CESBIC--Critical Enzymes for Sustainable Biofuels from Cellulose

Sugar can be fermented to ethanol which can then be distilled to give ethanol good enough to be used as a fuel. This is bioethanol. Bioethanol is a 'sustainable fuel' meaning that its production and use has a significantly lower impact on the environment than oil or gas. It can make a major contribution to meeting our future energy demands. Why then don't we make more use of bioethanol? The answer lies in the sugar that is needed to make it in the first place. To be sustainable the sugars need to come primarily from plants. However, plants only produce relatively small amounts of sugar. Most of a plant's energy is tied up in a material called cellulose (essentially the material that gives a plant its structure), which-whilst it is made up of individual sugar units-cannot be efficiently broken down into its individual sugar units and therefore cannot be fermented. This means that the vast majority of a plant is useless in producing bioethanol.

Therefore, there has been a global search for a means of converting cellulose to sugar which is both efficient and simple. Such a solution now appears to be within our grasp. It turns out that fungi do this chemistry all of the time, by secreting enzymes which attack the cellulosic plant material they are degrading. Very recently these enzymes were isolated and studied. They were shown to be unprecedented in terms of their structure and biochemical function. It is clear that we need to study these enzymes in much more detail to make the best use of them.

This project aims to do exactly that, not only by studying the enzymes themselves, but also seeing how these can be then used directly in industry to make bioethanol. The project brings together some of the leading investigators from the UK, Denmark and France. We will take what is called a 'genomics to catalyst' approach, where the genomics allows to understand the full range of fungal enzymes that are used to degrade cellulose, and the 'catalyst' part is developing this knowledge to working industrial catalysts.

Work described in this proposal aims to tackle head-on the single major limitation in sustainable bioethanol production. This limitation is their current inability to degrade effectively cellulose into glucose. As cellulose is, by far, the most abundant biopolymer synthesised in large quantities by all known plants (equivalent in energy to 20 times the global oil usage) its effective conversion to glucose and then into bioethanol via fermentation is of major importance. Indeed, all commentators on biofuels identify cellulose as the only really long-term sustainable source of bioethanol and that cellulose's recalcitrance to degradation is the limiting factor.

Of the more promising solutions that are being explored, the catalysed conversion of cellulose to glucose is the one that is attracting most commercial attention. In this context enzymatic degradation of cellulose by cellulases has been the focus of research for the last fifteen years, but it has been held back by a lack of understanding of how cellulose was initially attacked by oxidative enzymes, such that the cellulose is made accessible to more traditional cellulases. However, the field recently gained considerable momentum when the full structure of the fungal cellulose-degrading enzyme, GH61, was published in September 2011 gaining significant worldwide attention (10,000 downloads as of Jan 1 2012). This structure is vital as it now opens up the way for 1) understanding the key catalytic factors behind the enzymatic degradation of cellulose and 2) the development of biomimetic catalysts which carry out the same oxidative process. Given the importance of cellulosic bioethanol there is sure now to be a major worldwide effort to build on this discovery.

In this proposal we aim to take a genomics to catalyst approach to maximise the use of GH61s in the production of bioethanol.

Biofuels hold the potential to make an essential contribution to UK and global energy demands. One of the keys to their development is the efficient conversion of biomass, particularly lignocellulosic biomass, into bioethanol. In this regard cellulose is by far the most important biomass source; it is very highly abundant (100 billion kilogrammes are produced globally every year), is present in all plants, and is the principal component of plants that can be grown densely on marginal land, e.g. switchgrass. The production of bioethanol from cellulose is, however, faced with a single critical issue. This issue is the chemical recalcitrance of cellulose. This recalcitrance severely limits its conversion to bioethanol and has, so far, prevented all attempts to generate significant amounts of bioethanol from sustainable plant sources. Indeed, reflecting the thoughts of all commentators on this issue, the International Energy Agency says that bioethanol will only play a major role in meeting sustainable energy demands if the key technological barrier of cellulose recalcitrance can be overcome (IEA, World Energy Outlook 2006).

The work described in this proposal seeks to address this key issue head-on. It builds on a recent and important breakthrough in the field, which is the full determination of the structure of a fungal enzyme called GH61. This class of enzymes is the long-sought-for 'missing link' in the conversion of cellulose to sugars by fungi. The action of the enzyme class is to oxidise cellulose directly thus making it tractable to other enzymes which can then convert it into soluble sugars and onto bioethanol. The structure of GH61 now points chemists towards the key chemical features of a synthetic catalyst which could be used itself to degrade cellulose. The way is now open, therefore, to generate cellulosic bioethanol sustainably. As such, the work in the proposal offers significant potential to the biofuel industry and to meeting the future energy demands of society.