Molecular Basis for Controlled Transmembrane Electron Transfer

大多数生物可以通过呼吸产生能量。虽然像人类这样的生物仅限于呼吸氧气，但细菌要灵活得多，可以呼吸各种各样的分子，包括硝酸盐、硫酸盐和不同的金属氧化物。在呼吸过程中，细菌在其核心分解不同的碳化合物，并释放电子。然后这些电子在一个称为还原的过程中与目标分子结合。在呼吸过程中减少的大多数分子都很小，可以很容易地进入细胞，然而一些细菌能够进行一种被称为矿物呼吸的过程，字面上是“呼吸岩石”，其中电子被用来减少不溶性金属。这是一个挑战，因为细菌必须安全地将电子从细胞移到矿物质中；需要电子通过绝缘的外细胞膜传递的过程。矿物呼吸过程是最古老的呼吸形式之一，它改变了环境中许多金属的可用性，从必需元素铁和锰到剧毒的铀和砷。这些细菌有效地控制了底土和地下水中这些金属的含量，因此了解它们是如何工作的是环境修复的重要一步。这些过程在微生物燃料电池的开发中也被证明是有用的，在微生物燃料电池中，废物可以被分解，释放的电子要么用来发电，要么用来合成有用的产品，如苛性钠或乙醇。我们最近发现了这些细菌将电子移出细菌的一种方式：一种由三种不同蛋白质成分组成的生物线。两种叫做细胞色素的蛋白质含有十个铁原子，每个铁原子都包含在一个叫做血红素的辅助因子中。一种细胞色素存在于细胞表面，而另一种存在于细胞内部。第三种蛋白质在细胞表面的膜上形成一个中空管，两种细胞色素的尖端从相反的两端插入管中并在中间相遇，允许电子直接在细胞色素之间移动并产生分子线。电子从细胞内部到细胞表面的运动定义了这些细菌，对我们理解如何利用这些细菌至关重要。该项目旨在了解这些生物导线如何在膜中发挥作用，将电子传递到不同的矿物和电极中。我们要解决的这个过程的一个基本方面是确定细胞色素是专门为不同的矿物质量身定做的，还是它们只是将电子放电到附近的任何东西上。我们还会发现当细胞膜上的两种细胞色素分离时会发生什么；电子是否还能移动出去，或者电线是否以某种方式闭合。这项研究将使用许多技术，包括东安格利亚大学专门研究这些系统的一种新方法。这包括将复合物插入人造膜中，并测量电子在膜两侧的运动。我们还将发现不同类型的细胞色素是如何相互作用的，它们是聚集在一起形成带电区域还是分布在细胞表面，使用特殊的化学标签，当它们靠近时就会发生变化。最后我们将得到组成细胞色素的结构以及整个复合物。这项研究的结果将是对这种分子线最重要的特性的理解，这可能导致其在各种生物修复和生物技术中的应用。

Most organisms can generate energy through respiration, or breathing. While organisms such as humans are restricted to breathing oxygen, bacteria are much more flexible and can breathe a broad variety of molecules, including nitrates, sulfates and different metal oxides. During respiration, the bacteria break down different carbon compounds in their cores, and electrons are released. These electrons are then combined with a target molecule in a process known as reduction. Most of the molecules reduced during respiration are small and can easily move into the cell, however some bacteria are capable of a process known as mineral respiration, literally 'breathing rock', where the electrons are used to reduce insoluble metals. This poses a challenge because the bacteria have to safely move the electrons out of the cell and into the mineral; a process that requires electron transport through the insulating outer cell membrane. The process of mineral respiration is one of the oldest forms of respiration and alters the availability of many metals in the environment, from the essential elements iron and manganese to the extremely toxic uranium and arsenic. The levels of these metals in the subsoil and groundwater are effectively controlled by these bacteria, so understanding how they work is an important step in environmental remediation. These processes are also proving useful in the development of microbial fuel cells, where waste material could be broken down and the released electrons used either to produce electricity or to synthesis useful products such as caustic soda or ethanol. We recently identified one of the ways in which these bacteria move electrons out of the bacteria: a biological wire made of three different protein components. Two proteins called cytochromes that contain ten iron atoms which are each held in a cofactor know as a haem. One cytochrome is found on the surface of the cell, while the other is found on the inside. The third protein forms a hollow tube in the membrane at the surface of the cell and the tips of the two cytochromes insert into this tube from opposite ends and meet in the middle, allowing electrons to move directly between the cytochromes and generating a molecular wire. The movement of electrons from the inside of the cell to the cell surface is what defines these bacteria and is essential to our understanding of how these bacteria can be utilised. This project aims to understand how these biological wires function in the membrane to move electrons across and into different minerals and electrodes. One fundamental aspect of this process that we will address is to identify whether the cytochromes are specifically tailored to different minerals, or if they simply discharge electrons into whatever is nearby. We will also find out what happens when the two cytochromes held across the membrane disconnect; whether electrons can still move out, or if the wire is somehow closed off. This research will use a number of techniques, including a novel method developed at the University of East Anglia specifically for studying these systems. This involves inserting the complex into an artificial membrane and measuring the movement of electrons across the two sides of the membrane. We will also find out how the different types of cytochromes interact with each other, whether they cluster together to form charged areas or spread across the cell surface, using special chemical labels that change when brought close together. Finally we will obtain structures of the component cytochromes as well as the entire complex. The outcome of this research will be an understanding of the most important properties of this molecular wire, which could lead to its utilisation in a variety of bioremediative and biotechnological roles.