第一批细胞是如何从化学物质汤（大约三到四到四十亿年前）发展的？英国布里斯托尔大学原生植物研究中心的化学家兼主任斯蒂芬·曼（Stephen Mann）说：“即使生物学家比我们每个人都了解更多的细胞，但他们对细胞的了解更多。”化学家在很大程度上探讨了细胞寿命的出现。自1952年著名的Miller urey实验以来，他们已经展示了一些基本的化学构建基础是如何从简单的有机分子中综合的，但是它与复制细胞和生物学的存在之间仍然存在很大的差距生活。
Origin-of-life researchers are hoping to plug that gap by studying protocells. ‘We think protocells can be the potential path between non-living and living systems,’ says Yan Qiao, a chemist at the Chinese Academy of Sciences in Beijing, China. Protocells are cell-like compartments that attempt to mimic the earliest stages and functionality of cellular life. ’When we think about what makes something a protocell, rather than it just being a droplet or a sack of lipids, it should make use of energy, be able to grow and divide … and the most importantly, transmit information to its future generations,’ says biophysicist Anna Wang from the University of New South Wales in Australia. ‘No one’s been able to do all of those bits and piecest simultaneously in a lab yet, especially using materials that could have existed on early Earth,’ she adds.
最初由苏联生物化学家亚历山大（Alexander）歌剧在1930年代提出的一个想法表明，第一个细胞是没有膜的相分开的液滴，称为凝聚力。尽管他的1938年有关该主题的书a sceptical review inNature，包括对他经常引用弗里德里希·恩格斯的批评。
In the last decade Mann hasrevived this ideaand has opened up research into coacervates. They appear when two oppositely charged polyelectrolytes come together in solution, forming a separate condensed phase of highly enriched droplets, and often attracting other small molecules into the phase. ‘The polyelectrote can be RNA, DNA, proteins, peptides or polysaccharides like dextran sulfate,’ explains Qiao. The process is very fast, spontaneous and they form in a broad range of chemical conditions.
她一直在试图通过在合成的半渗透囊泡中创建称为蛋白质体的合成半渗透囊泡中的凝聚力来模仿细胞细胞器形成，该囊泡最初在曼的布里斯托尔实验室中开发。这些是由蛋白质的紧密挤满的单层制成的 - 在这种情况下，牛血清白蛋白和聚酰胺，它们会自发地自组装。Qiao通过最初用阳离子聚电解二乙基氨基乙基 - 脱去凝集蛋白体加载蛋白体，形成了凝聚酸酯，外部添加了一个偶氮苯分子，用两个负电荷的谷氨酸基团取代。该分子能够渗透到膜，内部形成凝聚液。她还发现，如果引入了单链DNA分子，它将变成划分为凝聚力.
Qiao was able to use her coacervate protocell to process binary information – perhaps the precursor to the complex signalling cascades that modern cells perform. Making use of the photoresponsive azobenzene molecule and the pH-sensitive nature of both molecules, she was able to use both UV light and changes in pH to disassemble the coacervate inside the protocell. The change from the顺式totransconfiguration that occurs in azobenzenes on illumination with UV light reduced the attraction between the two polyelectrolytes, as did an increase in pH, both causing the coacervate to collapse. By introducing various enzymes, she was also able to create chemical signalling, so that adding urea or a combination of lactose and oxygen would cause the coacervate to dissolve.
