Milk Composition & Synthesis
Major Milk Proteins
All of the major milk proteins (except serum albumin and immunoglobulins) are synthesized by epithelial cells in the mammary gland from amino acids (AA's) extracted from the blood.
Found in milk in the form of a Micelle (a dense protein granule). The phosphate groups covalently bound to the casein molecules are involved in binding Calcium (Ca) via ionic bonds. After caseins are phosphorylated, Ca++ binds to the phosphate to initiate polymerization of the micelle particles. This [casein-PO4- Ca++ - PO4-casein] structure is key to micelle formation.
The casein micelle functions as a source of nutrients for the neonate: supplying amino acids, calcium and phosphate.
Micelles are ~140 nanometers in diameter. They are composed of alpha- (as-), beta-, and kappa-caseins. a-Caseins are in multiphosphorylated forms (as2, as3, as4, as5, and as6). ß-Casein is a major casein in cow milk, but is the minor casein in human milk. k-Casein (a glycoprotein) is distributed throughout the casein micelle and acts to stabilize the micelle. Gamma-caseins (part of the proteose peptone fraction of milk) are C-terminal fragments of ß-casein, which are released by plasmin digestion, mostly while the milk is in the gland.
The image below shows several secretory vesicles containing electron-dense casein micelles.
The destabilization of the casein micelle structure and partial hydrolysis of casein decreases the quality of fluid milk and the yield of cheese from milk. Conversely, the destabilization of the casein micelle by proteases is part of the mechanism involved in milk digestion in the stomach and intestine. Controlled hydrolysis of casein is also the means of producing cheeses and other cultured milk products.
Major Whey Proteins:
ß-LACTOGLOBULIN (ß-LG): Is ~50 % of the total whey protein content in milk. Is the major whey protein in ruminants and pigs. It is not found in milk of many species. The function of ß-LG is unknown; it may be a fatty acid or lipid binding protein. It does have sequence similarities with retinol-binding proteins, but this may not be its function. Generally it is found in milk of species which transport high levels of immunoglobulins during colostrum formation, however the specific relationship between the presence of ß-LG and immunoglobulin transport remains unclear.
a-LACTALBUMIN (a-LA): Is ~25 % of the total whey protein content in milk. Is the "B" protein of the Lactose Synthase enzyme complex (see Lactose Synthesis Lesson). Therefore, it is of major interest in terms of the control of milk secretion. May have other nonspecific effects on the integrity of milk fat membranes. a-LA binds Ca and Zn.
Minor Whey Proteins:
SERUM ALBUMIN (SA) : Serum albumin comes from the serum; it is not synthesized in the mammary gland. It is presumed to enter the milk via "leakage" by the paracellular pathway, or by uptake with other components such as immunoglobulins. There does not seem to be a more specific mechanism of transport. Increases in milk concentration of serum albumin occur especially during mastitis and during mammary involution. Function of serum albumin in milk is unknown. It does bind to fatty acids, as well as other small molecules.
IMMUNOGLOBULINS (Ig): include IgG1, IgG2, IgA, IgM : Immunoglobulins are in very high concentrations in colostrum, but much lower concentrations in milk. Immunoglobulins are part of the passive immunity transported to the neonate via colostrum in many species. They are part of the mammary immune system. Secretory Component (SC) is the part of the IgA receptor which hydrolysed by a protease and remains attached to IgA as it is secreted from the cell.
Other Milk Proteins
Lactoferrin (LF) - Is an iron binding protein and has antibacterial properties. Relatively low concentrations during lactation in cow milk, but is increased during mastitis and involution. Lactoferrin may also be an immunomodulator. Lactoferrin is in human milk in high concentrations; in fact it is the major whey protein in human milk. Lactoferrin is the major nonspecific disease resistance factor found in the mammary gland. [See also the sections on Dry Period and Mammary Involution and Mastitis).
Lactoperoxidase - Is an enzyme that breaks down hydrogen peroxide. It has antibacterial properties.
Lysozyme - Is an enzyme that cleaves the carbohydrate polymers of the bacterial cell wall. High activity in human milk and possibly in horse milk, but very low activity in cow milk.
ß2-Microglobulin - Initially it was discovered as a crystalline precipitate in resuspended casein and initially was called lactolin. It is now know to be part of the Major Histocompatability Complex II (MHC II). While the function in milk is not known, it more recently has been found in the epithelial cell to be associated with a protien that binds to immunoglobulin G. It is thought that ß2-Microglobulin may be involved in the functionality of that IgG receptor in the process of IgG transport during colostrum formation. This latter function for ß2-Microglobulin remains controversial.
Milk has numerous other enzymes including: proteases, protease activators, nucleases, glycosidases, and others
Skim milk has cell membranes and their associated membrane-bound enzymes such as galactosyltransferase (the "A" protein component of Lactose Synthase; see the Lactose Synthesis section ).
Milk Composition & Synthesis