Positively harg canyon Figure 21-36 The active site of superoxide dismu- tase is located in a positively charged canyon(blue)that is surrounded by a ring of negatively charged residues (red). The superoxide anion is elec trostatically attracted to the positively charged active site. [Drawn from 3sod pdb. J. A. Tainer, E D. Getzoff, J.S. Richardson, and D.C. Richardson Nature306(1983):284.]