那么，凝聚力可以代表第一批原始细胞 - 像核糖体一样，细胞膜以后出现？并非每个人都相信。尽管它们很容易形成，但凝聚也会自发地合并并融合在一起。曼恩说：“他们天生不稳定。”他指出，活细胞的主要特征之一是利用膜的两侧利用化学梯度来利用能量。澳门万博公司‘在这方面，凝聚力是有问题的，因为在没有膜的情况下，它们与环境处于开放状态。”
其他人正在基于自组装脂肪酸囊泡的原始细胞追求替代模型 - 本质上是用来制作肥皂的表面活性剂分子。王说：“它们可以使这些非常动态且有趣的膜模仿模仿，但它们肯定不是我们在现代生物体中发现的膜。”但是，它们确实提供了一种创建一个划定的隔间的方法，与凝聚工不同。决定哪种模型更有可能看到化学家试图模仿活细胞中发现的某些特性，例如生长和分裂的能力。
‘日本广岛大学的生物化学家穆尼基·穆苏（Muneyuki Matsuo）说，‘获得原核细胞的增殖能力是“化学进化”和“达尔文进化论”之间缺失的联系。它在Protocell需要拥有的属性列表上一直很高。王研究了脂肪酸囊泡生长和分裂的特性和条件。她找到one method是从两个具有多个或单个脂质层的囊泡群开始，每个囊泡由八到18碳制成，由不同的长度脂肪酸制成。
’When you put two vesicles that are made of different lipids near each other, they’re going to start exchanging material… sometimes it’s dramatic enough that you can get some of them to cannibalise all the lipids off the other type of vesicle, [which] dramatically increases in surface area, [and this] can be really destabilising and lead to division,’ explains Wang. But whether this really constitutes growth is debatable. ‘The growth step is from some sort of reservoir of material,’ she admits.
Another challenge with protocells made from fatty acid vesicles is the route to cells containing information-carrying molecules such as RNA and DNA. A large proportion of origin-of-life researchers support the RNA world hypothesis which suggests that, before DNA, the first self-replicating, information-carrying molecule was RNA. As well as their role in protein synthesis, RNA-based catalysts called ribozymes (ribonucleic acid enzymes) are still involved in cellular processes including RNA splicing in gene expression, so the RNA world proposes that self-replicating RNA molecules existed before protein synthesis took over.
The problem with this idea is that ribozymes need the assistance of magnesium ions to function. The ion stabilises intermediate structures, allowing the RNA molecule to fold into the correct conformation. But magnesium ions also disrupt fatty acid membranes. The group of Jack Szostak at the Harvard Medical School in Boston, US, has been examining this problem. The 2009 physiology Nobel prize winner now focuses his research on understanding the origins of life and has tried to bypass this problem using citrate-chelated magnesium(II), which is able to assist in the templated copying of RNA using trimers of 2-aminoimidazoyl-activated RNA.添加氨基酰亚动物离开小组在核苷酸上，磷酸组激活RNA延伸。
The amino acid thioester structure used was inspired by the hypothesis that thioesters were probably abundant in prebiotic chemistry and, given the high energy of the thioester bond, they could have provided the basis of very early metabolic reactions. ‘In fact, ADP is converted into ATP by receiving energy from the thioester in modern organisms,’ says Matsuo. ‘These facts suggest that thioesters may have been the energy currency of primitive metabolism.’
几个小组还一直在研究RNA分子如何进入凝聚蛋白的原状以及它们的存活率和反应是否会得到增强。这包括来自德国德累斯顿的Max Planck分子细胞生物学与遗传学研究所的化学家Dora Tang。唐和她的合作者一直在尝试使用基于羧甲基脱氧钠盐和多肽聚l-赖氨酸的系统来提高凝聚力内部的RNA浓度，而在存在源自烟草Ringspot病毒的简单核酶。使用荧光探针，她表明核酶继续充当催化剂，并在凝聚液内裂解RNA分子。
Tang was also able to increase the uptake of RNA molecules into the coacervate. ‘We found that different lengths of RNA can be selectively retained within the coacervate,’ she explains. They found that longer RNA molecules, with 39 nucleotides, were比较短的保留更好of 12 nucleotides. ‘In the case of RNA this is probably associated with charge, so the longer the RNA, the more charge.’
We are a long way from mimicking basic cell behaviour
Her work on coacervate protocells has made Tang think about the various models for the early stages of chemical evolution in a different way. Competing with proponents of the RNA world are those who suggest that primitive protein synthesis developed first, before RNA or DNA, in what’s known as the peptide world. ‘We know that you can form coacervates from RNA and peptides. So is there a possibility that we don’t have disparate ideas, that these could have all accumulated within one droplet and started the onset of life?’ she asks.
