This first lecture on molecular oxygen has laid a number of chemical foundations for the study of oxygen reactivity in biological systems. Oxygen arrived on earth in a way by accident, being the product of photosynthesis, which uses water as an electron donor for the conversion of carbon dioxide into sugars. The origin of oxygen and the evolution of its concentration in the Earth's atmosphere were briefly discussed during this lecture, raising in particular the question of oxygen's sudden appearance, still poorly understood, some 2.2 billion years ago. However, its potential oxidizing properties have since been used extensively in a wide variety of metabolic and biosynthetic reactions. This use, which is thermodynamically very favorable, is not simple because, due to its paramagnetic nature, oxygen reacts with organic substrates extremely slowly (spin prohibition). It therefore involves very fine mechanisms for activating the oxygen or the substrates to be oxidized. These different mechanisms were briefly presented and illustrated with examples of the metalloenzymes involved in these activation processes.
In Abstract, activation of a substrate involves its transformation into intermediate radical species that are highly reactive towards oxygen, while activation of oxygen involves its reduction to peroxide and activation of the peroxide by metal ions (in particular iron and copper).
