Oxygen: Friend or foe? Archaeal superoxide dismutases in the protection ofintra- and extracellular oxidative stress

Both "environmental chemistry" and metabolic biochemical reactions can cons tantly generate in vivo free radicals and other oxygen-derived species that can cause severe damage to almost all biomolecules, especially to DNA, pro teins, and lipids. The superoxide anion has been shown to be the most readi ly generated and spread radical among organisms and it is a common intermed iate of oxidative stress processes in the cells. The antioxidant defense sy stem of superoxide dismutases (SOD) scavenges and minimizes the formation o f this radical, and thus plays a major role in reducing cumulative oxidativ e damage in different cell compartments both in aerobic and anaerobic cells . In the cell, cytosol SODs are constitutively present and induced by many oxidative agents able to raise the superoxide concentrations. Presence of S ODs, however, in extracellular cell-associated locations demonstrates how v aluable they are in maintaining the integrity of cells against oxidative st ress generated by the cell environment, particularly upon increased oxygena tion. Because SODs have recently been found in Archaea, which are prokaryot es, sometimes living in extreme environments, even in anaerobic ones, these enzymes can be considered essential: they may have allowed the evolution o f aerobic respiration starting from an ancient form of oxygen-insensitive l ife.