ABSTRACT
Atomic oxygen is the most abundant element in the earth’s crust; molecular oxygen in the atmosphere and water is required to support all forms of aerobic life. The present oxygen reservoir (37 Emol, 1 Emol=1018 moles) has built up as a result of photosynthesis, a process that liberates dioxygen from water. It is kept approximately constant by respiration, in which O2 is used as the ultimate electron acceptor. In addition, oxygen atoms are “fixed” into various organic molecules by a variety of enzymes (e.g. oxygenases) and non-enzymatic processes (Gilbert, 1981; Elstner, 1982, 1987). Aerobic organisms must, however, cope with the adverse effects of oxygen. At higherthanatmospheric concentrations, dioxygen may inhibit or inactivate certain enzymes and it also competes with photosynthetic CO2 fixation by ribulose-1,5-bisphosphate carboxylase/ oxygenase, increasing the energetic cost of photosynthesis. Still, the toxic effect of oxygen is mainly exerted by its reactive derivatives, whereas ground-state dioxygen is rather unreactive and can peacefully co-exist with organic matter. This characteristic is explained by the parallel spins of two unpaired electrons of dioxygen, imposing an energetic barrier on its reaction with non-radical compounds (the “spin restriction”). In order to become chemically reactive, dioxygen must be physically or chemically activated (Table 1).
