Mechanistic insights into lytic polysaccharide monooxygenases: an integrated structure/spectroscopy study

Metal-containing enzymes perform critical biochemical functions across the full breadth of biology. They are also found in many everyday products such as washing powders and cleaning products. Furthermore, these types of enzymes are used very widely by industry (e.g. paper manufacture). Therefore, any insight we can gain into their detailed biochemistry and chemistry has significant societal benefits.

In most of these enzymes the metal ion is an essential feature of the enzyme's active site, and in many cases the metal(s) at the active site is/are (a) transition metals, e.g. Fe, Mn, Cu. What is distinctive about such types of active site is that they are often capable of 'extraordinary' chemistry in which the conversion of normally unreactive substrates is catalysed. This is perhaps no truer than in the enzymes which Nature deploys to convert abundant biomass (e.g. wood, cellulose and methane respectively) through to bio-valuable products, where metal-containing enzymes carry out many of the key conversion steps.

As such, knowledge of and utilisation of these enzymes is essential for myriad industries, perhaps the most conspicuous of which is the nascent second-generation biofuel industry in which recalcitrant and abundant biomass is converted through to bioethanol through enzymatic degradation of the biomass feedstock. Indeed this projects concentrates on a brand new set of enzymes called lytic polysaccharide monooxygenase which contain copper. These enzymes are proving to be key factors in our ability to convert waste plant matter (e.g. leaves and stalks) through to biofuel. In fact, these enzymes are part of the reason why several new biorefineries have opened-up around the world which are able to convert grasses through to bioethanol. This project, therefore, seeks to understand in detail how the enzymes work and also to find ways to improve their conversion capabilities.

Plant-derived biomass is the major feedstock for myriad industries. However, despite its widespread nature, at the heart of all use of plant-based biomass is one grand challenge, a challenge which has bedevilled all research efforts to date, and one that-without a solution-presents major societal issues, particularly in fuel production. The challenge is to find a means of overcoming the sometimes enormous (and sometimes astonishing) chemical and physical recalcitrance of many plant-based biopolymers, most notably cellulose and lignin, such that the full calorific and chemical potentials of these forms of biomass can be sustainably realised.

Given the context of the challenge, it is no surprise that there have been multiple and wide-ranging efforts to find ways of sustainably deconstructing plant biomass. Various physical, biochemical and chemical methods have been proposed. Of these methods, however, enzymatic solutions offer the most promise not least because Nature has fine-tuned, through evolution, the most efficient methods of biomass utilisation, and that recent years have seen advances in our understanding of the consortium of enzymes acting in concert which is deployed by saprophytes in the degradation of biomass.

Many of these enzymes are metalloenzymes of which perhaps the most conspicuous are the copper-containing lytic polysaccharide monooxygenases (LPMOs). Indeed the discovery of LPMOs has overturned our understanding of biological biomass degradation. This proposal seeks to use a combined (and novel) structure/spectroscopy investigation into determining the definitive mechanism of action of LPMOs. At its heart the proposed work will use advanced spectroscopic methods to determine the electronics of the copper ion and in particular its ability to active molecular oxygen or peroxide to give a highly oxidative species. To this end we will use EPR, MCD, XAS and Raman spectroscopies, alongside structural information from XRD studies.

There can be no doubt that LPMOs have already made a very significant impact on the second generation biofuel industry. The most advanced second-generation bioethanol plants (mostly in Brazil and the US) now use enzyme cocktails which include LPMOs. The ability of this class of enzymes to help overcome the 'recalcitrance barrier' of abundant biomass is their key feature. As such, LPMOs are opening up the possibility of fermenting some of the most abundant biomass through to commodity products. The key question behind all of this, however, is: 'do we know the full extent of the action and efficiencies of LPMOs?' Indeed, it is only 6 years since their discovery and much is still to be learned about these enzymes. Each extra insight we gain then feeds directly through to their commercial use, in particular the types of substrate upon which they can act and--most importantly--the key kinetic and biochemical parameters which will allow scientists/technologists to maximise their use. This aim lies at the heart of the current proposal. We seek to provide the key detailed information about how LPMOs work at a molecular level and then to make this information available through publications and through the CAZy database.

Accordingly, the impact of our work will be on the ability of second-generation biorefineries to achieve commercial and environmental sustainability, with all of the attendant effects this will have, including reduced carbon emissions, reduced pressure on arable farming land, greater utilisation of natural resources and, finally, the social/economic effects of allowing nations rich in biomass to convert this resource through to liquid fuel.