Caseins

• Casein Properties:
  
  

• Unique physical properties distinct from globular proteins.

• Influences behavior and properties of other milk constituents.

• Low solubility at pH 4.6, allowing separation from whey proteins.

• Protein Structure:
  
  

• High molecular weight polymers of α-amino acids.

• Primary structure: polypeptide chain linked by peptide bonds.

• L-configuration for all amino acids except glycine.

• Casein Composition:
  
  

• Contains ester-bound phosphate (absent in whey proteins).

• αs1- and β-caseins lack cysteine; αs2- and κ-caseins have two cysteine residues.

• High proline content (αs1: 17%, αs2: 5%, β: 17%, κ: 12%).

• Structural Features:
  
  

• No organized secondary structure.

• Contains short α-helix or β-sheet structures.

• Ionizable groups are accessible for titration and reactions.

• Denaturing agents and heat do not affect secondary structure.

• High proline content prevents closely packed, orderly conformation.

• Aggregation & Charge Distribution:
  
  

• Four caseins differ in charge distribution.

• Varying tendency to aggregate in presence or absence of Ca²⁺ ions.

αs1-CASEINS

The polypeptide chain of αs1-casein consists of two predominantly hydrophobic regions (residues 1-44, and 90-199) and a highly charged polar zone (residues 45-89).All but one of the phosphate groups is in the 45-89 residues segment, and the prolines are distributed at intervals in the hydrophobic segments.

Thus, this protein can be visualized as a rather loose flexible polypeptide chain.

Self-association of αs1-casein depends markedly on its concentration and on the pH, ionic strength, and kind of ion in the medium, but it is relatively independent of temperature.

Physical measurements such as light scattering, sedimentation, and viscosity indicate that αs1- casein is completely dissociated to a flexible random chain monomer at neutral pH and 0.01 ionic strength.

It associates at neutral pH and higher ionic strength, the extent of association depending on protein concentration, it binds about 8 moles Ca2+ per mole near pH 7, probably to the ester phosphate groups.

It aggregates and precipitates at very low concentrations of Ca2+. A small amount of peptides, sometimes called A-casein, is present in milk; these appear to originate from proteolysis of αs1-casein.

αs2-CASEIN

αs2-casein has a remarkable dipolar structure with a concentration of negative charges near the N-terminus and positive charges near the C-terminus.

Its properties have not been investigated as thoroughly as those of the other caseins, but certainly, it binds Ca strongly and is even more sensitive to precipitation by Ca2+ than αs1-casein.

It self-associates at neutral pH in the absence of Ca2+, and the association depends markedly on ionic strength and is at a maximum at an ionic strength of about 0.2.

β-CASEIN

β-casein has a strong negatively charged N-terminal portion.

The outstanding characteristic of the association of β-casein in both the absence and the presence of Ca2+ is its strong dependence on temperature.

In the absence of Ca2+, only monomer is present at 4°C, but large polymers (20-24 monomers) are formed at room temperature.

β -casein tightly binds about 5 Ca2 + per mole, consistent with its ester phosphate content.

γ-CASEINS

A group of caseins designated as γ-caseins have been known for some time to correspond to C terminal portions of the β -casein sequence.

These are formed by cleavage of β-casein by the enzyme plasmin.

κ – CASEINS

About one-third of the κ casein molecules are carbohydrate-free and contain only one phosphate group.

The N-terminal residue of κ-casein is glutamic acid.

κ-casein as isolated from milk consists of a mixture of polymers probably held together by intermolecular disulfide bonds.

κ-casein is rapidly hydrolyzed at the Phe (l05)-Met (l06) bond by the enzyme chymosin and by other proteases, yielding an N-terminal fragment called para- κ casein, which contains the two cysteine residues.

κ-casein binds about 2 moles Ca2+per mole of protein at neutral pH but differs markedly from the other caseins in its solubility over a wide range of Ca2 ++ concentrations.