The preparation of k -casein

The role of glycosylation in amyloid fibril formation of bovine κ-casein

In order to explore the functions of glycosylation of κ-Casein (κ-CN) in bovine milk, unglycosylated (UG) and twice glycosylated (2G) forms of κ-CN B were purified by selective precipitation followed by anion exchange chromatography from κ-CN BB milk and tested for their amyloid fibril formation and morphology, oligomerisation states and protein structure. The diameter of self-assembled κ-CN B aggregates of both glyco-form were shown for the first time to be in the same 26.0-28.7 nm range for a 1 mg mL^-1 solution. The presence of two bound glycans in the protein structure of 2G κ-CN B led to a greater increase in the maximum amyloid fibril formation rate with increasing protein concentration and a difference in both length (82.0 ± 29.9 vs 50.3 ± 13.7 nm) and width (8.6 ± 2.1 vs 13.9 ± 2.5 nm) for fibril morphology compared to UG κ-CN B. The present results suggest that amyloid fibril formation proceeds at a slow but steady rate via the self-assembly of dissociated, monomeric κ-CN B proteins at concentrations of 0.22-0.44 mg mL^-1. However amyloid fibril formation proceeds more rapidly via the assembly of either aggregated κ-CN present in a micelle-like form or dissociated monomeric κ-CN, packed into reorganised formational structures above the critical micellar concentration to form fibrils of differing width. The degree of glycosylation has no effect on the polarity of the adjacent environment, nor non-covalent and disulphide interactions between protein molecules when in the native form. Yet glycosylation can influence protein folding patterns of κ-CN B leading to a reduced tryptophan intrinsic fluorescence intensity for 2G compared to UG κ-CN B. These results demonstrate that glycosylation plays an important role in the modulation of aggregation states of κ-CN and contributes to a better understanding of the role of glycosylation in the formation of amyloid fibrils from intrinsically disordered proteins.

The cysteine content of caseion micelles

Following enzymic digestion with pronase, masked SH groups in micellar casein became available for titration in disaggregating media with mercurial reagents. The content of cystine and cysteine was also estimated after reduction with (a) sulphite, and (b) borohydride, and also by reaction in alkaline conditions with Cd(OH)2. The results show that the micelles contain mainly cysteine, and that it is likely that cystine is not present.

Simple methods for the purification of crude κ-casein and β-casein by treatment with calcium phosphate gel

Batch methods applicable on a large scale are described for the purification of crude κ- and βκ-Casein, dissolved in urea-containing buffer, was freed from αs- and β-caseins by treatment with calcium phosphate gel and recovered in about 60% yield. β-Casein was freed from most impurities by adsorption on to calcium phosphate gel at pH 7·8 in the presence of urea and elution with 6 M-urea–N-NH4OH at 4°C. The recovery was about 50%.

The effect of preparative conditions on the composition of the k-casein complex

The effects of changes in the conditions of preparation on the composition of the k-casein complex have been studied and a method of preparation is suggested which embodies the optimal conditions. The varied conditions include temperature of precipitation of the acid casein, the pH, temperature, CaCl2 concentration and duration of the CaCl2 treatment, and the conditions of centrifuging. Changes in composition caused by alcohol fractionation are reported and the results are discussed from the point of view of k-casein as a complex of three or more proteins.

An electron microscope study of the internal structure of casein micelles

A study of casein micelles was made with the electron microscope, using very thin sections cut from micelles embedded in Araldite. The micelles appear to be built up of units that are approximately spherical, about 100 Å in diameter and of about 300000 molecular weight.

Comparative study of methods for the isolation and purification of bovine kappa-casein and its hydrolysis by chymosin

