Rennin: Occurrence, Reaction & Coagulation

Rennin, also known as chymosin, is an enzyme that digests protein and converts it into soluble casein in milk. It occurs only in the fourth stomach of animals which chew cud, including cows. A young animal's stomach is able to retain milk longer after consuming it because of its action. Milk is coagulated by pepsin in animals that lack rennin, just as it is in humans. In dairy products such as cheese and junket, the commercial form of rennin, rennet, is used.

Key Takeaways: Rennin, Chymosin, Milk, Enzyme, Rennet, Curd, Fermentation

What is Rennin?

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Chymosin is another name that is sometimes used for Rennin. A protein-digesting enzyme, Rennin, is responsible for curdling milk by converting caseinogen into insoluble casein. Cud-chewing animals, such as cows, only have it in their fourth stomach. It allows young animals to retain milk longer in their stomachs as a result of its action. Pepsin action can coagulate milk in animals that lack rennin, as rennin does in humans. A commercial form of rennin, Rennet, is used to separate junket from cheese. People and other living organisms can benefit from rennin's actions. 

Rennin Chemical Structure

Occurrence

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Several species of tetrapod’s produce chymosin, but it is especially best known to be found in the abomasum lining of ruminant animals. The gastric chief cells within newborn mammals can produce chymosin, which is a digestive enzyme that curdles milk by allowing it to stay in the intestines and to facilitate better absorption. The chymosin-like enzyme is found in some human infants too, but the results of other studies have not replicated this specific finding. Pigs, seals, and chicks are other non-ruminant animals that contain chymosin. 

Despite having a pseudogene for chymosin on chromosome 1, humans do not produce a protein that corresponds to it. Besides chymosin, humans also have lipase and pepsin for digesting milk. A few other mammals, in addition to primates, have become extinct due to loss of the chymosin gene.

Enzymatic Reaction

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During cheese-making, chymosin can be used to cause curd formation and extensive precipitation. K-casein is the native substrate of chymosin, which is cleaved down to residue 105, 106, methionine, and phenylalanine. The result is calcium phosphor caseinate. 

The hydrophobic (para-casein) groups of casein unite when a particular linkage between the hydrophilic (acidic glycopeptide) and hydrophobic (para-casein) groups of casein is broken, causing the aqueous phase of milk to get trapped. 

The charge interactions acting between glutamates and histidine on kappa casein and the aspartates of chymosin help enzymes bind to the substrate. The ‘flap’ forms a hydrogen bond with the active site and disables further substrate binding when chymosin isn't the binding substrate.

Recombinant Chymosin

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Productors looked for alternatives to animal and microbial rennet due to their imperfections and scarcity. It has become possible to convert fungi, yeasts, and bacteria to produce chymosin when they ferment. This wouldn’t be possible without genetic engineering. Fermentation-Produced Chymosin (FPC), which is used by cheese producers, contains no ingredients or genetically modified components because the bacteria, that are genetically modified, can be killed after fermentation. 

Despite being produced in an efficient way, FPC contains the same chymosin as animal sources. Furthermore, FPC products have been on the market since 1990, and the US Food and Drug Administration recognized FPC as the first artificial enzyme to be approved for sale and registration. It has around 80% market share for rennet on the global scale, and around 60% of US hard cheeses were made in 1999 using FPC.

Coagulation of Milk

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Chymosin coagulates milk by combining milk proteins, making it vital to understand what milk proteins are. There are four different types of casein found in milk: alpha-s1, alpha-s2, beta, and kappa. Beta casein and alpha casein are both hydrophobic proteins that precipitate readily when exposed to calcium - the normal milk calcium concentration is far more than enough to precipitate these proteins. Kappa casein, on the other hand, is a distinctly varied molecule that cannot precipitate in calcium. Micelles, which are formed by the self-association of the caseins, prevent the precipitation process by interacting with the alpha and beta caseins. By keeping the bulk of milk protein soluble as well as preventing it from spontaneously coagulating, kappa casein prevents milk protein from coagulating.

Rennin helps in the process of cheese making

In turn, chymosin converts the kappa casein into a smaller protein called macro peptide by proteolytically cutting and inactivating it. Conversely, beta-casein is incapable of stabilizing calcium-insoluble precipitates and micellar structures that form a curd.

It is also used in cheesemaking. Traditionally, chymosin was derived from dried calf stomachs. However, with the expansion of the cheese industry, it was no longer feasible to use calf stomachs (note that these must come from young calves). Additionally, it has been found that several proteases are capable of coagulating milk by converting casein to paracasein. All enzymatic preparations that clot milk are called "rennet."

Things to remember

• Coagulating milk is a function of a stomach enzyme.
• Hydrochloric acid activates prorenin, which is secreted by the gastric glands in an inactive form.
• Caseinogen (milk protein) is converted by Rennin into insoluble casein when calcium ions are present.
• Rennin differs from rennin by having a different pH. optimal and by the fact that rennin has mild proteolytic activity compared to rennin.
• A protein-digesting enzyme, Rennin is responsible for curdling milk by converting caseinogen into insoluble casein.