Biochemical Characteristics of Milk Proteins

Table adapted from RD Bremel course notes, University of Wisconsin.

Protein Processing and Milk Proteins

GLYCOSYLATION

Glycosylation of proteins occurs in the RER and Golgi. Membrane-bound enzymes (glycosyltransferases) are responsible for adding sugars to AA's (Asp, Ser, Thr). This involves a complex set of reactions where sugars are put on and taken off of the proteins. Galactosyltransferase (part of the lactose synthase complex) is one example of this type of glycosyltransferase.

Glycoproteins are very common. The carbohydrate moieties of glycoproteins are involved in many functions, including:

• stabilization of protein conformation,
• increases the half-life of serum proteins,
• involved in membrane-membrane interactions,
• act as a signal for the intracellular routing of proteins to subcellular compartments.

Glycosylation does not necessarily have anything to do with protein function:

• Carbohydrate free a-lactalbumin is equally as active as glycosylated a-lactalbumin.
• k-Casein is the major glycosylated milk-specific protein synthesized by the epithelial cells. It is very heterogeneously glycosylated.
• Generally, k-casein from colostrum has a higher carbohydrate content than that from mature milk.

PHOSPHORYLATION

Phosphorylation is a ubiquitous biological process. It is involved in many regulatory mechanisms. It occurs after completion of the polypeptide chain. It can occur at Ser, Thr, Glu, Asp, His, Lys, and Tyr. Many enzymes will phosphorylate proteins with ATP, but they differ in specificity. These are called Kinases. Kinases are membrane-bound in the smooth endoplasmic reticulum and Golgi.

Caseins are phosphoproteins.

• k-Casein is not extensively phosphorylated- probably because of competition between the glycosyltransferases and the kinases for sites on the protein.
• Caseins are in multiphosphorylated forms - the same casein polypeptide chain may be phosphorylated at varying sites.
• Probably due to the tremendously high rate of protein synthesis which exceeds the phosphorylating capacity of the membrane-bound kinases.

A whey protein found in the milk of rats (WAP, whey acidic protein or P-protein) has been found in 4 phosphorylated forms- 1, 2, 3, or 4 phosphates. A similar protein in mouse milk is not phosphorylated.