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Sources, isolation and properties of protein ingredients - Caseins

6 important questions on Sources, isolation and properties of protein ingredients - Caseins

What is the cause of caseins being random-coil proteins?

Large amount of proline residues in the amino acid sequence. The proline residue induces a specific rotation of the peptide backbone, hindering the formation of alpha-helices and beta-sheets.

Why do caseins associate in micelles?

Because of the random-coil structure, caused by proline residues.
Unequal distribution of hydrophobicity and charge over the primary sequence

How big is a 'native' casein macro-micelle?

300 nm

Describe a casein macro-micelle

On the outside are k-caseins, which contain a larger carbohydrate moiety. In the macro-micelle, calcium (Ca2+) ions are present. These ions can interact with the phospho-serine groups on the casein molecules, Ca-phosphate bridges. Also, free phosphate is present in complexes with the Ca2+-ions.

What is done with casein protein molecules (e.g. Modification)?

They are phosphorylated. The normally non-charged serine residues are modified (post-translational modification) by addition of an acidic phosphate group. Results in a decrease of the pI. Phosphate groups can have a strong interaction with calcium (Ca2+). Alpha-s1 and alpha-s2 are the most heavily phosphorylated.

Why do caseins not precipitate at pH 6.7 and what would change if they did precipitate?

Because there is electrostatic repulsion and the carbohydrate chains of the k-casein result in positive interaction with the water.

Lowering the pH or addition of rennet which removes the carbohydrate chain will result in precipitation of the caseins.