Aggregation of -Lactoglobulin Regulated by Glucosylation
Broersen, K. ; Elshof, E. ; Groot, J. de; Voragen, A.G.J. ; Hamer, R.J. ; Jongh, H.H.J. de - \ 2007
Journal of Agricultural and Food Chemistry 55 (2007)6. - ISSN 0021-8561 - p. 2431 - 2437.
heat-induced aggregation - amyloid formation - fibril formation - protein - glycoprotein - stability - hydrophobicity - glycosylation - calreticulin - denaturation
A large number of proteins are glycosylated, either in vivo or as a result of industrial processing. Even though the effect of glycosylation on the aggregation of proteins has been studied extensively in the past, some reports show that the aggregation process is accelerated, whereas others found that the process is inhibited by glycosylation. This paper investigates the reasons behind these controversial results as well as the potential mechanism of the effect of glucosylation on aggregation using bovine -lactoglobulin as a model. Glucosylation was found to inhibit denaturant-induced aggregation, whereas heat-induced aggregation was accelerated. It was also found that the kinetic partitioning from an unfolded state was driven toward refolding for glucosylated protein, whereas aggregation was the preferred route for the nonglucosylated protein. Keywords: Aggregation; glucosylation; -lactoglobulin; hydrophobicity; electrostatic repulsion; unfolding/refolding
Structure and rheological properties of acid-induced egg white protein gels
Weijers, M. ; Velde, F. van de; Stijnman, A. ; Pijpekamp, A. van de; Visschers, R.W. - \ 2006
Food Hydrocolloids 20 (2006)2-3. - ISSN 0268-005X - p. 146 - 159.
heat-induced aggregation - induced gelation - beta-lactoglobulin - functional-properties - light-scattering - denatured whey - disulfide bond - ionic-strength - ovalbumin - lysozyme
This study compares the rheological properties of acid-induced gels prepared of industrial spray-dried egg white proteins (EWP) with the acid-induced gels prepared of ovalbumin (OA) and whey protein isolate (WPI). Also we aimed to form transparent gels of EWP by means of the cold-gelation process. We showed that it was not possible to prepare cold-set gels because ovotransferrin (OT), present in EWP, was found to interfere with fibril formation. Therefore, we developed a new purification method in which first Or was selectively denatured by a heating step, subsequently precipitated by acidification and removed by centrifugation. Finally, the supernatant was desalted by ultra filtration. This resulted in a preheated EWP preparation, which mainly contains OA (> 80%). By removing OT using this new preheat procedure transparent gels were obtained after acid-induced gelation. Fracture properties of various EWP preparations were determined and compared with those of acid-induced gels of OA and WPI. Gels formed from different EWP preparations were weak (fracture stress 1-15 kPa, fracture strain 0.3-0.7), and the networks consisted of thin strands with hardly any additional disulphide bonds formed during the gelation step. In conclusion, the microstructure of the aggregates formed in the first step of the cold-gelation process and the amount of additional disulphide bonds formed during the second step appeared to be the determining factors contributing to the hardness and deformability of acid-induced gels of egg white proteins.
Do sulfhydryl groups affect aggregation and gelation properties of ovalbumin?
Broersen, K. ; Teeffelen, A.M.M. Van; Vries, A. de; Voragen, A.G.J. ; Hamer, R.J. ; Jongh, H.H.J. de - \ 2006
Journal of Agricultural and Food Chemistry 54 (2006)14. - ISSN 0021-8561 - p. 5166 - 5174.
