A biological perspective on the structure and function of caseins and casein micelles

Abstract

Caseins belong to a larger group of secreted calcium phosphate‐binding phosphoproteins that have a natively unfolded conformation. Nearly all members of the group are involved in aspects of calcium phosphate biology and nearly all have recognition sites for phosphorylation by the Golgi protein kinase. In the caseins these are often close together in the primary structure, forming the so‐called phosphate centres. Certain highly phosphorylated phosphopeptides derived from the calcium‐sensitive caseins will combine with amorphous calcium phosphate to form defined chemical complexes called calcium phosphate nanoclusters. Both the substructure of casein micelles and the partition of salts in milk can be explained quantitatively by the ability of the calcium‐sensitive caseins to sequester calcium phosphate and form nanocluster‐like structures. A simple stability rule for milk can be derived by applying equilibrium thermodynamics to the process of calcium phosphate sequestration. In principle, the stability rule can be extended to problems of instability encountered in milk‐processing operations and to the formulation of other types of high calcium foods.

Seasonal variation, method of determination of bovine milk stability, and its relation with physical, chemical, and sanitary characteristics of raw milk

“…High values for SCC are probably related to low milk stability due to proteolysis, low casein contents, high values of sodium and chlorine contents, and micelle destabilization (Chavez et al, 2004;Horne, 2015); on the other hand, high values for TBC are related to low milk stability, probably due to proteolysis and lactose degradation, which leads to lower pH and calcium phosphate values and further increase in ionic calcium concentrations and micelle destabilization (Auldist and Hubble, 1998;Chavez et al, 2004;Bueno et al, 2008;Lewis and Deeth, 2009). The increase in the paracellular flow of components between the blood stream and the milk when SCC is elevated (Stelwagen et al, 2000) leads to elevated levels of sodium, chloride, and phosphorus in milk (Smyth et al, 2004) and might be related to low stability. However, Donatele et al (2003) and Kolling (2012) found no relationship between TBC, SCC, and milk stability.…”

The evolution of milk secretion and its ancient origins

“…The caseins are phosphorylated during synthesis, and aggregate into large micelles containing calcium bound to phosphorus in calcium phosphate nanoclusters (Smyth et al, 2004). Multiple caseins participate in these micelles; however, k-casein plays a particularly important role in stabilizing the micelle in secreted milk.…”

“…By an unknown evolutionary process, the different caseins became involved in the formation of complex micelles stabilized by calcium and phosphate bonds (Smyth et al, 2004). This transformation of ancestral SCPPs into a complex of micelle-forming proteins was essential in converting milk from an egg supplement to a major source of nutrients for the suckling young.…”

Formation and stability of poly-L-lysine/casein multilayers

“…β-casein comprises 38% of total casein contents. The function of these caseins is to store and transport bio-available metal ions (especially Ca(II) and Mg(II)) by sequestering and transporting them from mother to the neonate (Alaimo et al 1999;Smyth et al 2004;Horne 2002). The function of κ-casein is to stabilize casein micelles (Horne 2002).…”