Human beta-casein

Study on the mechanism of natural polysaccharides on the deastringent effect of Triphala extract

This present study investigated the masking effect of high methoxyl pectin, xanthan gum, and gum Arabic on the astringency of the traditional herbal formula Triphala and further examined the mechanism of polysaccharide reducing astringency. Results of sensory evaluation and electronic tongue illustrated that 0.6 % pectin, 0.3 % xanthan gum, and 2 % gum Arabic had a substantial deastringent effect. The polyphenols in Triphala are basically hydrolysable tannins, which with high degree of gallic acylation may be the main astringent component of Triphala. Moreover, the three polysaccharides can combine with β-casein through CO and NH groups to form soluble binary complexes and decrease the secondary structure of β-casein. When polysaccharides were added to the Triphala-protein system, polyphenol-protein precipitation was also diminished, and they were capable of forming soluble ternary complexes. Consequently, the competition between polysaccharides and polyphenols for binding salivary proteins and the formation of ternary complexes help decrease the astringency of Triphala.

Discrete phosphorylation generates the electrophoretic heterogeneity of ovine β-casein

The heterogeneity of ovine β-casein, apparent after discontinuous gel electrophoresis at alkaline pH and gel isoelectric focusing either by staining with Coomassie blue or immunoblotting, was investigated in detail. Electrospray mass spectrometry was used to characterize ovine casein by direct examination of whole casein samples. β-Casein in individual milks consisted of several components which were characterized through the accurate determination of their molecular masses. It was demonstrated that protein species occurring simultaneously in individual ovine milks differed mainly in the number of phosphate groups.

The effect of conserved residue charge reversal on the folding of recombinant non-phosphorylated human β-casein

A short stretch of 13 amino acids in the central portion of human beta-casein contains four positively charged conserved residues, three Lys and one Arg. We changed these individually to Glu, reversing their charge, and compared the resulting recombinant proteins to the wild-type recombinant, monitoring thermal aggregation with turbidity as well as using the fluorescence of the intrinsic Trp, of hydrophobically bound ANS and fluorescence resonance energy transfer from Trp to ANS to detect differences in structure. The results demonstrate the need to maintain the actual or functional identity of these conserved charged amino acid residues in order to attain the protein folding and functional properties of the wild-type human beta-casein molecule. They emphasize the probability that native human beta-casein has a unique folding pattern that is important for its function of suspending minerals and delivering the protein and minerals to the neonate in a readily ingestible form.

Comparative aspects of milk caseins

The caseins comprise the major protein component of milk of most mammals and are secreted as micelles that also carry high concentrations of calcium. They are phosphoproteins that represent the products of four genes, equivalent to those that encode the bovine alpha s1, alpha s2, beta, and kappa-caseins. There is considerable variation in the relative proportions of the particular caseins across species. The primary sequences of the alpha s1, alpha s2, and beta-caseins also show considerable species variation consistent with rapidly evolving genes that are proposed to have a common precursor. In contrast, the kappa-caseins exhibit features that demonstrate a separate origin and function where they are proposed to stabilise the micelle structure. This review focuses on comparative aspects of the caseins across a number of species for which information is now available.

Monomer characterization and studies of self-association of the major beta-casein of human milk

The casein form that has four organic phosphoryl groups, beta-casein (CN)-4P, is the major constituent (approximately 35%) of the beta-CN fraction of human milk and should play an important role in micelle structure and formation. In 3.3 M urea, the monomer is present with a molecular mass of 24,500 Da and a sedimentation coefficient of 1.3 S (Svedberg units, 10(-13)s). In 0.02 M NaCl and 0.01 M imidazole (low salt buffer) at pH 7, the sedimentation coefficient was 1.5 S, which increased to 14 S at 37 degrees C. Laser light scattering in low salt buffer and 9 mg/ml of protein indicated monomers with a radius of about 4 nm at 4 degrees C. The size of the radius increased as temperature increased, and, at 37 degrees C, the radius was about 12 nm. The molecular mass suggested the presence of about 47 monomers per polymer. In 0.25 M NaCl and with 10 nM Ca2+ prior to precipitation, the polymer attained a maximum radius of about 15 nm, which perhaps is the size of the smallest human milk micelles. The low value for reduced viscosity of 8.2 ml/g for the calcium-induced polymer was independent of protein concentration, suggesting a spherical shape and fixed size. Calcium apparently binds strongly to the phosphates; the dissociation constant was 8.1 x 10(-4) M. Other constituents of milk, such as inorganic orthophosphate, may contribute to differences in the manner by which beta-CN, with various phosphorylation levels, participate in micelle formation.

