Oxygen bound molecular, reactivity

It should be mentioned that electron transfer to the quinone pool, both by PS II and by the reaction centers of purple bacteria, now proceeds via a two-electron gating mechanism after one electron has arrived at the temporarily bound quinone Qb, the semiquinone remains in its unprotonated, negatively charged form and tightly bound to the reaction center only after a second photoreaction does its full reduction, protonation and release as a quinol take place [19]. This procedure may also have played a role in the selection of the dimeric reaction center structure, but its importance most likely has to do with the reactivity of semiqui-nones with molecular oxygen and in that case it probably appeared much later. 

Compounds such as superoxide anion and peroxides do not directly interact with lipids to initiate oxidation they interact with metals or oxygen to form reactive species. Superoxide anion is produced by the addition of an electron to the molecular oxygen. It participates in oxidative reactions because it can maintain transition metals in their active reduced state, can promote the release of metals that are bound to proteins, and can form the conjugated acid, perhydroxyl radical depending on pH, which is a catalyst of lipid oxidation (39). The enzyme superoxide dismu-tase that is found in tissues catalyzes the conversion of superoxide anion to hydrogen peroxide. 

The two flavins present in AbzCoA-M/R are not equivalent. The flavin responsible for the reductase activity is bound weakly compared to the other and can dissociate during purification. The flavins also differ in their reactivity. In the absence of 2-aminobenzyl-CoA, only one flavin reduces rapidly ( 70s ) when the enzyme is mixed with NADH. However, when CoA substrate is present, both flavins reduce rapidly ( 70 and 2 s ), indicating that the presence of substrate stimulates reduction of the hydroxylase flavin. In addition, the reduced hydroxylase flavin reacts quickly with molecular oxygen, while the reductase flavin does not. While the two flavins are quite different kinetically, they are indistinguishable thermodynamically. Only a single flavin redox potential can be measured for the enzyme, indicating that the flavins have identical potentials. The redox potential of the free enzyme is —219 mV, while in the presence of 2-aminobenzyl-CoA it is -202 mV. ... 

With certain transition-metal complexes, oxygen adducts may be formed, sometimes reversibly (see Sections 24-A-l, 25-E and 25-F). Although the Oz entity remains intact, the complexes may be described as having coordinated 02 or O2- ions these may be bound to the metal in a three-mem-bered ring or may act as a bridging group. Coordinated molecular oxygen is more reactive than the free molecule, and various substances not directly oxidized under mild conditions can be attacked in presence of metal complexes. 

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