PhD defence by Lise Friis Christensen

Principal supervisor

• Professor Egon Bech Hansen

Co-supervisor

• Associate Professor Paolo Marcatili

Examiners

• Professor Peter Ruhdal Jensen, National Food Institute, Technical University of Denmark
• Adjunct Professor Herwig Bachmann, Faculty of Science, Systems Biology, Vrije Universiteit and Expertise Group Leader Fermentation at NIZO food research, Amsterdam, The Netherlands
• Professor Lotte Bach Larsen, Department of Food Science, Aarhus University, Denmark

Chairperson at defence

• Sr. Scientist Martin Steen Mortensen, National Food Institute, Technical University of Denmark

Resume
Protein scissors from lactic acid bacteria have potential to cleave plant proteins into new food ingredients.

Lactic acid bacteria are utilized to develop and preserve food products. The protein-degrading abilities of lactic acid bacteria can be used to change food texture, taste, and shelf life as they cleave proteins into smaller protein fragments using their rich catalog of protein scissors – known as proteases. The ability of proteases to alter protein functionality in food is important as part of the ongoing transition from a predominantly animal-based to a more plant-based protein-rich diet.

This PhD thesis contributes to new knowledge about the prevalence and diversity of proteases found on the cell surfaces of lactic acid bacteria. Additionally, as part of the PhD project, different methods have been developed to study the protease properties for degrading plant proteins.

The National Food Institute hosts a growing collection of bacteria strains that have been sampled from various ecological niches, including plant sources. This PhD study has searched within the DNA material of these bacterial strains, which has led to the discovery of new cell surface proteases of lactic acid bacteria derived from plant-based materials. The diversity of these proteases has been compared to similar and well-studied proteases, and the protease structures has been predicted by artificial intelligence technology. This study reveals greater structural diversity in these proteases than previously known.

Two methods are presented to investigate if and how proteases degrade potato protein, which is a sustainable and food relevant plant protein. The first method shows how the growth of lactic acid bacteria can be used to directly assess how well their proteases degrade potato protein. The second method uses a dye to label the potato protein, which allows small amounts of purified proteases to be detected. This method is suitable for performing multiple protease tests as a way to save time when searching for proteases that can degrade potato protein.

This PhD thesis concludes that lactic acid bacteria from various environments can be a resource for new cell surface proteases with diverse abilities to cleave different types of proteins, including plant proteins. The understanding of these protease structures can be used in combination with the laboratory methods presented for further characterization of the identified proteases, or as a basis for bioengineering to modify protease functionality.