A Model for the Denaturation and Aggregation of β‐Lactoglobulin

Abstract

A quantitatively correct kinetic model for the temperature-induced denaturation and aggregation of P-lactoglobulin is presented. The model recognizes an initiation, a propagation and a termination step by analogy with polymer radical chemistry. The decrease in native P-lactoglobulin is predicted to follow order 312, in agreement with experimental results. The size of the protein polymer particles is predicted to be proportional to the square root of the initial P-lactoglobulin concentration. The scattered light intensity is proportional to the product of concentration and size of the protein polymer particles. The initial increase in scattering intensity of the particles therefore scales with the initial squared P-lactoglobulin concentration. The influence of other reaction conditions, e.g. ionic strength and pH, can be incorporated via the reaction constants of the reaction kinetic pathway.Like many other proteins, the milk serum proteins P-lactoglobulin and a-lactalbumin are sensitive to heating at 60-100 "C. During heating, intramolecular and intermolecular changes and reactions occur, which are generally denoted as denaturation and aggregation. There is a vast amount of literature on this topic, much of which is heuristic in nature.Knowledge of the precise mechanisms of the processes is still inadequate, and there is no clear physical picture on which to base predictions. Such predictions are of great interest for industrial (food) processing of these proteins, because they can aid in the understanding and control of the functionality and quality of food products.Although a quantitative model for denaturation and aggregation is still lacking, much information on milk serum proteins has been obtained. Upon heating, the proteins undergo conformational changes, which are thought to result in an unfolding of the molecules. The whole process is described as denaturation. These changes are reversible in principle, but experimentally denaturation often appears to be irreversible [ 1, 21. Renaturation may be hampered by secondary effects like electrostatic and hydrophobic interactions between newly exposed parts of the molecules, which lead to aggregation of denatured molecules. Native molecules are inert as long as solvent conditions are non-critical and aggregation does not seem to occur before denaturation has taken place.The appearance of heated P-lactoglobulin dispersions is a very intriguing feature. It can vary from a transparant appearance to an opalescent and milk-white turbid appearance, and not only dispersions but also firm gels may be formed The final appearance of heated protein solutions strongly depends on the medium conditions such as ionic strength, type of ions (especially Caz+) and pH. Transparent dispersions and gels are found at low ionic strength and a pH distinct from the isoelectric pH (e.g. above pH 6.5). In a rheological characterization, transparent gels exhibit elastic properties [2, 31 quite similar to those of polymer gels. In a recent study on ovalbumin [6], similar features were obse...

Partially folded structure of monomeric bovine β‐lactoglobulin

“…2d. A sigmoidal curve, indicative of a cooperative mechanism, cannot be expected under these conditions to be due to an additional irreversible association that can take place at high temperature and involves intermolecular chemical reactions through the formation of new disulphide bonds among denaturated molecules [14,18,19]. This is confirmed by the sudden decrease of ellipticity on going from 70 to 90°C.…”

“…The association behaviour of [3-LG is largely influenced by pH changes: the two most common variants of bovine 13-LG, [3-LG-A and -B (which differ in two amino acids), are in a monomer-dimer equilibrium at pH 3.0 and appear to form higher aggregates above pH 3.5 [14]. Since of these two forms it is only the monomer that can be amenable to a detailed NMR study, we undertook a search for the conditions under which the equilibrium is completely shifted towards the formation of the monomer.…”

The influence of bovine serum albumin on β-lactoglobulin denaturation, aggregation and gelation

“…A mechanism for -lg denaturation under conditions similar to those used in this study has been proposed previously (Roefs & de Kruif, 1994). However, the rapid denaturation of BSA during the initial stages of heat denaturation in this study could alter the mechanism of the protein aggregation.…”

Role of Whey Protein-Casein Complexes on Yoghurt Texture

“…For instance, it was reported that intermolecular S-S bridge formation depends on the concentration of -lactoglobulin at temperature below 75 C, but S-S bridge formation does not depend on the concentration of -lactoglobulin at temperature higher than 75 C (Iametti et al, 1996). Other researchers (Roefs and De Kruif, 1994;Hoffmann and vanMil, 1997) also found same results upon addition of -lactoglobulin that the average size of the protein complexes (S-S bridge formation) was increased with increasing the concentration of -lactoglobulin from 10 to 50 mg/mL during heating at temperature up to 65 C.…”