Casein, Alpha

C.A.S.: 9000-71-9

The caseins are a group of proteins whose hydrophobicity and relatively high charge differentiate them from most other proteins. alpha_s1-Casein is the major protein fraction of bovine milk. A phosphoprotein, it exists as an equilibrium of soluble and complex colloidal aggregates (micelles). The casein is solubilized by dialyzing skim milk against phosphate buffer or by dilution. Bovine caseins include alpha_s1, alpha_s2, beta, and kappa-casein. The following information pertains to primarily bovine alpha_s1-casein.

History:

In 1925 Linderstrom-Lang and Kodama first demonstrated the caseins to be composed of two discrete fractions instead of a homogeneous substance as they were initially regarded. In 1939 Mellander used zone electrophoresis to demonstrate three components he designated alpha-, beta-, and gamma-caesin.

Beginning in the 1950s, several models for the structure of casein association colloids (micelles) were proposed (Waugh and von Hippel 1956, Hill and Wake 1969, Bingham et al. 1972, Creamer et al. 1973, and Swaisgood 2003). In 1961, Wake and Baldwin refined the electrophoresis and found even more components. The tight linkage of alpha_s1-casein and beta-casein genes was first demonstrated in 1964 (Grosclaude et al. 1964), and the order of the three casein loci was first proposed in 1973 (Grosclaude et al. 1973).

As molecular biology techniques became available in the 1970s and 1980s, it was determined that the heterogeneity is in fact due to effects of post-translational processing, genetic polymorphisms, and alternative splicing of the gene product. Bovine caseins were confirmed as alpha_s1, alpha_s2, beta, and kappa-casein (Ng-Kwai et al. 1992, and Ginger and Grigor 1999).

Recently, investigators have studied the effects of selective serotonin reuptake inhibitor (SSRI) administration on inhibition of milk protein gene expression in vitro and in vivo and the consequent reduction of milk yield (Hernandez et al. 2011). Researchers have also characterized the caseins of goat milk to evaluate its suitability as an alternative for subjects allergic to cow’s milk (Ballabio et al. 2011).

Molecular Characteristics:

The gene that encodes alpha_s1-casein (csn1s1) is located on chromosome 6 in Bos taurus (GenBank accession: 282208). The as1-casein amino acid sequences of various species including rabbit (Devinoy et al. 1988), guinea-pig (Hall et al. 1984), mouse (Henninghausen and Sippel 1982), and human (Johnsen et al. 1995) have been found to contain a highly conserved 15 residue signal sequence but divergent mature chain sequences. The C-terminal region of bovine and rat contain only 29% identity, while bovine and pig share 48% identity. The alpha_s2-Caseins and beta-caseins also contain the highly conserved 15 residue signal sequence and divergent mature chain sequences (Ginger and Grigor 1999). kappa-Casein contains a 21 residue signal sequence (Bonsing et al. 1988).

Composition:

alpha_s1-Casein is the most abundant protein of bovine milk. It is highly phosphorylated and exists as a major and minor form. The major form contains 8 bound phosphates and the minor 9 bound phosphates (Eigel et al. 1984). Phosphorylation occurs at Ser/Thr-X-Y motifs (Mercier 1981).

alpha_s2-Casein is also highly phosphorylated. That of bovine milk consists of four isoforms that contain between 10 and 13 phosphates (Brignon et al. 1977, and Eigel et al. 1984).

Bovine beta-casein exists in only one form, which contains 5 phosphates per mole. The beta-caseins of other species including human (Greenberg and Groves 1979), goat (Richardson and Mercier 1979), and possum (Ginger et al. 1999) contain multiple forms. Bovine milk has also been found to contain fragments of minor peptides including gamma₁, gamma₂, and gamma₃-caseins. The proteose peptone components 5 and 8-fast were initially found to be soluble in acid conditions and thought to be whey proteins. However, they were eventually determined to be products of the partial proteolysis of beta-casein by plasmin (Gordon et al. 1972, Groves et al. 1973, Eigel 1977, and Eigel 1981).

kappa-Casein is the only casein soluble in the presence of calcium ions. It also has the smallest amount of phosphate, with phosphorylation sites being present only in the C-terminal region. kappa-Casein is the only casein to contain carbohydrate moieties (Jollés et al. 1978).

Except for short alpha-helical regions, caseins have little secondary or tertiary structure (Huppertz et al. 2006). The caseins of milk exist in the form of colloidal complexes called micelles. The micelles contain an amorphous micellar calcium phosphate core, surrounded by a phosphopeptide (casein) shell (Swaisgood 2003).

Protein Accession Number:

P02662

Molecular weight:

• 27 kDa (McKenzie 1967)
• 23.6 kDa (Mercier et al. 1971)

Isoelectric Point:

4.91 (Theoretical)

Extinction Coefficient:

• E_{1%, 280} = 10.1 (Thompson and Kiddy 1964)

Applications:

• Casein allergenicity studies
• Marker peptides for the detection of milk allergen in food samples
• Substrate in protease activity assays (beta-casein) (Mandl et al. 1953)