and Zn(II) are coordinated by the same histidine. Additionally, the Cu is bound by a solvent molecule and three other histidines, and the Zn is coordinated to two other histidines and an Asp. Once the Cu is reduced, the shared His ligand is released and the Cu moves about 1 Å to react with superoxide. The Zn does not participate in the mechanism for the disproportionation of superoxide, and it is believed it undergoes no redox reaction. The Zn has been shown to be important for the stabilization of the protein, but the delivery mechanism remains unknown. Zn(II) is a d molecule, and as such, very little probing of the Zn site can be done. The Cu, however, undergoes an oxidation state change and can be examined more easily. Due to the difficulty of probing Zn and the convenience of a second metal site (Cu), studies to determine the role of Zn are often paired with Cu, or the Zn is replaced with the typically paramagnetic Co (d).Here, what little is known about the Zn is put forth, as well as a discussion of the better understood Cu.