ABSTRACT
The amount of molecular oxygen in the atmosphere began to increase as the result of the evolution of cyanobacteria, which, using light energy, produced molecular oxygen from water and carbon dioxide. Then molecular oxygen was used by respiring organisms as the terminal electron acceptor in their energy-producing electron transfer chains, a process that involves generation of partially reduced, potentially toxic oxygen by-products. Intra- and extracellular nonspecific reductases that can act on paraquat have been identified in Saccharomyces cerevisiae. Evidence that oxygen radical species are continuously generated during aerobic life implies that aerobes evolved some strategies to survive in the presence of oxygen. The presence of copper at the catalytic site of Cu,ZnSOD suggests that copper availability can limit enzymatic activity, thus exerting a regulation at the posttranslational level. This is indeed the case in anaerobic S. cerevisiae , which contains a proenzyme requiring copper for activation.
