Engineering amyloid and amyloid-like morphologies of β-lactoglobulin

Background: Depending on environmental conditions, almost all proteins can form amyloid and amyloid-like aggregates that have unique functional properties. This opens numerous applications for designed aggregates in materials, medical and food applications. However, it is poorly understood how the amyloid (-like) aggregation and their resulting morphology is induced or influenced by various environmental and processing conditions. Scope and approach: We identified and summarized conditions under which amyloid (-like) aggregates are formed and their impact on aggregate morphology. The focus is on β-lactoglobulin, but generic effects on other proteins are discussed, in order to elucidate common mechanistic properties. Key findings and conclusions: The flexibility of linear aggregates can be evaluated by comparing the persistence (L_p) and contour length (i.e., length when completely stretched; L_c). Shorter and more flexible amyloid-like aggregates (L_p < L_c) usually occur from a relatively fast assembly (e.g., low repulsion, high concentration, solvents). Longer, semi-flexible amyloid aggregates (L_p ∼ L_c) based on fibrillization-prone peptides that are slowly formed and assembled (e.g., high repulsion, low concentration). In either case, the aggregation kinetics increases through protein destabilization (e.g., heating, zinc addition, solvent and hydrolysis effects) and decreases through stabilization (e.g., glycerol addition). Post-processing (e.g., mechanical or interfacial stress) fragments aggregates into stiffer rods (L_p > L_c). Semi-flexible morphologies can align in liquid crystalline phases or interact with linear polysaccharides; while flexible aggregates can entangle. This allows for various possibilities to build higher order fibril or hybrid networks for various applications, such as bundles, coatings/films, or gels. This knowledge is crucial to produce specific morphologies for applications and to draw conclusions about how morphologies will be affected during processing (e.g., shearing).