Oxidation is a versatile method to modify cellulose material properties, used in range of applications from cosmetics to papermaking. Biocatalytic oxidation, using enzymes as catalysts, is an interesting opportunity due to the low energy consumption and reduced waste formation, as well as environmental safety of the protein-based catalysts. However, for a long time there were no known enzymes able to do this, although indications that presence of oxygen can have positive effect on cellulose degradation by microbial enzyme cocktails existed. Around 10 years ago, a new enzyme type was discovered: lytic monosaccharide monooxygenases (LPMOs), enzymes which can oxidize not only cellulose, but also chitin, hemicelluloses and starch, depending on the enzyme variant. This discovery had not only scientific, but also technical impact, since these enzymes can have a huge positive effect on efficiency of lignocellulose saccharification needed in biofuel biorefineries.
Beyond the biofuel and chemical sector, the LPMOs offer tools to biocatalytic oxidation of cellulose for material applications. As in any new technology, many fundamental aspects need to be understood before the targets of application requirements can be reached. These have been elucidated in a FinnCERES-funded project Enzymatic modification of lignocellulose building blocks & cellulose surfaces modelling and related project From fundamentals to valorization: Enzymatic oxidation of cellulosic fibres and underlying mechanisms (FunEnzFibres) funded in European Co-fund action ERA-Net ForestValue.
Figure 1: Biocatalytic oxidation of cellulose for material applications can be obtained by lytic monosaccharide monooxygenases (LPMOs). Computational modelling reveals molecular level interactions and changes resulting from the enzyme processing.
Molecular modelling figure credits: Maisa Vuorte, Soft Materials Modelling Group, Aalto University. Photographs of fermentors&pulps: VTT image bank. Trichoderma culture: Nina Aro. Nanocellulose & dissolving fibre: Panu Lahtinen & Ulla Holopainen-Mantila.
The nature is full of enzymes, but their use in industry requires availability in large quantities, free from any interfering impurities. VTT has developed a synthetic biology based method for LPMO production, enabling grams of enzyme production and significantly less impurities compared to convention production systems (Read more here). The same production system has also been exploited in screening of new LPMOs from tropical fungi (Indzyme project), as a joint effort by VTT and Vivekananda Institute of Tropical Mycology.
Cellulose is structurally organised at several levels: from polymer to crystals, nanofibrils and finally into fibres. The LPMO enzymes act on molecular level on the cellulose polymer, but the effects of the oxidation reflect into higher organisational levels and material behaviour.
Computational modelling offers a powerful tool for analysing molecular level interactions and changes in the cellulose structures. We have examined the effect of LPMO oxidation defects and attachment of the enzymes via cellulose binding modules (CBMs) on cellulose fibril surfaces. The modelling work shed light on how oxidation influences the fibril surfaces and their water interactions, revealing both the mechanism of desorption of glucose chain fragments and connection of the defects to enhanced interactions between fibrils that can be expected to show viscoelastic response in solution.
Indeed, at the level of nanofibrils, the LPMO catalysed oxidation, rheological investigation showed that the enzymatic oxidation loosens the fibril network and makes the material more liquid-like. At the level of fibres, it was seen that the weakening of the fibre structure by enzymatic oxidation led to better solubility of cellulose, and easier disintegration of the fibres under mechanical shear, which have significance in production of, for example, textile fibres and cellulosic nanomaterials. They have been studied utilising novel analytical tools developed at BOKU University (Read more here). In general, choice of the LPMO variant affected the outcome of the application trials, indicating that also this technology follows ‘one size does not fit all’ principle. The modelling work involved in the efforts extends the interpretation of the effect of enzymes and provides microscale-to-macroscale engineering approaches to affect the material properties.
In conclusion, our findings contribute to building the recipes on which the technology can emerge as mature biovalorization means. The LPMOs exhibit a large diversity in nature, and their story in lignocellulose valorisation is clearly just in its beginning.
Additional information:
Industrial biotechnology
VTT Technical Research Centre of Finland Ltd
Departments of Chemistry and Materials Science and Bioproducts and Biotechnology
Aalto University
Check out below a short video clip on enzymatic oxidation of cellulosic fibres extracted from the FinnCERES science documentary Fibre and Beyond.
