Quantification of the forces that mediate electron transfers between proteins

点击翻译按钮获取中文摘要

Electron transfer reactions are the basis of photosynthesis and respiration, which power all life on Earth. In essence energy directly provided by the sun or from foodstuffs is used to move electrons along a chain of proteins; some of these proteins can move freely, shuttling back and forth carrying their cargo of electrons to and from other proteins that are held in position within a thin sheet of membrane. The mystery is how a freely-moving protein finds its way to a particular membrane-attached protein, how it docks at the membrane surface, releases its electron and then manages to undock, all in a few milliseconds. Yet without hundreds of these electron transfer reactions happening every second, life on Earth could not be sustained. Somehow these pairs of proteins balance two conflicting requirements: they have to come together quickly and specifically to transfer electrons, yet they also have to be able to separate rapidly afterwards. So whatever forces brought the proteins together in the first place can be switched into reverse - how is this possible? What is this switch? Finding this out is the purpose of the proposed research, and it has important implications for all energy-yielding electron transfers on Earth. Up until now, electron transfer reactions between proteins have been studied by looking at the collective behaviour of billions of protein molecules. The light-absorbing properties of these proteins changes when electrons move between them; this is because these proteins contain a coloured haem molecule, as in haemoglobin in blood. Past work, monitoring the colour of the proteins and therefore their cargo of electrons, has shown how whole populations of molecules behave, but proteins are individuals just like humans; every molecule is slightly different from the others. We need to understand these biological reactions at the level of individual proteins so we can measure the forces that bring them together. The problem is that we don't know how individual protein molecules behave, and more importantly we don't know anything about the attractive forces that bring the proteins together and the repelling forces that separate them after the electron has jumped between them. To measure these forces, and to discover the reversible switch that allows docking/undocking, we developed a method to attach one protein partner, the one that receives the electrons, to a glass surface. The other protein, the one carrying the electron, was attached to the tip of a probe that was brought closer and closer to the surface-attached protein until the electron jumps between them. This probe is part of a highly sensitive instrument called an atomic force microscope (AFM). When we retracted the AFM probe from the surface with the electron accepting proteins we were surprised to find that we met a resistance. Why would this happen? Surely the tip-attached and surface-attached proteins would be easy to pull apart once the electron has transferred. It looks as if we had jumped the gun - pulled too early - and we had not waited long enough for the proteins to reorganise themselves for the separation event. So the reversible switch that allows docking, then electron transfer, then undocking had not been activated yet. We are now in the position where we can use our AFM to find out how single protein molecules attract each other in the first place and how they change after electron transfer in order that they can undock and separate. Moreover we can use an electron-accepting protein that only works when we shine light on it so we can control exactly when these reactions occur. Finally, we can make proteins with altered contact zones to find out which parts of the protein are important for docking/undocking. We think that these measurements, the first of their kind, will tell us how electron transfers, essential for plant photosynthesis and for our respiration, work so quickly and efficiently.