Cellular defenses against superoxide and hydrogen peroxide

Abstract

Life evolved in an anaerobic world; therefore, fundamental enzymatic mechanisms and biochemical pathways were refined and integrated into metabolism in the absence of any selective pressure to avoid reactivity with oxygen. After photosystem II appeared, environmental oxygen levels rose very slowly. During this time, microorganisms acquired oxygen tolerance by jettisoning enzymes that use glycyl radicals and exposed low-potential iron-sulfur clusters, which can be directly poisoned by oxygen. They also developed mechanisms to defend themselves against superoxide (O(2)()) and hydrogen peroxide, partially reduced oxygen species that are generated as inadvertent by-products of aerobic metabolism. Contemporary organisms have inherited both the vulnerabilities and the defenses of these ancestral microbes. Current research seeks to identify these, and bacteria comprise an exceptionally accessible experimental system that has provided many of the answers. This manuscript reviews recent developments and identifies remaining puzzles.

Fig. 1

The redox states of oxygen with standard reduction potentials. The standard concentration of oxygen was regarded as 1 M.

Fig. 2

Sources of oxidative stress for bacteria include (1) intracellular enzyme autoxidation, (2) environmental redox reactions, (3) H₂O₂ released by competing microbes, (4) phagosomal NADPH oxidase, and (5) redox-cycling antibiotics.

Fig. 3

Known mechanisms by which H₂O₂ and O₂⁻ injure cells include Fenton-mediated damage to proteins and DNA, the oxidation of solvent-exposed [4Fe-4S] clusters, apparent inhibition of transketolase, and disruption of the sulfur assimilatory pathway. The details of inactivation of transketolase and of sulfur metabolism remain unclear.

Fig. 4

Metal metabolism is altered during H₂O₂ stress. The Suf system restores cluster assembly, Dps sequesters unincorporated iron, Fur represses iron import, and ferrochetalase (Fch) ensures continued heme activation. The basis of protection by induced manganese import is uncertain.