Effects of chemical phosphorylation of bovine casein components on the properties related to casein micelle formation (original) (raw)
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A biological perspective on the structure and function of caseins and casein micelles
International Journal of Dairy Technology, 2004
Caseins belong to a larger group of secreted calcium phosphate-binding phosphoproteins that have a natively unfolded conformation. Nearly all members of the group are involved in aspects of calcium phosphate biology and nearly all have recognition sites for phosphorylation by the Golgi protein kinase. In the caseins these are often close together in the primary structure, forming the so-called phosphate centres. Certain highly phosphorylated phosphopeptides derived from the calcium-sensitive caseins will combine with amorphous calcium phosphate to form defined chemical complexes called calcium phosphate nanoclusters. Both the substructure of casein micelles and the partition of salts in milk can be explained quantitatively by the ability of the calciumsensitive caseins to sequester calcium phosphate and form nanocluster-like structures. A simple stability rule for milk can be derived by applying equilibrium thermodynamics to the process of calcium phosphate sequestration. In principle, the stability rule can be extended to problems of instability encountered in milk-processing operations and to the formulation of other types of high calcium foods.
Study of the dissociation of β-casein from native phosphocaseinate
Le Lait, 1994
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Studies of the biological function and structure of casein micelles, and future implications
At the heart of the milk system are the colloidal casein-calcium-transport complexes termed the casein micelles. Despite extensive research in more than half a century, accomplished by the application of a wide range of physical techniques such as light, neutron, and X-ray scattering, and Electron Microscopy (EM), the molecular details of casein micelles and the main contributing forces that stabilize them in milk remain controversial and still sustain interest and effort in the dairy science research community. Among various proposed molecular models, two main confl icting theories about the internal structure of casein micelles have emerged. One places the emphasis on the four main protein constituents, α s1 -, α s2 -, βand κ-casein in the micelles, while the other proposes that inorganic calcium phosphate nano-clusters are the dominant players in holding the micelles together. In this chapter, casein micelles are examined in the light of recent advances in understanding protein-protein interactions (associations) and protein structurefunction relationships. The biological signifi cance of casein micelles, in relation to their unique construct, allows for an effi cient transit through the mammary secretory apparatus and this is critically assessed, in addition to the existing overwhelming amount of evidence supporting the argument that proteinaceous complexes act as the formative agents in the synthesis of casein micelles in mammary tissue; in other words, that protein-protein interactions are paramount in the formation and stabilization of casein micelles.
Composition, Structure, and Integrity of Casein Micelles: A Review
Journal of Dairy Science, 1984
This review is an attempt to bring what is known about casein micelles together into a coherent summary. Some of the earlier models and theories no longer appear workable, and much is left to be concluded on this subject. It is hoped with what we now know that those who already have contributed and others will continue to pursue the understanding of casein micelles. PHYSICAL PROPERTIES OF CASEIN MICELLES The biological function of bovine casein micelles is to provide efficient nutrition to the young calf. It does not require inherently a high degree of ordered structure but, rather, an effective mechanism for secretion of a highly concentrated solution of protein, calcium, and phosphate. During the past 20 yr research has been extensive to determine the composition and structure of casein micelles and to identify forces that maintain their integrity. The approximate composition of bovine casein micelles is in Table 1. In cows' milk, casein micelles occur in colloidal dispersion. Structure and properties of the casein group of proteins, which comprise over 90% of the mass of casein micelles, have been reviewed (11, 13, 76, 89, 90, 98). Casein micelles are highly hydrated and spongelike colloidal particles containing about 3.7 g H20/g protein (6, 47, 48). Relatively little of this water (.5 g H20/g protein) is bound to the protein. The remainder is occluded within the micelle and moves with the micelle during hydrodynamic experiments. Numerous models describing casein micelles have been
Purification and properties of a major casein component of rat milk
Biochimica et biophysica acta, 1981
A casein component (C2-casein) was purified by ion-exchange and gel filtration chromatography from rat milk, and the properties of this protein were examined. The molecular weight of C2-casein, as determined by Sepharose 4B gel filtration in 6 M guanidine hydrochloride, was 34 000 +/- 1000. The average hydrophobicity calculated from the amino acid composition showed that C2-casein is a rather hydrophilic protein. The alpha-helix content obtained from optical rotatory dispersion experiments was about 12%. In ultracentrifugation analyses, monomer and polymer peaks of C2-casein were both seen, and the monomer-to-polymer ratio was not affected by changing temperature conditions. C2-casein was precipitated by the presence of 2.5 mM CaCl2, and the precipitability was greatly decreased by the dephosphorylation of the protein. C2-casein was stabilized from Ca2+-dependent precipitation by the addition of another rat casein component (C3-casein) or of bovine kappa-casein.
