Attacking different biomass components
Biomass from non-edible crops could be a good source of glucose and other small sugars that can in turn be converted into bioethanol. But these sugars are hard to convert as they are embedded in difficult to digest crystalline molecules, like cellulose. Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes, which oxidatively cleave polysaccharides like cellulose, thus helping other enzymes access these hard to break materials and convert them to glucose. Because of this, LPMOs have received a lot of attention, as they may be key to complex plant biomass degradation in industry and in nature.
As well as cellulose, plant biomass is formed by an intricate matrix of other complex polysaccharide molecules. It has been known that several LPMOs are able to cleave oxidatively both cellulose and other hemicellulose polysaccharides in this complex matrix and may do so in nature. The European CESBIC team used a variety of techniques to look in detail at the substrate preference of two related LPMOs and the work has been published in the prestigious journal Nature Communications on 20/10/2017. The crystallography group at the Department of Chemistry at the University of Copenhagen (UCPH) has used X-rays to probe the atomic structure of the LPMOs as they attack different polysaccharides, while co-workers at Novozymes A/S, Marseille, Cambridge and York have selected enzymes for the study, produced them, characterized in detail the product of the reaction, the substrate preference and reductant dependence and probed the active site using spectroscopy.
A graphical representation of the experiments in which crystals of the LPMO are infused in the polysaccharide fragments.
Crystals of enzymes contain water filled channels through which smaller molecules can be infused and bind. By analyzing the crystals after binding of different model substrates, an atomic view of the enzyme in the crystal as it binds its target molecule can be captured. Our study showed for example, that fragments of cellulose and xylan, another plant polysaccharide, which has quite a similar chemical structure, bind rather differently to the LPMO under study. The different atomic arrangement is an indicator that the process of degradation follows different routes, although both cellulose and xylan can be cleaved by this LPMO. This was also indicated by a different dependence of the degradation on an external electrons source for the two polysaccharides, and by a different electronic structure round the copper as shown by EPR spectroscopy by British coworkers in Cambridge and York.
The Department of Chemistry UCPH team consisted of PhD student Kristian E.F. Frandsen. MSc student Tobias Tandrup and lab manager Jens-Christian N. Poulsen, led by Associate Professor Leila Lo Leggio (also member of ISBUC). The essential X-ray experiments were carried out at the MAX-Lab synchrotron in Lund, Sweden, and the ESRF in Grenoble, France. The project was funded by DSF as part of the ERA-IB program. In addition to UCPH’s Department of Chemistry and Novozymes A/S, the company that identified and produced the enzyme for the experiments, the partners of the international CESBIC consortium were the University of Cambridge, Aix-Marseille Université and the University of York. Leila Lo Leggio’s team will continue its work on LPMOs as part of the new HOPE project funded by the Novo Nordisk Foundation.
Leila Lo Leggio