How do multi-heme cytochromes form transmembrane wires and conduct electrons between the cell and environment?

人类通过呼吸氧气来获得生命所需的能量。这一过程包括使用从我们所吃的食物中提取的电子将氧气转化为水，这一过程被称为氧气还原。自由能在这个过程中被释放，我们用它来制造ATP，这是生命的通用能源货币。我们对氧气的依赖使我们产生了需氧菌--失去氧气，我们就会死亡。因此，人类被限制在地球表面生活，那里可以自由获得氧气。然而，地球上的大部分宜居环境并不是被人类开发的，而是被包括细菌在内的各种微生物所利用，这些微生物可以在没有氧气的情况下生存。真正令人惊叹的是，这些细菌中的一些可以生活在地球地下深处，并通过“呼吸岩石”生存下来。这是因为地球地下环境中一些最丰富的呼吸底物是不溶矿物，特别是铁的矿物。这些矿物质使一些土壤呈微红色，它们也可以被视为裸露悬崖上的红色接缝。事实上，“铁呼吸”是缺氧区最普遍的呼吸过程之一，因此具有广泛的环境意义。例如，它直接影响氮、硫和碳循环等几个生物地球化学循环的平衡，进而影响一氧化二氮等强有力的温室气体的排放。它还可能导致地下或海底石油管道的溶解，从而对石油工业造成不利影响。在某些方面，细菌呼吸矿物质铁的方式类似于它们呼吸氧气的方式，使用电子来“减少”呼吸底物。因此，细菌细胞内新陈代谢产生的电子被传递给铁，铁通过带负电荷的电子将其电子状态从所谓的铁态改变为铁态。然而，由于铁矿物是一种大的不溶颗粒，它不能自由地扩散到细菌细胞中。因此，如果细菌能够利用一种铁矿物质作为呼吸电子受体，它必须对一个令人困惑的问题有一个分子答案。“当电子由细胞内的新陈代谢产生时，细菌如何将电子转移到矿物质所在的细胞外？”对于所谓的革兰氏阴性细菌来说，这是一个非常具有挑战性的问题，因为它们被两个密封的细胞膜包围，内膜和外膜，而不溶的矿物质铁位于外膜之外。这个问题的部分解决方案在于特殊的“电子转移蛋白”，它们实际上位于细胞的外部，在那里它们可以将电子传递给细胞外的不溶矿物质。然而，这并不是全部的解决方案，因为仍然需要一个特殊的电子转移系统来使电子穿过外膜，以中介电子从细胞外传递到这些细胞表面蛋白质。这种电子从细胞外转移到所谓的“微生物-矿物界面”的机制尚不清楚。它代表了环境丰富的细菌组的生物化学研究中的一个主要问题。回答这个问题将为细菌的能量过程提供新的见解。它还将产生重要的生物技术影响，因为有可能在生物修复过程中使用矿物氧化物呼吸细菌，以清理受有毒有机污染物(如石油泄漏)或放射性金属(如铀(VI))污染的环境。它们在微生物燃料电池中的应用也在探索中，在微生物燃料电池中，细菌可以利用电极作为固体细胞外电子受体来产生电流。

Humans obtain the energy they need for life by respiring ('breathing') oxygen. This process involves using electrons extracted from the food we eat to convert oxygen to water in a process known as oxygen reduction. Free energy is released in this process and we use this to make ATP, which is the universal energy currency of life. Our dependency on oxygen makes us obligate aerobes -take away the oxygen and we die. Thus humans are confined to living on the surface of planet Earth where oxygen is freely available. However, the vast proportion of Earth's habitable environments are not exploited by humans, but by a diversity of micro-organisms, including bacteria, that can live in the absence of oxygen. What is truly amazing is that some of these bacteria can live deep in the Earth's subsurface and survive by 'breathing rocks'. This is because some of the most abundant respiratory substrates in the Earth's subsurface environments are insoluble minerals, particularly minerals of iron. Such minerals give some soils a reddish colour and they can also be seen as red seams in exposed cliffs. In fact 'iron respiration' is amongst the most widespread respiratory process in anoxic zones and so has wide environmental significance. For example it directly impacts on the balance of several biogeochemical cycles such as the nitrogen, sulphur and carbon cycles and this can in turn influence the release of potent greenhouse gases, such as nitrous oxide. It can also be detrimental to the oil industry through contributing to the dissolution of subsurface or submarine oil pipes. In some aspects the way bacteria respire mineral iron is similar to the way in which they respire oxygen, using electrons to 'reduce' the respiratory substrate. Thus, electrons generated by metabolism inside the bacterial cell are passed to the iron, which changes its electronic state from a so-called 'ferric state' to a 'ferrous state' by the negatively charged electron. However, because the ferric iron mineral is a large insoluble particle it cannot freely diffuse into bacterial cells. Consequently, if a bacterium is to be able to utilise an iron mineral as a respiratory electron acceptor it must have a molecular answer to a perplexing question. 'How can the bacteria move electrons to the outside of the cell where the mineral is located when the electrons are generated by cellular metabolism inside the cell?' This is a very challenging problem for a so-called Gram negative bacteria since they are surrounded by two sealed cell membranes, the inner membrane and the outer membrane, and the insoluble mineral iron lies outside of this outer membrane. Part of the solution to the problem lies in special 'electron transfer proteins' that actually sit on the outside of the cell where they can pass electrons to extracellular insoluble minerals. However, this is not the whole solution, since there still needs to be a specialised electron transfer system to take the electrons across the outer membrane to mediate the passage of electrons out of the cell to these cell-surface proteins. The mechanism by which this electron transfer out of the cell to the so called 'microbe-mineral interface' occurs is still not known. It represents a major question in the study of the biochemistry of an environmentally abundant group of bacteria. Answering it will provide new insights into bacterial energetic processes. It will also have important biotechnological impacts since there is potential for using mineral oxide respiring bacteria in bioremediation processes for the clean up of environments contaminated with toxic organic pollutants (e.g. oil leaks) or radioactive metals, such as Uranium (VI). Their use in microbial fuel cells where the bacteria can be used to generate electric currents using electrodes as solid extracellular electron acceptors is also being explored.