Kinetics of fibrilar aggregation of food proteins

In this thesis we study the kinetics of fibrilar aggregation of two model proteins widely used in the food industry -b-lactoglobulin (b-lg) and hen egg white lysozyme (HEWL). The kinetics of protein aggregation is studied mostly experimentally and, when possible, theoretically. The process of fibrilar (or linear) protein aggregation is the process of formation of elongated structures from otherwise compact (globular) proteins. Studying the kinetics of this process for different proteins can lead to a better understanding of the mechanism of the process and to a possible generalisation of this mechanism. The investigation of the morphology of the formed aggregates at different stages of the process of aggregation could also lead to a more complete picture of the detailed mechanism of the process. Last, but not least, is the influence of the protein stability on the type of the formed aggregates and the kinetics of fibrilar aggregation.

The specific aims of this thesis are the following: 1) To investigate the kinetics of heat-induced fibrilar aggregation of two model proteins, bovineb-lg and HEWL, in as much detail as possible; 2) To study the morphology of the fibrils formed from both proteins; 3) To study the influence of the environment such as temperature, pH, and ionic strength on the kinetics of fibrilar aggregation and the morphology of the formed fibrils.

The heat-induced fibrilar aggregation ofb-lg is investigated at pH 2.0, 80 °C, and at various ionic strengths. Fibril formation is followed in situ using static (SLS) and dynamic light scattering (DLS), small angle neutron scattering (SANS), and proton NMR techniques. The fibrils that form after short heating periods (up to a few hours) disintegrate upon slow cooling, whereas fibrils that form during long heating periods do not disintegrate upon subsequent slow cooling. Even after prolonged heating, an appreciable fraction of the protein molecules is incorporated into fibrils, only when theb-lg concentration is above some critical concentration that is ionic strength dependent.The linear aggregation ofb-lg upon prolonged heating at pH 2.0 at80 °Cappears to be a multistep process. Competing reactions lead to two products: long linear aggregates and low molecular weight "dead end" species. The "dead end" species comprises monomeric non-native protein molecules and cannot form fibrils. Fibril formation involves at least two steps: the reversible formation of linear aggregates, followed by a slow process of "consolidation" after which the fibrils no longer disintegrate upon subsequent slow cooling.

Based on the obtained experimental data we have derived a kinetic model for the heat-induced aggregation ofb-lg at pH 2.0. The model involves a nucleation step and a simple addition reaction for the growth of the fibrils as well as a side reaction leading to the complete denaturation and inactivation of a part of the protein molecules. An analytical solution of the model for the early stages of the aggregation is obtained. The model describes very well the experimental data obtained by in situ SLS. It allows us to obtain molecular parameters for the kinetics of fibrilar aggregation ofb-lg as a function of the ionic strength. It gives us an expression for the apparent critical concentration for fibril formation due to the competition between the complete denaturation of the protein molecules and the formation of long fibrils. We also obtain the size of the critical nucleus for the fibril formation as a function of the ionic strength. In the case of a 13 mM ionic strength the critical nucleus consists of ca. 4 monomers; for all the other ionic strengths studied it is a dimer. This shows the important role that the non-specific electrostatic interaction has for the fibrilar aggregation ofb-lg at pH 2.0. It affects the rate of aggregation: the higher the ionic strength, the faster the aggregation. It also affects the detailed mechanism by which the aggregation takes place: the size of the critical nucleus increases when decreasing the ionic strength from 50 mM to 13 mM.

We have also shown that time-resolved SANS can be used with success in studying protein aggregation and that with enough additional information for the aggregation process one can in practice obtain complete information about the aggregation kinetics of the process.

Tapping mode atomic force microscopy results indicate that the fibrils formed at pH 2.0 upon heating at 80 °Chave a periodic structure with a period of about 25 nm and a thickness of one or two protein monomers. The main difference between the fibrils observed at different ionic strengths is their length and curvature. Fibrils obtained at higher ionic strength are shorter and more curved as opposed to longer and straighter fibrils obtained at lower ionic strengths. In case of higher ionic strength the fibril formation is faster, more fibrils are formed and as a result the mean length of the fibrils is shorter. Fibrils obtained at all ionic strengths exhibit similar type of periodic morphology, which suggests that the detailed mechanism of fibril formation might be independent of the ionic strength, but specific forb-lg.

In the case of HEWL we study the effect of pH and temperature on the fibril formation. Fibril formation is promoted by low pH and temperatures close to the midpoint temperature for protein unfolding (detected using far-ultraviolet circular dichroism (CD)). The stability of HEWL toward heat treatment is greatly influenced by the pH. The lower the pH, the lower the stability of the protein is. The conditions at pH 2.0 are unique in promoting the fibrilar aggregation of HEWL since heating of solutions at pH 3.0 and 4.0 to temperatures just above the midpoint of the unfolding transition of the molecule does not lead to the appearance of fibrilar aggregates.

HEWL fibrils are formed after a lag time that is practically concentration independent. This means that the governing process for the fibril formation is the change in the structure of single protein molecules caused by a prolonged exposure to a temperature close to the midpoint of the unfolding transition. Nucleation presumably involves a change in the conformation of individual lysozyme molecules. Indeed, long term CD measurements at pH 2.0, T = 57°C show a marked change of the secondary structure of lysozyme molecules after about 48 h of heating.

The fibril morphology is complex. The fibrils formed at pH 2.0 are long and straight with a length of the order of 5mm and predominant thickness of about 4 nm and consist of stiff rod-like subunits with length either 124 or 157 nm. On smaller scale the fibrils consist of a coiled structure with a period of ca. 30 nm that gives the appearance of the rod-like subunits probably because of defects occurring every 4 or 5 turns.

The fibrils consist mostly of full-length HEWL, although, some fragments due to hydrolysis at pH 2.0 and 57°C are probably incorporated into the fibrils. At any rate the hydrolysis of the protein is not the cause of the aggregation since at pH 3.0 no hydrolysis is detected but fibrils do form.

In conclusion we can say that for a full and general description of the processes of fibrilar aggregation of globular proteins the type of specific interaction responsible for the aggregation must be identified. The interacting parts of the protein must also be identified. The last and most difficult task is to characterise the conformation of the protein in solution at conditions suitable for aggregation.