Copper-zinc superoxide dismutase catalysis

Smirnov V. V. Roth J. P. Mechanisms of electron transfer in catalysis by copper zinc superoxide dismutase. J. Am. Chem. Soc. 2006, 128, 16424—16425. 

The generation of O2 from potassium superoxide was also applied to stop-flow procedures. In this method O2 was dissolved in dimethyl sulfoxide and stabilized in 18-crown-6-polyether. This method is useful for mechanistic studies indeed, McClune and Fee (1976) were able to obtain catalytic rate constants for bovine copper/zinc superoxide dismutase as a function of pH in various buffers. More recently the mechanism of catalysis and of anion inhibition of iron superoxide dismutase from E. coli have been examined by this method using a specially constructed stop-flow spectrophotometer (Bull and Fee, 1985). A limitation of the method is that the pre-equilibrium state cannot be properly investigated because of the time resolution of the stop-flow equipment (== 5 msec). 

Zinc usually binds to proteins via residues of cysteine and histidine. Sometimes, zinc is bound to residues of glutamate or aspartate. The zinc ion sometimes plays a catalytic role and sometimes a structural role. In the latter case, it helps maintain the three-dimensional structure or conformation of the protein. For example, carboxypeptidase A contains two atoms of zinc. One is required for catalytic activity and is boimd to cysteine and histidine. The other, which plays a structural role, is bound only to cysteine. Cytoplasmic superoxide dismutase is a dimer. It contains one atom of Cu " and one of Zn per subunit. The zinc is boimd via three residues of histidine and one residue of aspartate. It is buried deep within the enzyme and serves a structural role. The copper atom is bound via four residues of histidine. It resides close to the surface of the protein and participates in the chemistry of catalysis. 

The dismutation of O2 by iron superoxide dismutase was found to be similar to that for the copper/zinc bovine superoxide dismutase. The results obtained by Lavelle et al. (1977) showed that catalysis of dismutation of O2 by the iron superoxide dismutase from Photobacterium leiognathi is first order with respect to substrate concentration for all ratios of substrate to enzyme concentrations reported. Although the enzyme is stable between pH 6.0 and 10.0, the value of the rate constant decreases as the pH increases. The second-order rate constant for the reaction be-... 

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