Structural studies on casein micelles of human milk: dissociation of beta-casein of different phosphorylation levels induced by cooling and ethylenediaminetetraacetate

Information on the structure of human casein micelles has been obtained from dissociation of beta-casein (CN). Two approaches were used: cooling at 4 degrees C and addition of EDTA. An initial loss of about 80% of the protein optical density occurred upon cooling to 4 degrees C. Dissociation was time dependent, and at > or = 24 h about 10% remained. However, mean size and voluminosity of micelles increased, as indicated by laser light scattering and viscosity measurements. This process was reversible, and 95% of the protein reentered the micelles upon incubation for 3 h at 37 degrees C. Upon cooling, amounts of nonphosphorylated beta-CN increased, and singly phosphorylated beta-CN levels were almost constant relative to the total beta-CN in micelles. Upon addition of EDTA (0 to 5 mM), the forms with three to five phosphates were the major dissociating constituents; EDTA that was added by dialysis produced similar results but at lower concentrations. These data suggest that, in the absence of significant amounts of alpha s1-CN, nonphosphorylated and singly phosphorylated human beta-CN may form a framework, as proposed for alpha s1-CN for bovine milk, along with the colloidal calcium phosphate for the development of the final micelle structure by addition of the more highly phosphorylated forms. The results also indicate that human casein micelles have a less rigid structure than those of other species.

Interaction properties of doubly phosphorylated beta-casein, a major component of the human milk caseins

Doubly phosphorylated beta-casein constitutes nearly 30% of the total human beta-caseins and is thus one of the major components of that fraction. The properties and mode of association of doubly phosphorylated beta-casein are therefore important determinants of the structure and function of the human casein micelle. Doubly phosphorylated beta-casein has an absorbency of 6.2 and a partial specific volume of .74. The protein precipitated at room temperature when 10 mM Ca2+ was added but produced a clear solution in 1 M NaCl. Equilibrium dialysis produced an average of 2.06 major Ca(2+)-binding sites at 37 degrees C with a dissociation constant of 12.1 x 10(-4) M. The monomer at 20 degrees C was calculated to have a solvation of 2.1 g of H2O/g of protein and an axial ratio of 6.8, suggesting a prolate ellipsoid of about 11 by 2 nm. At high ionic strength, evidence exists for a spherical structure with a molecular weight of 2.25 x 10(6). This structure would represent a polymer of about 90 monomers with a radius of 14.8 nm and a solvation of 1.93 g of H2O/g of protein. This association behavior is similar to that of other phosphorylated human beta-caseins but differs from the nonphosphorylated form. It changes when both Ca2+ and inorganic orthophosphate are present.

Interactions of triply phosphorylated human beta-casein: monomer characterization and hydrodynamic studies of self-association

The triply phosphorylated form of human beta-casein comprises about 15% of that fraction and is thus a significant component about midway between the two extremes of zero and five phosphoryls. Its partial specific volume, v, of 0.74 +/- 0.01 and absorbancy, E1% 1 cm, 280 nm, of 6.2 +/- 0.2 are almost identical to the other human beta-caseins. Equilibrium dialysis gave an average of 3.1 +/- 0.4 major Ca2+ binding sites at 37 degrees C with Kdiss = 8.6 x 10(-4) M. Sedimentation and viscosity at low temperatures or in 3.3 M urea suggested a prolate ellipsoidal monomer with 1.4 g H2O/g protein, 10 nm in length and 1.4 nm in width. The concentrated charge of the phosphoryls may be near one end of the ellipsoid, allowing the molecules to align with the flow in the viscometer at low concentration but, due to intermolecular electrostatic interactions, not when concentration is high. This would provide a reason for the heretofore unexplained curvature in the plots of reduced viscosity, eta red, vs beta-casein protein concentration. Self-association increased with temperature. At 37 degrees C in low salt buffer, s20,W was 16 S, which increased to about 33 S as ionic strength, I, was increased to 0.2 and above. At the same time, eta red in low salt buffer decreased from about 22 ml/g at 4 degrees C to a constant value of about 5 ml/g above 23 degrees C. A similar value for eta red at 37 degrees C, which was almost independent of protein concentration, was obtained at I greater than 0.25, giving an extrapolated intrinsic viscosity value of [eta] = 4.0 ml/g. Using this value and assuming a spherical aggregate, calculations suggest a radius of 9 nm with about 48 monomers and 0.86 g H2O/g protein.

Enzymic dephosphorylation of bovine casein to improve acid clotting properties and digestibility for infant formula

To improve acid clotting properties, enzymic dephosphorylation of caseins with calf intestinal alkaline phosphatase (CAP) or potato acid phosphatase (PAP) was investigated. Greater dephosphorylation was achieved using alpha s1- or beta-casein as substrates, compared to whole casein or skim milk. Electrophoresis of PAP-modified caseins revealed bands with lower mobility and a multibanded pattern in the beta-casein region which was similar to that of human beta-casein. On the other hand, CAP modification produced electrophoretic bands having lower mobility of the beta-casein component, but with higher mobility in the alpha s1-casein component as well as increased net negative charge in the CAP-casein. PAP-casein formed a fine dispersion upon acidification to pH 4, with a microstructure similar to that of acidified human casein. Greater initial rates of hydrolysis by pepsin at pH 4 were observed for both CAP- and PAP-modified caseins, compared to bovine and human caseins. The rate and extent of hydrolysis remained high for CAP-casein but tended to level off with PAP-casein during sequential digestion with pepsin and pancreatin. There may be advantages in the use of partial dephosphorylation to improve acid clotting and digestibility properties of bovine casein for infant feeding.

