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Flavoproteins are variably able to catalyze both one- and two- electron redox reactions. The ring portion involving nitrogens 1 and 5 and carbon 4a reflects sequential addition or loss of electrons and hydrogen ions in one-electron processes and addition or loss of other transitory adducts with two-electron processes. There are nine chemically discernible redox forms, that is, three levels of oxidation-reduction and three species for acid, neutral, and base conditions. Of these only five have biological relevance because of pH considerations [2]. These biological forms are summarized in Figure5.3. It should also be noted that free flavin semiquinones (radicals) are quite unstable, and the free flavia hydroquinones react very rapidly with molecular oxygen to become reoxidized. The latter is kinetically dissimilar to reduced nicotinamide coenzymes, which reoxidize more slowly with O2 . Binding to specific enzymes markedly affects the kinetic stability of the half-reduced and reduced forms of flavocoenzymes and again allows for diverse reactions under biological situations. An example of a flavoprotein undergoing a one-electron transfer is with the microsomal NADPH-cytochrome P450 reductase. This enzyme contains both FAD and FMN, and the latter cycles between a neutral semiquinone and fully reduced form[9]. The FAD in the electron-transferring flavoprotein, which mediates electron flow from fatty acyl coenzyme A (CoA) to the mitochondrial electron transport chain, cycles between oxidized quinone and anionic semiquinone. In addition, a single-step, two-electron transfer from substrate can occur in the nucleophilic reactions shown in Figure5.4. Such cases as hydride ion transfer from reduced nicotinamide coenzymes or the carbanion generated by base abstraction of a substrate proton may lead to attack at the flavinN-5 positions; some nucleophiles, such as the hydrogen peroxide anion, are added on the C-4a position with its frontier orbital. [ Last edited by yxfei1220 on 2009-10-7 at 20:43 ] |
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2楼2009-10-02 11:06:30
wanglu3353
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3楼2009-10-03 07:37:34
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| 黄素蛋白是variably既能够催化单，双电子氧化还原反应。环部分涉及氮原子1和5，碳连续第4A反映除了在单电子过程，增加或其他暂时的加合物的损失或两电子过程的电子和氢离子的损失。有9个明显的氧化还原化学形式，即三个层次的氧化还原和3酸性，中性物种和基础条件。其中只有5个因素，因为pH值[2生物学意义]。这些生物的形式总结了Figure5.3。还应该指出，自由黄素半醌（自由基）很不稳定，自由弗拉维亚苯二酚分子氧发生反应非常迅速成为reoxidized。后者是动力学不同烟酰胺辅酶减少，这reoxidize缓慢与O2。绑定到特定的酶显着影响了一半动力学稳定性，减少和flavocoenzymes形式和减少重新生物的情况下允许不同的反应。的一个经历一个单电子转移黄素蛋白的例子是与微粒NADPH -细胞色素P450还原酶。这种酶包含基金会和黄素，中立之间醌，后者周期完全简化形式[9]。在电子风潮转移黄素蛋白，介导电子从脂酰辅酶A（CoA）流入线粒体电子传递链，循环氧化之间醌和醌阴离子。此外，单步，我们两基电子转移可能发生在Figure5.4所示的亲核反应。从减少烟酰胺辅酶或基片基抽象所产生的质子氢离子负离子转移这种情况可能会导致攻击的flavinN - 5的位置;例如过氧化氢负离子一些亲，被添加在C - 4A条位置其前线轨道 |
4楼2009-10-07 17:48:35