2021年12月，唐与来自德国路德维希·马克西米利大学慕尼黑的合作者Dieter Braun，published new resultssuggesting how coacervates may have evolved in the sorts of gas bubbles found inside heated rock pores – one of the suggested locations where life may have begun. Using a temperature gradient to recreate the out-of-equilibrium conditions, they found that they could form cocervate droplets from polyanionic and polycationic species, and that they would accumulate at the gas–water interface where they underwent continued fragmentation that prevented the droplets coalescing into a single phase. The non-equilibrium interface conditions allowed coacervates to form with different compositions to those found in the bulk liquid phase, which Tang sees as a clue to the driving force for the early evolution of coacervates.
但是，大多数创建可以在内部生长或复制分子生长或复制分子的原始尝试的尝试几乎是可持续的。王说：“现在，似乎很难拥有几个周期的事情……总是需要某种人类干预。”她创建的脂肪酸囊泡似乎最终会变得越来越小，直到消失。缺少的是能够从周围环境中使用燃料 - 一种新陈代谢。‘最终，他们正在逐渐消退，而生活当然具有这种平衡状态。曼恩说，我们如何[重现]这将是一个巨大的挑战。”曼恩说。“我们离真正模仿这种基本细胞行为还有很长的路要走。”唐同意。‘如果您要从头开始建造一些东西，那么您就没有那样的复杂性，可以使您真正维持摆脱平衡行为。因此，这是我们面临的工程挑战。’
建立原核细胞的问题不是个体组成部分，而是了解它们的组装方式。曼恩问：“即使您拥有所有组件，它们如何以综合的方式或系统方式进行合作行动，以产生您可能认为生活的基本代表？”“这并不明显 - 您可以拥有所有的组件，就像您可以拥有计算机的所有零件一样，但是如果它们不正确地集成和电源，那么它们就会像硅一样坐在那里一样。’
Burden of proof
Whether it will ever be possible to prove the exact mechanism by which cellular life came about four billion years ago is clearly debatable. Wang says most of the origin-of-life research community are just hoping for a way to ‘connect the dots’ of each increasingly more complex step. ‘Then I think some people would say “OK, I’m satisfied, we found one way to get there” – [though] not everyone.’
Mann, who is sceptical that we can ever know what was essentially a historic event, is more interested in the general scientific questions protocell research brings up and the tantalising possibility of discovering if there is life beyond biology – a ‘life 2.0’ that might help us understand the transition between non-living and living matter. ‘In my opinion, that’s much more realistic, although I don’t think we’ll answer [it] in the next 50 years.’ He suspects the key is down to information flow. ‘You could argue that the difference between non-living and living is that information has the upper hand in a living system… information is in the executive position, it’s somehow controlling the matter [of] which it’s made.’
I don’t think we’re going to have protocells climbing out of the test tube
Those working on protocells to discover how life began are limited to scenarios that are biologically plausible, using the molecules we know are found in biology today, but Mann’s search for life 2.0 has allowed him to be a bit more flexible in his design criteria and find applications for these systems. ‘I don’t think we’re saying that we’re going to have protocells climbing out of the test tube, moving along the desk, but we can establish behaviours in these artificial systems, which are representative of the [properties of life], like growth, division, and motility.’
Mann has designed synthetic protocells that can chemically communicate with living cells and tissue, in one case being able to release nitric oxide, a vasodilator. He has created bio-compatible proteinosomes using haemoglobin-containing lipid membrane fragments that are able to circulate in the blood stream. These encapsulate polysaccharide–polynucleotide coacervates, also containing the enzyme glucose oxidase. In the presence of glucose and hydroxyurea, the enzyme catalyses a cascade of reactions leading to the release of nitric oxide, which causes the blood vessels to dilate. The system was tested on rabbits by injecting the protocells and hydroxyrea into their carotid artery. Making use of the animals’ own blood glucose, they were ableto produce sufficient levels of nitric oxide诱导动物血管周长的增加。