Interactions in human casein systems: self-association of nonphosphorylated human beta-casein

Since caseins were originally defined as phosphoproteins, nonphosphorylated beta-casein, comprising nearly 5% of the total beta-casein in the isoelectric precipitate from human milk, appears to be unique. Despite the relatively small amount present, its properties suggest that it may play an important role in micelle formation and structure. It has a partial specific volume, v, of 0.749 +/- 0.008 and an absorbance, E1% 1 cm,280 nm of 6.2 +/- 0.2. Sedimentation and viscosity data yield a solvation of 3 g H2O/g protein and an axial ratio of about 5 for the monomer. This would be consistent with a prolate ellipsoid of 10 nm length and 2 nm width. Equilibrium in the system is attained quite slowly and the temperature-dependent polymerization was found to be reversible. With calcium, the solubility behavior reflects an increased hydrophobicity and lower electrostatic repulsion in the molecule. There is essentially no strong calcium binding to this protein but there is evidence which strongly suggests that calcium binds to nonphosphate groups at higher concentrations. Increasing the temperature from 4 to 37 degrees C causes an apparent conformational change and an increase in protein aggregation which is further increased by addition of NaCl at 37 degrees C until a limiting size is reached at about 0.1 M NaCl. This limiting size polymer contains about 75 monomers and is nearly spherical with a radius of about 12 nm and a solvation of 1.5 g H2O/g protein. Laser light scattering measurements on the solution in 0.25 M NaCl revealed a relatively homogeneous particle size with a corrected diffusion coefficient, D20,w, of 2.8 X 10(-7) cm2/s.

A 20,000-dalton casein fragment in human milk

A new peptide of 20,000 daltons was found in human milk as a constituent of the casein micelle. Enzymic digestion with plasmin or trypsin revealed that the peptide was identical with a degradation product of human beta-casein. The amino acid composition of the degradation product and the previously reported sequence in the N-terminal region of human beta-casein suggested that the peptide was a fragment of beta-casein lacking the C-terminal region. The thermal sensitivity of this peptide was higher than that of beta-casein, but the peptide lost the property of calcium-dependent precipitation, which intact beta-casein possesses.

Rat beta casein cDNA: sequence analysis and evolutionary comparisons

The complete sequence of a 1072 nucleotide rat beta-casein cDNA insertion in the hybrid plasmid pC beta 23 has been determined. Primer extension was employed to determine the sequence of an additional 82 5'-terminal nucleotides in beta-casein mRNA. Rat beta-casein mRNA consists of a 696 nucleotide coding region, flanked by 52 nucleotide 5' and 406 nucleotide 3' noncoding regions, including a 40 nucleotide poly(A) tail. The derived 216 amino acid sequence of rat beta-casein was compared to the previously determined sequences of beta-caseins from several other species. Approximately 38% of the amino acids have been conserved among the rat, ovine, bovine and human sequences and these conserved amino acids occurred in clusters throughout the protein. One such cluster containing the majority of the potential casein phosphorylation sites was located near the amino terminus. Contrary to the considerable divergence observed for the processed beta-casein, 14 of 15 amino acids in the signal peptide sequence of the precasein were identical between the rat and ovine caseins.

Phosphorylation of caseins, present evidence for an amino acid triplet code posttranslationally recognized by specific kinases

The fifty of so phosphorylated hydroxyamino acid residues hitherto investigated in caseins from different species have been found to occur in tripeptide sequences -Ser/Thr-X-A- where X represents any amino acid residue and A is an acidic residue. This strongly suggests that phosphorylation of caseins involves basically the stepwise enzymatic recognition of primary and secondary anionic amino acid triplets where the determinants are dicarboxylic residues and phosphoseryl residues, respectively. Studies of genetic variants of bovine caseins have provided clear-cut evidence for the actual occurrence of the former recognition sites. The occurrence of the above tripeptide sequences is a necessary but not a sufficient condition for phosphorylation of caseins to occur. Possible factors of constraint such as different intrinsic properties of both phosphate acceptor residues and acidic determinants, the characteristics of the local environment in terms of overall charge and hydrophilicity, secondary structure and steric hindrance, an insufficient available pool of casein kinase(s)... are discussed. All evidence now available supports the concept that phosphorylation of caseins is a posttranslational event and it is suggested that the process may occur during the transfer of completed polypeptide chains from the smooth endoplasmic reticulum to the Golgi apparatus where most of phosphate incorporation is presumably carried out. This organelle is rich in membrane-bound specific cyclic AMP-independent kinase(s) able in vitro to rephosphorylate specifically although not completely phosphatase-treated caseins and caseinophosphopeptides.