heat-induced aggregation - whey-protein isolate - beta-lactoglobulin - thermal aggregation - disulfide bonds - acetylmercaptosuccinic anhydride - rheological properties - interchange reactions - globular-proteins - high-pressure
The aim of this work is to evaluate the impact of sulfhydryl groups on ovalbumin aggregation and gelation. Ovalbumin was chemically modified to add sulfhydryl groups in various degrees. The rate of aggregation was not affected by the introduction of sulfhydryl groups, and disulfide bond formation was preceded by physical interactions. Hence, disulfide interactions may not be the driving force for the aggregation of ovalbumin. Investigation of the aggregates and gels by electron microscopy and rheology suggested that a critical number of sulfhydryl groups can be introduced beyond which the microstructure of the aggregates transforms from fibrillar into amorphous. Rheological studies further suggested that covalent networks, once formed, do not have the possibility to rearrange, reducing the possibility to attain a stronger network. These results show that, even though aggregation of ovalbumin may be primarily driven by physical interactions, formed disulfide bonds are important to determine the resulting aggregate morphology and rheological properties
Number of thiol groups rather than the size of the aggregates determines the hardness of cold set whey protein gels
Alting, A.C. ; Hamer, R.J. ; Kruif, C.G. de; Paques, M. ; Visschers, R.W. - \ 2003
Food Hydrocolloids 17 (2003)4. - ISSN 0268-005X - p. 469 - 479.
heat-induced aggregation - beta-lactoglobulin - induced gelation - interchange reactions - disulfide bonds - denatured whey - isolate - ph - sulfhydryl - polymers
Variation of protein concentration during heating resulted in the formation of protein aggregates with clearly different structural and chemical characteristics. Heating conditions were chosen such that differences in the degree of aggregation were excluded. Acid induced gelation of dispersions of these aggregates resulted in gels with clearly different hardness. Although gel hardness seemed to correlate with the different structural aggregate features as reported before in literature, the differences in hardness could for the most part be cancelled by blocking of the thiol groups. Application of thiol-blocked protein aggregates enabled us to make a distinction between the effect of structural- and chemical-properties of the aggregates. Formation of larger disulfide cross-linked protein structures paralleled the increase in gel hardness and dominated the effect of structural characteristics on mechanical properties of cold-set gels. In addition, the effect of the presence of native non-aggregated protein on the final gel properties can be excluded, since in our gel-experiments most protein (>95%) participated in the formation of a protein network. Therefore, we can conclude that the hardness of cold set whey protein gels is determined by the number of thiol groups rather than by the size of the aggregates or other structural features.
Physical and chemical interactions in cold gelation of food proteins
Alting, A.C. ; Jongh, H.H.J. de; Visschers, R.W. ; Simons, J.W.F.A. - \ 2002
Journal of Agricultural and Food Chemistry 50 (2002)16. - ISSN 0021-8561 - p. 4682 - 4689.
heat-induced aggregation - whey-protein - beta-lactoglobulin - disulfide bonds - denatured whey - set gels - isolate - ph
pH-Induced cold gelation of whey proteins is a two-step process. After protein aggregates have been prepared by heat treatment, gelation is established at ambient temperature by gradually lowering the pH. To demonstrate the importance of electrostatic interactions between aggregates during this latter process, -lactoglobulin aggregates with a decreased iso-electric point were prepared via succinylation of primary amino groups. The kinetics of pH-induced gelation was affected significantly, with the pH gelation curves shifting to lower pH after succinylation. With increasing modification, the pH of gelation decreased to about 2.5. In contrast, unmodified aggregates gel around pH 5. Increasing the iso-electric point of -lactoglobulin via methylation of carboxylic acid groups resulted in gelation at more alkaline pH values. Comparable results were obtained with whey protein isolate. At low pH disulfide cross-links between modified aggregates were not formed after gelation and the gels displayed both syneresis and spontaneous gel fracture, in this way resembling the morphology of previously characterized thiol-blocked whey protein isolate gels (Alting, et al., J. Agric. Food Chem. 2000, 48, 5001-5007). Our results clearly demonstrate the importance of the net electric charge of the aggregates during pH-induced gelation. In addition, the absence of disulfide bond formation between aggregates during low-pH gelation was demonstrated with the modified aggregates