活性氧和硫物种:犯罪伙伴

N. Blackstone
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摘要

复杂性的出现需要合作,但选择通常有利于不合作的脱北者。这种进化冲突可以通过各种机制来缓解,从而使复杂性得以出现。化学渗透就是这样一种机制。在共生关系中,化学渗透伙伴仅从出口产品中获益。不这样做可能导致电子载流子高度减少和有害量的活性氧物种。然而,这种机制在生命史上的作用(例如,真核生物起源于原核生物)似乎值得怀疑,因为大气中的氧气水平要低得多,海洋基本上是厌氧的。在这种情况下,应考虑硫的作用。最后一个真核生物共同祖先(LECA)是兼性需氧菌。在厌氧条件下,LECA可能进行各种形式的厌氧代谢。例如,苹果酸歧化,其中苹果酸既被氧化又被还原,允许NADH的再氧化。末端电子受体富马酸盐在还原时形成琥珀酸酯。当存在氧气时,过量的琥珀酸酯会导致电子逆向流动,形成高水平的活性氧。在厌氧条件下,可能已经形成了反应性硫物种。因此,即使在元古界的低大气氧气水平下,消除最终产物也可能具有选择性优势。
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Reactive Oxygen and Sulfur Species: Partners in Crime
The emergence of complexity requires cooperation, yet selection typically favors defectors that do not cooperate. Such evolutionary conflict can be alleviated by a variety of mechanisms, allowing complexity to emerge. Chemiosmosis is one such mechanism. In syntrophic relationships, the chemiosmotic partner benefits simply from exporting products. Failure to do this can result in highly reduced electron carriers and detrimental amounts of reactive oxygen species. Nevertheless, the role of this mechanism in the history of life (e.g., the origin of eukaryotes from prokaryotes) seems questionable because of much lower atmospheric levels of oxygen and a largely anaerobic ocean. In this context, the role of sulfur should be considered. The last eukaryotic common ancestor (LECA) was a facultative aerobe. Under anaerobic conditions, LECA likely carried out various forms of anaerobic metabolism. For instance, malate dismutation, in which malate is both oxidized and reduced, allows re-oxidizing NADH. The terminal electron acceptor, fumarate, forms succinate when reduced. When oxygen is present, an excess of succinate can lead to reverse electron flow, forming high levels of reactive oxygen species. Under anaerobic conditions, reactive sulfur species may have formed. Eliminating end products may thus have had a selective advantage even under the low atmospheric oxygen levels of the Proterozoic eon.
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