Molecular Basis for Controlled Transmembrane Electron Transfer

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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.