Metal-hydrido intermediates in enzymes: atomic level mechanistic insight and technological applications of hydrogenases

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Renewable hydrogen offers us a future free of fossil fuels, not just because we might use hydrogen in our cars, but because hydrogen is the primary chemical formed upon water energisation by sunlight and is used to make other energy-rich chemicals. Hydrogen, the simplest of molecules is also one of the most important industrial chemicals and more than 70 million tonnes are used worldwide. It is a raw material for some of the most important chemical processes, particularly in making ammonia, the essential fertiliser. At present, most hydrogen is produced from fossil fuels; however, hydrogen is also the greenest and most easily renewable of future fuels and raw materials because sunlight and water are earth-abundant resources. We are familiar with the electrolysis experiment in the school laboratory where hydrogen is formed along with oxygen because electrical energy is converted to chemical energy: explosive recombination of the hydrogen and oxygen, initiated by a spark, releases back much of the original electrical energy as heat and light. Solar energy also, can be stored as hydrogen; indeed, green plants do this in a disguised way in photosynthesis (consider that 'hydrogen' is 'stored' by combining it with carbon dioxide to give hydrocarbons and carbohydrates). Energy from the sun is easily able to convert water from the oceans into hydrogen and oxygen yet we this does not happen at any detectable rate: converting water to hydrogen requires not only systems for absorbing radiative energy (pigments, semiconductors) but also catalysts that will accelerate the chemical reactions. This research project is about the catalysts, produced by microorganisms, that convert water into hydrogen, and vice versa, at rates of many thousands per second at normal temperatures. These catalysts are giant molecules - enzymes known as hydrogenases - and they are of great importance for understanding and designing the chemistry of future hydrogen technologies. It is through hydrogenases that microbes thrive in all kinds of different environments and produce hydrogen (biohydrogen) for human benefit. Conversely, hydrogenases are important for the action of some notorious pathogens. Use of hydrogen in current industrial applications requires high temperatures and expensive resources. In terms of performance, the best catalysts available to industry are based on platinum, a limited, expensive element. In contrast, hydrogenases catalyse the interconversion between hydrogen gas and protons (water) at rates and efficiencies higher than platinum but using the common elements iron and nickel. To achieve these rates, nickel and iron are 'dressed up' in special atomic environments that are also buried to shield them from water and other small molecules that may disrupt or destroy the special environment. One of the specific aims of this research is to establish how the special structures of the active sites of the two kinds of hydrogenase (one contains only iron, the other contains iron and nickel) perfected by biology during over two billion years of evolution, lead to such high activity. How does hydrogen interact with the atoms of the active site, how important is the exact positioning of different atoms, how important are bond strengths and mobilities of different groups, what special properties of other small molecules enable them to block the normal reactions with hydrogen ? With this information we can (a) determine definitive rules for the design of synthetic catalysts, based on iron or nickel having activities as high as platinum, (b) engineer hydrogenases so that they can survive oxygen, leading eventually to sustainable, large scale photosynthetic hydrogen production by whole organisms, (c) engineer hydrogenases so that they can be applied, as isolated enzymes, in special technologies such as fuel cells and in continuous 'cofactor regeneration' a technical requirement for enzyme-based synthesis of some expensive chemicals.