The primary structure of proteins consists of a polypeptide chain of amino acids residues joined together by peptide linkages, which may also be cross-linked by disulphide bridges. Amino acids contain both a weakly basic amino group, and a weakly acid carboxyl group both connected to a hydrocarbon chain, which is unique to different amino acids. The three-dimensional organization of proteins, orconformation, also involves secondary, tertiary, and quaternary structures. The secondary structure refers to the spatial arrangement of amino acid residues that are near one another in the linear sequence. The alpha-helix and ß -pleated sheat are examples of secondary structures arising from regular and periodic steric relationships. The tertiary structure refers to the spatial arrangement of amino acid residues that are far apart in the linear sequence, giving rise to further coiling and folding. If the protein is tightly coiled and folded into a somewhat spherical shape, it is called a globular protein. If the protein consists of long polypeptide chains that are intermolecularly linked, they are called fibrous proteins. Quaternary structure occurs when proteins with two or more polypeptide chain subunits are associated.
Milk Protein Fractionation
The nitrogen content of milk is distributed among caseins (75%), whey proteins (18%), miscellaneous proteins (2%) and non-protein nitrogen (5%). This nitrogen distribution can be determined by the Rowland fractionation method:
- Precipitation at pH 4.6 - separates caseins from whey nitrogen
- Precipitation with sodium acetate and acetic acid (pH 5.0) - separates total proteins from whey NPN
Ninety-five percent of the nitrogen is associated with protein. The average concentration of proteins in milk is as follows (although there can be considerable natural variation):
|grams/ litre||% of total protein|
|Total Whey Proteins||6.4||19.4|
Caseins, as well as their structural form - casein micelles, whey proteins, and milk enzymes will now be examined in further detail.