European Biophysics Journal, 2018
The white appearance of skim milk is due to strong light scattering by colloidal particles called casein micelles. Bovine casein micelles comprise expressed proteins from four casein genes together with significant fractions of the total calcium, inorganic phosphate, magnesium and citrate ions in the milk. Thus, the milk salts are partitioned between the casein micelles, where they are mostly in the form of nanoclusters of an amorphous calcium phosphate sequestered by caseins through their phosphorylated residues, with the remainder in the continuous phase. Previously, a salt partition calculation was made assuming that the nanoclusters are sequestered only by short, highly phosphorylated casein sequences, sometimes called phosphate centres. Three of the four caseins have a proportion of their phosphorylated residues in either one or two phosphate centres and these were proposed to react with the nanoclusters equally and independently. An improved model of the partition of caseins and salts in milk is described in which all the phosphorylated residues in competent caseins act together to bind to and sequester the nanoclusters. The new model has been applied to results from a recent study of variation in salt and casein composition in the milk of individual cows. Compared to the previous model, it provides better agreement with experiment of the partition of caseins between free and bound states and equally good results for the partition of milk salts. In addition, new calculations are presented for the charge on individual caseins in their bound and free states.
Agricultural and Biological Chemistry, 1982
Bovine casein micelles were fractionated on controlled pore granule (CPG-10/3000) chromatography by size and the chemical properties of the fractionated micelles were compared. The results indicated the presence of two types of micelles distinguishable as large and small micelles. In skim milk, 72.7% ofcasein was calculated to be in the form of small micelles, 13.6% in the form of large micelles and 13.8% in non-micellar casein form. The asl-casein content decreased, but /?-and fc-casein content increased as the micelle size becamesmaller. K>Casein in large micelles had a much higher sialic acid content than in small micelles. It was found that this difference in sialic acid content was due to the presence of nonglycosylated /c-casein in small micelles. In large micelles, non-glycosylated K-casein was almost undetectable. The addition of wheat germ lectin to micelles resulted in the formation of aggregates through intermicellar bridges between the carbohydrate chains of jc-casein located on the surface of the micelles. Both large and small micelles formed aggregates after the addition of wheat germ lectin. Large micelles were more sensitive to wheat germ lectin than small ones.
Calcium-induced aggregation of bovine caseins: effect of phosphate and citrate
Colloid and Polymer Science, 2006
The formation of colloidal particles by Ca 2+ precipitation of whole caseinates in the presence of phosphate (Pi), citrate (Cit), or both of the anions in concentrations found to be effective in previous works was followed comparing the colloidal particle size and the ionic and proteic composition of the precipitates obtained. Ca 2+ was incorporated to the precipitate and colloidal particles in a different way than Pi, differences which were related to the presence of Pi and/or Cit in the media. A sequential salting-out process due to progressive Ca 2+ binding to at least two kinds of sites was observed. The precipitation curves were fitted, and the affinity constants and binding site numbers were calculated with a modification of the Farrell's equation based on the concept of Wyman's linked functions. Precipitates obtained at low total Ca 2+ concentrations in different conditions varied their casein composition. Colloidal particles appeared at the beginning of the second salting-out step, in different amount, and in average size according to the presence or absence of Pi and/or Cit in the media. Consideration of these differences showed that Cit favored the formation of bigger colloidal particles, acting especially in the first steps of the casein aggregation and conditioning the mechanism of this process.
Composition and size distribution of bovine casein micelles
Biochimica et Biophysica Acta (BBA) - General Subjects, 1980
Chromatography of glutaraldehyde-fixed skim-milk on controlled-pore glass (CPG-10, 300 nm) gave three micellar fractions whose averaged diameters, measured by electron microscopy, decreased progressively with increasing elution volume. Casein mieelles with diameters up to 680 nm were detected. The casein composition of the same fractions from unfixed skim-milk was determined. As the fraction elution w)lume increased, K-casein varied from 7.7 to 11.4% of total casein, giving as/~ ratios of 6.1, 4.7 and 3.3. A plot of g-casein content versus mieelle surface-to-volume ratio for skimmilk and the column fractions approximated to a straight line. Re-calculation of the published results from two other studies also gave linear relationships between u-casein content and surface area for artificial micelles. The three regression lines thus obtained had small intercepts. It was concluded that the data indicated the same fundamental structure for casein micelles, with a predominant surface location for ~-casein, whether the micelles are natural or artificial and whether they are aggregated by Ca "-+ alone or by Ca 2÷ together with calcium phosphate-citrate complex.