Bicarbonate-enhanced peroxidase activity of Cu, Zn SOD: is the distal oxidant bound or diffusible?

The Cu,Zn SOD catalyzes the bicarbonate-dependent oxidation of a wide range of substrates by H2O2. A mechanism in accord with this activity has been described. It involves the generation of a strong oxidant (Cu(I)O, Cu(II)OH, or Cu(III)) by reaction of the active site Cu with H2O2, followed by oxidation of bicarbonate to CO3− that in turn diffuses from the active site to oxidize the various substrates in free solution. Recently, an alternative mechanism, entailing firmly bound HCO3− and CO3−, has been proposed [J. Biol. Chem. 278 (2003) 21032–21039]. We present data supporting the diffusible CO3− and discuss the properties of this system that can be accommodated in this way and that preclude bound intermediates.