22-9 Oxidation-Reduction Processes in Nature Dids-Nhier Re amgga2sa0a0ran 忽e8 eprotect cell membraefrom +4n+4e一2H0 PROPAGATION STEPI H R 的 0。 10 10 The double bonds in α,β-unsaturated ketones also undergo cycloadditions to dienes: 22-9 Oxidation-Reduction Processes in Nature Ubiquinones mediate the biological reduction of oxygen to water. Nature utilizes the benzoquinone-hydroquinone redox couple in the reversible oxidation reactions of the cascade of steps by which molecular oxygen is used in biochemical degradations. The ubiquinones, collectively called coenzyme Q, CoQ or Q, are an important series of compounds used for this purpose. An enzyme system that utilized NADH, a biological reductant, converts Q into its reduced form, QH2. QH2 then participates in a series of reactions involving the cytochromes (electron-transporting iron containing proteins) and ending with the reduction of O2 to water: Phenol derivatives protect cell membranes from oxidative damage. Two intermediates in the conversion of molecular oxygen into water are the highly reactive radicals superoxide, O2 - ·, and the hydroxyl radical, ·OH, which arises from the cleavage of H2O2. An example of the damage caused by reactive radicals is illustrated by the reaction with a phosphoglyceride containing an unsaturated fatty acid: Cleavage of the relatively weak O-O bond gives rise to an alkoxy radical which may decompose (β-scission) to an unsaturated aldehyde. Related, but more complex mechanisms, yield unsaturated hydroxyaldehydes, such as trans-4-hydroxy-2-nonenal and propanedial