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These proteins, which include αS1‐, αS2‐, β‐, and k‐casein, have a primary amino acid sequence different from each other and occupy different positions in micelle and perform specific functions. Casein proteins contain 32–42% non‐polar amino acids which makes them highly hydrophobic but due to the presence of high number of phosphate and sulfur‐containing amino acids and carbohydrates in case of k‐casein, they are quite soluble in aqueous solvents [2]. © Copyright 2020 Hearst Communications, Inc. Identifying chaperonic function of other proteins present in milk will have many industrial and clinical insights. This actually contains two diametrically opposite theories. This model places steric restraints upon k‐casein which posses few secondary structures. Various investigators have found that αS1‐, β‐ and k‐caseins diffuse out of the micelle at low temperature due to decrease in hydrophobic interactions [42–44]. Disulfide bonds between cysteine residues during folding of pleated sheet structures, helical segments, and unordered structures leads to the formation of tertiary structure. The lack of phosphoserine cluster to bind calcium in k‐casein makes it to interact hydrophobically and act as a propagation terminator. Caseins constitute about 75–80% of total protein and precipitate at pH 4.6 at 30°C. Thus, we tread a variation of a well-worn path in biology by exploring a structure-function trichotomy. αS‐Caseins are the major casein proteins with 8–12 seryl phosphate groups, while β‐casein contains about 5 phosphoserine residues and is more hydrophobic than αS‐caseins and k‐casein [49]. The first three casein subtypes are calcium-sensitive (all except for k-casein). Casein Proteins: Structural and Functional Aspects, Milk Proteins - From Structure to Biological Properties and Health Aspects, Isabel Gigli, IntechOpen, DOI: 10.5772/64187. k‐casein at the other end is not only calcium insoluble, but it also interacts with other calcium‐sensitive caseins and stabilizes them thereby initiates the formation of the stable colloidal state. Casein micelles of most species appear quite similar at the ultra structural level. The phosphoprotein casein is made up of four different subunits -- αs1-casein, αs2-casein, b-casein and k-casein -- which together form a complex with phosphorus, calcium, water, and enzymes to make a sphere called a micelle. Earlier principle protein of bovine milk was considered to be homogenous protein casein. Calcium phosphate sequestration also depends upon the formation of phosphate centers in the primary sequence by clustering of phosphorylated residues [60]. There are many potential sites for strong ion bonding in apolar environment that might play a role in the stabilization of casein micelles. Under the normal pH, ionic strength, and temperature of milk, the major protein components of milk are insoluble. Monomers of αS1‐ or β‐caseins with charged phosphate loop form caseinate core due to addition of calcium ions. Casein in milk in its native state exists as large associate of macromolecules in colloidal dispersion with a mass of ∼ 108 Da and ∼200 nm in size [62]. According to Payens (1966) model based on his experimental data on the association of caseins, the densely folded αS1‐caseins remain adhered to loose network of β‐caseins to form micelle core. Despite the variations in casein components, the αS1‐ and αS2‐caseins are calcium sensitive, whereas β‐ and k‐casein are not sensitive to calcium. The calcium‐soluble protein would be readily split by chymosin which leads to its coagulation due to destabilization of colloid. Ambedkar Centre for Biomedical Research, University of Delhi, North Campus, New Delhi, India. According to model proposed by Holt, the casein micelle forms a tangled web of flexible casein networks forming a gel‐like structure with C‐terminal region of k‐casein extending to form a hairy layer and microgranules of colloidal calcium phosphate at center. However, the exact structure of casein and its micelles is still under debate. Due to the lack of well‐defined structure, crystallization of casein proteins to provide a three‐dimensional crystal structure is not possible, but at the other end, this lack of structure helps to facilitate proteolysis and therefore ready absorption of amino acids and small peptides in the intestine [2, 78]. Our team is growing all the time, so we’re always on the lookout for smart people who want to help us reshape the world of scientific publishing. Linderstrom‐Lang in 1929 postulated that mixture of calcium‐insoluble proteins stabilized by calcium‐soluble protein form the colloidal milk complex [39]. It is an especially good source of lysine, an amino acid that is often low in grain-based diets. Carroll et al. Caseins proteins possess very little three dimensional structure but possess some secondary structure [79]. However, the exact structure and nature of these casein micelles are still under debate. Casoxins peptide possesses opoid antagonist, whereas casoplatelin inhibits platelet aggregation. According to this model, αS1‐monomers attach to chain like polymers of β‐casein which are self‐associated from β‐casein monomers. (1971) used urea, EDTA, sodium fluoride, and sodium lauryl sulfate for the disruption of micelles and found particles ∼8 to 12 nm in diameter [28]. By Mohd Younus Bhat, Tanveer Ali Dar and Laishram Rajendrakumar Singh, Submitted: September 29th 2015Reviewed: May 11th 2016Published: September 7th 2016, Home > Books > Milk Proteins - From Structure to Biological Properties and Health Aspects. k‐casein contains only one or two phosphoseryl residues and is only casein which is glycosylated [53]. Casein proteins are homologous in all the species as has been found by various protein and gene sequencing studies [80]. A model was proposed for the native casein micelle which consists of numerous loosely packed calcium caseinate complex units joined in association by a combination of calcium and colloidal calcium phosphate–citrate linkages. This protein has two cysteine residues with no known carbohydrate. αS1‐ and β‐caseins self‐associate by hydrophobic interactions as a result of formation of train–loop–train and tail–train like structures, respectively, upon adsorption at hydrophobic interfaces. Casein micelles are composed of four main types of proteins: αS1‐casein, αS2‐casein, β‐casein, and k‐casein. Casein proteins, which form about 80% of the bovine milk proteins, form large colloidal particles with calcium phosphate to form casein micelles, which for many years have been an important subject of interest. They found very little or no concentration of k‐casein protein on the outer surface of the casein micelles as was suggested by previous workers. A brief summary of the various types of bonding forces responsible for the stabilization of protein structure will be discussed. Carroll and Farrell in 1983 also found that the location of k‐casein is indeed related to casein micelle size using ferritin‐labeled double‐antibody technique coupled with electron microscopy [33]. It has been suggested that the calcification of the mammary gland is prevented by the formation of casein‐micelle complex with calcium phosphate. 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