Superoxide dismutases: studies of structure and mechanism.

Abstract

Superoxide dismutases are enzymes of defense which serve to protect respiring cells against a product of their own respiration. This is achieved by a catalytic scavenging of the superoxide radical which, surprisingly, is a rather commonplace intermediate of the reduction of oxygen. The reaction catalyzed is: $$O_2^-+O_2^-+2{H^+ }H{}_2+O_2$$ Enhancing the rate of this particular reaction may appear to “be a peculiar undertaking for an enzyme, since this reaction is reasonably rapid, even in the absence of catalysis and since several transition metal cations serve as effective catalysts. Nevertheless superoxide dismutases are indispensable because they operate with ultimate efficiency and are present in abundance, whereas free transition metal cations are less effective and are not plentiful inside cells. It is a fact that all of the superoxide dismutase activity, which can be measured in crude extracts of such diverse materials as erythrocytes, mammalian liver or Escherichia coli, can be accounted for in terms of the superoxide dismutases which they contain. There is thus no significant superoxide dismutase activity in these extracts save that due to these enzymes.