K-Casein

K-Casein (or Kappa-casein, k casein, kappa casein) is a mammalian milk protein involved in a number of important physiological processes. In the gut, the ingested protein is split into an insoluble peptide (para kappa-casein) and a soluble hydrophilic glycopeptide (caseinomacropeptide). Caseinomacropeptide is responsible for increased efficiency of digestion, prevention of neonate hypersensitivity to ingested proteins, and inhibition of gastric pathogens.

Caseins are a family of phosphoproteins (αS1, αS2, β, κ) that account for nearly 80% of bovine milk proteins and that form soluble aggregates known as "casein micelles" in which κ-casein molecules stabilize the structure. There are several models that account for the special conformation of casein in the micelles. One of them proposes that the micellar nucleus is formed by several submicelles, the periphery consisting of microvellosities of κ-casein Another model suggests that the nucleus is formed by casein-interlinked fibrils. Finally, the most recent model proposes a double link among the caseins for gelling to take place. All 3 models consider micelles as colloidal particles formed by casein aggregates wrapped up in soluble κ-casein molecules. Milk-clotting proteases act on the soluble portion, κ-casein, thus originating an unstable micellar state that results in clot formation.

Chymosin (EC 3.4.23.4) is an aspartic protease that specifically hydrolyzes the peptide bond in Phe105-Met106 of κ- casein and is considered to be the most efficient protease for the cheesemaking industry. However, there are milk-clotting proteases able to cleave other peptide bonds in the κ-casein chain, such as the endothiapepsin produced by Endothia parasitica. There are also several milk-clotting proteases that, being able to cleave the Phe105-Met106 bond in the κ-casein molecule, also cleave other peptide bonds in other caseins, such as those produced by Cynara cardunculus or even bovine chymosin. This allows the manufacture of different cheeses with a variety of rheological and organoleptic properties.

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