Teaching the fundamentals of electron transfer reactions in mitochondria and the production and detection of reactive oxygen species

Mitochondria fulfill a number of biological functions which inherently depend on ATP and O(2)(−•)/H(2)O(2) production. Both ATP and O(2)(−•)/H(2)O(2) are generated by electron transfer reactions. ATP is the product of oxidative phosphorylation whereas O(2)(−•) is generated by singlet electron reduction of di-oxygen (O(2)). O(2)(−•) is then rapidly dismutated by superoxide dismutase (SOD) producing H(2)O(2). O(2)(−•)/H(2)O(2) were once viewed as unfortunately by-products of aerobic respiration. This characterization is fitting considering over production of O(2)(−•)/H(2)O(2) by mitochondria is associated with range of pathological conditions and aging. However, O(2)(−•)/H(2)O(2) are only dangerous in large quantities. If produced in a controlled fashion and maintained at a low concentration, cells can benefit greatly from the redox properties of O(2)(−•)/H(2)O(2). Indeed, low rates of O(2)(−•)/H(2)O(2) production are required for intrinsic mitochondrial signaling (e.g. modulation of mitochondrial processes) and communication with the rest of the cell. O(2)(−•)/H(2)O(2) levels are kept in check by anti-oxidant defense systems that sequester O(2)(−•)/H(2)O(2) with extreme efficiency. Given the importance of O(2)(−•)/H(2)O(2) in cellular function, it is imperative to consider how mitochondria produce O(2)(−•)/H(2)O(2) and how O(2)(−•)/H(2)O(2) genesis is regulated in conjunction with fluctuations in nutritional and redox states. Here, I discuss the fundamentals of electron transfer reactions in mitochondria and emerging knowledge on the 11 potential sources of mitochondrial O(2)(−•)/H(2)O(2) in tandem with their significance in contributing to overall O(2)(−•)/H(2)O(2) emission in health and disease. The potential for classifying these different sites in isopotential groups, which is essentially defined by the redox properties of electron donator involved in O(2)(−•)/H(2)O(2) production, as originally suggested by Brand and colleagues is also surveyed in detail. In addition, redox signaling mechanisms that control O(2)(−•)/H(2)O(2) genesis from these sites are discussed. Finally, the current methodologies utilized for measuring O(2)(−•)/H(2)O(2) in isolated mitochondria, cell culture and in vivo are reviewed.