kappa-Casein was purified from a single batch of whole acid casein (kappa-A variant) using different methods in order to compare their merits in producing a purified material with a carbohydrate and phosphate heterogeneity representative of the whole kappa-casein complement in milk. Ion-exchange methods of purification gave products of higher purity than precipitation techniques involving final purification by ethanol fractionation, but all methods resulted in kappa-caseins of apparently similar heterogeneity and chemical composition. The purified kappa-caseins were hydrolysed with chymosin and the derived macropeptides isolated. These were all virtually identical as determined by reversed-phase chromatography and gel electrophoresis. Some observations on chymosin hydrolysis of kappa-casein were made. In addition to formation of the major para-kappa-casein (Glu1-Phe105) and macropeptide (Met106-Val169), chymosin hydrolysis at pH 6.6 also resulted in two minor para-kappa-caseins with N-termini corresponding to Phe18 and Ser33 of kappa-casein. At pH 5.5 and 4.5 para-kappa-casein was rapidly hydrolysed into at least six fragments, one of which had an N-terminus corresponding to Trp76 of kappa-casein. At pH 6.6, 5.5 and 4.5 the kappa-casein macropeptide was stable to chymosin, but at pH 2.3 it was hydrolysed by chymosin into fragments with N-termini corresponding to Met106, Ile125, Ala138, Val139, Thr145 and Glu147 of kappa-casein.

Proteolysis of bovine alpha s2-casein by chymosin

Proteolysis of bovine alpha s2-casein by chymosin (E. C. 3.4.23.4) in solution in 100 mM Na phosphate buffer, pH 6.5, at 30 degrees C was studied by reversed-phase (RP)-HPLC and urea-polyacrylamide gel electrophoresis (PAGE). Chymosin hydrolyzed alpha s2-casein in solution to eight peptides detectable by urea-PAGE. Peptides soluble in acetate buffer, pH 4.6, were isolated by RP-HPLC on a C18 column using an acetonitrile/water gradient and identified from their N-terminal amino acid sequence. The chymosin cleavage sites were at the bonds Phe88-Tyr89, Tyr95-Leu96, Gln97-Tyr98, Tyr98-Leu99, Phe163-Leu164, Phe174-Ala175 and Tyr179-Leu180. Chymosin cleavage sites were restricted to the hydrophobic regions of the molecule. The bond-type in alpha s2-casein cleaved by chymosin was in agreement with that found to be susceptible to chymosin in other caseins. The primary site of chymosin action on alpha s2-casein appeared to be at Phe88-Tyr89.

Proteolytic specificity of chymosin on bovine alpha s1-casein

The proteolytic specificity of chymosin (EC 3.4.23.4) on bovine alpha s1-casein at 30 degrees C in phosphate buffer, pH 6.5 and at pH 5.2 in the presence of 5% (w/v) NaCl was investigated. Peptides (pH 4.6-soluble) were isolated by reversed-phase HPLC and identified from their amino acid sequence; the identity of some peptides was confirmed by mass spectrometry and/or amino acid composition. The small peptides produced at pH 6.5 were Arg1-Phe23, Phe24-Phe28, Phe24-Leu40(?), Phe150-Phe153, Phe150-Leu156, Tyr154-Tyr159, Tyr154-Trp164, Asp157-Trp164 and Tyr165-Trp199. The same peptides, except Tyr154-Trp164, were produced at pH 5.2 in the presence of NaCl and, in addition, the peptides Arg1-Leu11, Phe24-Phe32, Lys102-Leu142, Ala143-Leu149 and Tyr165-Phe179. The rates of production of individual peptides differed under the two conditions studied but Arg1-Phe23 and Tyr165-Trp199 were the first and second peptides produced under both conditions. Pathways are proposed to interpret the proteolysis of alpha s1-casein in solution under the conditions of this study.

Structural aspects of the milk clotting process. Comparative features with the blood clotting process

The enzyme chymosin and its substrate, a casein fraction called k-casein, are involved in the milk clotting process. Recent data concerning the structure (peptide and sugar moieties) of various k-caseins and their role in casein micelles formation and stabilization are presented. The molecular events occurring during the primary phase of chymosin action on k-casein are discussed. Finally some structural features concerning more particularly the caseinoglycopeptides and the fibrinopeptides as well as the action of chymosin and thrombin involved in the milk and blood clotting processes are compared. Three examples of sequences of portions of k-caseins and fibrinogen presenting homology are presented.

A rennin-sensitive bond in alpha-s1 B-casein

Rennin acts on a specially sensitive bond in αs1B-casein to produce a basic peptide containing residues 1–23 of the original protein. At pH 6·4 and 30°C, the action is specific and rapid, the kinetic constants beingKm4·5×10−4M,Kcat3·8 s−1, andkcat/Km0·85×104s−1M−1. Pepsin, and a protease impurity in the acid phosphatase from wheat germ, have a similar action.