The artificial amyloid: fundamentals of formation and applications of food protein nanofibrils

QC 20240927

Abstract

Proteins are one of the fundamental building blocks of life as we know it. They are central to various biological processes and pivotal parts of essential procedures in the healthcare, food and sustainability industries. Over the past few years, significant research efforts have been made to employ bio-based strategies as alternative pathways to shift away from human dependency on petroleum-based polymers, placing protein as central building blocks for sustainable material development. In this framework, this thesis explores a specific class of material building blocks derived from the food we eat, referred to as protein nanofibrils (PNFs). Particular attention is paid to PNFs from soy protein isolate (SPI) and whey protein isolate (WPI) formed at low pH and high temperature, mirroring processes commonly found in food processing and cooking. The presented work focuses explicitly on how these fibrils assemble from acid hydrolysis of the initial food proteins into smaller aggregation-prone species, how aggregation occurs and how we can potentially process fibrils as valuable materials for society. 

A critical step in forming fibrils at low pH and high temperature is the heat-induced acid hydrolysis of protein chains into shorter peptides. The work first uses SPI as a model protein source to understand PNF formation. By determining the activation energy for SPI hydrolysis and comparing it with the lower activation energies for the aggregation processes of one of the peptide building blocks, BB2, we establish that hydrolysis is the with the highest energy of activation. Computer simulations are then employed to replicate the hydrolysis of proteins to model peptide production and degradation from rates of protein hydrolysis. With this, a connection between monomer production and fibrillation kinetics can be established. With this, numerical simulations of BB2 at 90 °C and pH two are then attempted, demonstrating that aggregation of this peptide alone does not replicate the fibrillation behaviour of SPI.

The thesis continues focusing on the fibrillation of food proteins and their potential safety for humans, now exploring how food fibrils affect the aggregation behaviour of Aβ42 aggregation, a hallmark of Alzheimer’s disease. The results show that seeds from food amyloids do not accelerate Aβ42 aggregation, with lysozyme- and oat-derived fibrils even delaying the aggregation of Aβ42. Further kinetic analysis reveals that aggregation inhibition is likely due to interactions between Aβ42 aggregates and food-derived fibrils, likely affecting the pathway for secondary nucleation in Aβ42 assembly. 

In contrast, seeding accelerates fibrillation in WPI, leading to the formation of longer fibrils that can reach the percolation threshold necessary for gelation and material formation. However, the results also indicate that percolation theories might need to be refined to better frame and predict the behaviour of PNFs and how they interact to form macromolecular structures.

Finally, the usage of food-derived WPI fibrils in material applications is also investigated. Fibril-based aerogels exhibit distinctive microstructural differences from aerogels without fibrils and enhanced pollutant adsorption capabilities, represented using the model molecule ibuprofen. Additionally, preliminary results show that fibrillar hydrogels can be used as electrolyte matrices for energy storage, displaying a broad operational potential window and excellent rate performance. 

Combined, the results provide important insights regarding the formation of fibrils at low pH and high temperature, their potential non-hazardous nature for human consumption, and their application in areas such as water purification and energy storage.

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