Digestive System > Stomach

Chymosin (Rennin) and the Coagulation of Milk

"Blessed are the cheesemakers" Monty Python

Chymosin, known also as rennin, is a proteolytic enzyme related to pepsin that synthesized by chief cells in the stomach of some animals. Its role in digestion is to curdle or coagulate milk in the stomach, a process of considerable importance in the very young animal. If milk were not coagulated, it would rapidly flow through the stomach and miss the opportunity for initial digestion of its proteins.

Chymosin efficiently converts liquid milk to a semisolid like cottage cheese, allowing it to be retained for longer periods in the stomach. Chymosin secretion is maximal during the first few days after birth, and declines thereafter, replaced in effect by secretion of pepsin as the major gastric protease. Chymosin is secreted in the neonatal stomach of ruminants (cattle, goats, camels), pigs, cats, and rats. Animals including humans, chimps, and horses have inactivating mutations in their chymosin gene and do not secrete the enzyme.

Chymosin is secreted as an inactive proenzyme called prochymosin that, like pepsin, is activated on exposure to acid. Chymosin is also similar to pepsin in being most active in acidic environments, which makes sense considering its mission.

In order to understand how chymosin coagulates milk, one needs to know something about milk proteins. The majority of milk protein is casein and there are four major types of casein molecules: alpha-s1, alpha-s2, beta and kappa. The alpha and beta caseins are hydrophobic proteins that are readily precipitated by calcium - the normal calcium concentration in milk is far in excess of that required to precipitate these proteins. However, kappa casein is a distinctly different molecule - it is not calcium-precipitable. As the caseins are secreted, they self-associate into aggregates called micelles in which the alpha and beta caseins are kept from precipitating by their interactions with kappa casein. In essence, kappa casein normally keeps the majority of milk protein soluble and prevents it from spontaneously coagulating.

Enter chymosin. Chymosin proteolytically cuts and inactivates kappa casein, converting it into para-kappa-casein and a smaller protein called macropeptide. Para-kappa-casein does not have the ability to stabilize the micellar structure and the calcium-insoluble caseins precipitate, forming a curd.

Aside from its physiologic role, chymosin is also a very important industrial enzyme because it is widely used in cheesemaking. In days gone by, chymosin was extracted from dried calf stomachs for this purpose, but the cheesemaking industry has expanded beyond the supply of available calf stomachs (remember that these have to be from young calves). It turns out that many proteases are able to coagulate milk by converting casein to paracasein and alternatives to chymosin are readily available. "Rennet" is the name given to any enzymatic preparation that clots milk.

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