A reduction-oxidation (redox) reaction is a process in which changes in oxidation states or oxidation numbers take place. Many transition metals exist in several stable oxidation states, which render them particularly interesting also in biological redox chemistry. Redox reactions play a central role in biochemistry; pertinent examples are photosynthesis and respiration where cascades of electron transfer reactions are coupled to synthesis of high-energy molecules like ATP and similar compounds. However, one of the expenses for living under oxygen rich conditions is the danger of unwanted radical formations. Oxygen easily gets reduced to hydrogen peroxide, and in the presence of reducing metal ions like Fe2+ or Cu+ further reactions may take place like the Fenton reaction, generating hydroxyl radicals:
Fortunately, the organism possesses a number of effective chelates, proteins like albumin, transferrin, and the like that, to a certain limit, will sequester redox-active iron- and copper ions.
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