My research focuses primarily on enzymes that degrade the cell walls of plants into their simple sugars. Why did/does this topic interest me? The answer is that when I secured my first tenured position at the University of Newcastle in the UK I joined a department that was interested in animal nutrition, particularly ruminants such as sheep and cattle. For these animals to thrive they need to extract the maximum nutritive value from the plant cell wall, which is the major component of their diet, and this is achieved through the action of the microbes that live in their intestines, primarily the rumen. Thus it was for this reason that I became interested in the enzymes that perform this function.

Once I started actually doing experiments with these enzymes I became amazed at their complexity and fascinated by their biochemistry. Indeed their complexity is related to the problems presented by their target substrateswhich are insoluble and chemically very complex and, as a result, are highly recalcitrant to biological attack. To overcome these challenges the enzymes have evolved highly complex structures with many of these biocatalysts contain regions, in addition to their catalytic core, which assist in accessing specific structures within the cell wall.

Thus, while I have always been aware of, and interested in, the practical applications of these plant cell wall degrading enzymes, the primary motivation for my research has always been to try and understand how these complex molecules work,  and it this that has sustained me throughout my career.

Currently it is an extremely exciting time to be working on plant cell wall degrading enzymes for many reasons. From a scientific perspective the advent of new technologies such as genomics, transcriptomics, together with the advances made in structural biology, has revolutionized our ability to make rapid advances in our understanding of how these enzymes work. For example the determination of microbial genomes has revealed a bewildering number of enzymes that make up the plant cell wall degrading apparatus of these organisms. High throughput transcriptomics  has provided insights into how the production of these enzymes are regulated while structural biology is providing important molecular details of how these hydrolases actually work. It is indeed a very exciting time to be working on plant cell wall degradation.

Not only are plant cell wall hydrolases scientifically fascinating but they have tremendous practical applications, particularly in the renewable or bioenergy sector. It is well established that supplies of oil have a finite life and with increased global industrialization these supplies are likely to be rapidly exhausted. An exciting and attractive environmentally friendly, and sustainable, alternative is to use plant biomass as the source of sugars used by yeast to generate bioenergy molecules such as ethanol. Plant biomass, rather than corn, is more attractive because it is more abundant, and thus sustainable, and does not compete, but actually acts in synergy, with the human food chain. To provide some insight into the scale of plant biomass, it is estimated that around 10 to 100 billion tons of plant biomass is degraded and synthesized every year in nature. Furthermore, if all the energy stored in plant biomass could be exploited then this will equate to 680 billion barrels of oil which is close to the annual consumption of energy on the planet.

It is evident, therefore, that the potential use of the plant cell wall within the bioenergy industry is significant but what limits its use, compared to corn, is that the enzymes work very slowly on these structures, for the reasons discussed above, reducing the economic potential of the process.  However, deploying our ever increasing understanding of how these enzymes work will underpin our capacity to produce novel enzymes with increased activity against the plant cell wall. These new biocatalysts will increase the economic viability of using plant biomass to produce bioenergy. Not only will this reduce the deleterious environmental impact of producing liquid fuels but it will also provide energy security and independence.

My vision is to harness the avalanche of knowledge on how these enzymes work to generate novel variants with increased activities against highly complex substrates. We can now see the critical features of different enzymes that enable them to do their particular job efficiently. We can also see how we might be able to combine these beneficial features into single enzymes to produce “super-biocatalysts” that have elevated activity against particular structures in the plant cell wall. What is particularly exciting is that we may be able to generate these biocatalysts partly through rational design but, with the advent of massive high throughput screening capability, which has resulted from the marrying of biological sciences with robotics and nanotechnology, we can now use forced protein evolution approaches to generate the required enzymes. It is my opinion that there has never been such an exciting time to be involved in biological research that is designed to produce a new generation of biocatalysts that convert the plant cell wall into useful sugars for the bioenergy industry.

Recent work coming out of my laboratory has focused on the mechanism of plant cell wall recognition. Recently we discovered that a particular protein substructure or module is found in a range of different enzymes with very different activities. After a detailed structural and biochemical analysis of these highly related proteins we discovered that they all bind to particular structures within pectins that are generated during their degradation. As pectins are the first polysaccharides to be attacked during plant cell wall degradation these protein domains, rather than directing their attached enzymes to the chemical structures that they specifically attack, recruit a series of different enzymes to regions of the cell wall that is undergoing rapid degradation. In short these enzymes are delivering their enzyme partners to highly susceptible regions of the plant. These studies have resulted in a paradigm shift in how we view the role of these adhesions in the plant cell wall degrading process and how they might be exploited to improve the industrial efficiency of the process. The work will shortly be published in the proceedings of the National Academy of Sciences USA.

In addition to the adhesions we are also very interested in knowing how the enzymes actually work particularly the structure of the transition state, which is a high energy form of the substrate that is rapidly attacked by its target enzymes. By using chemistry, structural biology in harness with biochemistry, we have shown that particular enzymes can distinguish their target substrates from other molecules, not by their structure but by the geometry adopted by the transition state during the reaction. This has led people to radically rethink our understanding of the nature of enzyme specificity. This information will again be invaluable in our quest to generate novel enzymes with elevated activities against highly complex molecules such as the plant cell wall. The work was published in Nature Chemical Biology in 2008.

Georgia presents a novel opportunity for people interested in studying complex carbohydrates, which is essentially what the plant cell wall consists of. The analysis of the enzymes that attack plant cell walls requires expertise in structural biology, chemical synthesis, enzymology, carbohydrate chemistry, plant cell wall synthesis and biological modeling. Georgia is unique, at a world level, in developing a single center that contains all this expertise. By coming to the CCRC at UGA I am able to collaborate with a myriad of distinguished scientists working in complementary fields to my own. Working in such a stimulating and knowledgeable environment gives me new research opportunities that were not available in the UK or, for that matter, anywhere in the world.