Oxidative challenge in biology Antioxidant

the structure of antioxidant vitamin ascorbic acid (vitamin c).

a paradox in metabolism that, while vast majority of complex life on earth requires oxygen existence, oxygen highly reactive molecule damages living organisms producing reactive oxygen species. consequently, organisms contain complex network of antioxidant metabolites , enzymes work prevent oxidative damage cellular components such dna, proteins , lipids. in general, antioxidant systems either prevent these reactive species being formed, or remove them before can damage vital components of cell. however, reactive oxygen species have useful cellular functions, such redox signaling. thus, function of antioxidant systems not remove oxidants entirely, instead keep them @ optimum level.

the reactive oxygen species produced in cells include hydrogen peroxide (h2o2), hypochlorous acid (hclo), , free radicals such hydroxyl radical (·oh) , superoxide anion (o2). hydroxyl radical particularly unstable , react rapidly , non-specifically biological molecules. species produced hydrogen peroxide in metal-catalyzed redox reactions such fenton reaction. these oxidants can damage cells starting chemical chain reactions such lipid peroxidation, or oxidizing dna or proteins. damage dna can cause mutations , possibly cancer, if not reversed dna repair mechanisms, while damage proteins causes enzyme inhibition, denaturation , protein degradation.

the use of oxygen part of process generating metabolic energy produces reactive oxygen species. in process, superoxide anion produced by-product of several steps in electron transport chain. particularly important reduction of coenzyme q in complex iii, since highly reactive free radical formed intermediate (q·). unstable intermediate can lead electron leakage , when electrons jump directly oxygen , form superoxide anion, instead of moving through normal series of well-controlled reactions of electron transport chain. peroxide produced oxidation of reduced flavoproteins, such complex i. however, although these enzymes can produce oxidants, relative importance of electron transfer chain other processes generate peroxide unclear. in plants, algae, , cyanobacteria, reactive oxygen species produced during photosynthesis, particularly under conditions of high light intensity. effect partly offset involvement of carotenoids in photoinhibition, , in algae , cyanobacteria, large amount of iodide , selenium, involves these antioxidants reacting over-reduced forms of photosynthetic reaction centres prevent production of reactive oxygen species.