Rennin is also known as chymosin. Rennin meaning is given as a protein-digesting enzyme that curdles milk by transforming the caseinogen into insoluble casein. It is found only in the cud-chewing animal's fourth stomach, like cows. Its action extends the period where milk is retained in the young animal's stomach. In animals, which lack rennin, milk can be coagulated by the pepsin action, as is the case rennin in humans. Rennet, which is a commercial form of rennin, is used in the manufacturing of preparing junket and cheese.
There are many useful pepsin and rennin actions that may be applied to humans and other living organisms.
The chymosin can be found in a diverse range of tetrapods, although it is one of the best-known ones to be produced by ruminant animals in the abomasum lining. Chymosin can be produced by gastric chief cells in newborn mammals to curdle the milk that they ingest by allowing a longer residence in better absorption and the bowels. Cats, pigs, seals, and chicks are non-ruminant animals that contain chymosin.
A study that is reported finding a chymosin-like enzyme in a few human infants (rennin in infants), but the others have failed to replicate this specific finding. Humans contain a pseudogene for the chymosin, which does not generate a protein found on chromosome 1. Also, humans have other proteins for milk digestion, such as lipase and pepsin.
In addition to the primate lineage leading up to humans, a few other mammals have also lost the chymosin gene.
Chymosin can be used to bring about curd formation and extensive precipitation in cheese-making. Chymosin's native substrate is K-casein that is particularly cleaved at the peptide bond between the amino acid residues 105 and 106, methionine, and phenylalanine. Calcium phospho caseinate is the resultant product. When a particular linkage between the hydrophilic (acidic glycopeptide) and hydrophobic (para-casein) groups of casein is broken, the hydrophobic groups get to unite and form a 3D network, which traps the milk's aqueous phase.
Charge interactions between the glutamates and histidines on the kappa-casein and the aspartates of chymosin initiate enzymes, which is binding to the substrate. When chymosin isn't the binding substrate, a beta-hairpin known as "the flap" will hydrogen bond to the active site, covering it and preventing further substrate binding.
Due to the imperfections and scarcity of animal and microbial rennets, producers sought replacements. With genetic engineering development, it became possible to extract the rennet-producing genes from the stomach of animals and insert them into certain fungi, yeasts, or bacteria, to make them form chymosin during the fermentation process.
The microorganisms, which are genetically modified, can be killed after the fermentation, and chymosin is isolated from the fermentation broth so that Fermentation-Produced Chymosin (FPC), which is used by the cheese producers, does not contain any ingredient or GM component. FPC has identical chymosin as the animal source but is produced in an efficient way. Also, the FPC products have been on the market since 1990, and they are considered the ideal milk-clotting enzyme.
FPC was given as the first artificially produced enzyme to be registered and allowed by the United States Food and Drug Administration. About 60% of US hard cheese was made in 1999 with FPC, and it has around 80% of the global market share for rennet.
Coagulation of Milk
In order to understand how to coagulate milk by chymosin, one should know something about milk proteins. Casein is the most abundant protein in milk, and it comes in four different types: alpha-s1, alpha-s2, beta, and kappa. Both the alpha and beta caseins are hydrophobic proteins, which are precipitated readily by calcium - the normal calcium concentration in the milk is far more than required to precipitate these proteins.
But, kappa casein is a distinctly varied molecule, and it is not calcium-precipitable. As the caseins get secreted, they self-associate into the aggregates, known as micelles, where the alpha and beta caseins are kept from the precipitating process by their interactions with kappa casein. In essence, kappa casein preserves the bulk of milk protein soluble while also preventing it from coagulating spontaneously.
Enter chymosin. The chymosin then proteolytically cuts and inactivates kappa casein by converting it into para-kappa-casein and a smaller protein known as macropeptide. On the other side, para-kappa-casein does not hold the ability to stabilize the calcium-insoluble caseins precipitate and the micellar structure, forming a curd.
Besides its physiologic role, chymosin is also a more important industrial enzyme because it can be widely used in cheesemaking. Chymosin was extracted from dried calf stomachs in days gone by; for this purpose, however, the cheesemaking industry has expanded beyond the supply of available calf stomachs (note that these have to be from the young calves).
Also, it turns out that several proteases are able to coagulate milk by converting the casein to paracasein and alternatives to chymosin are available readily. "Rennet" is the term given to any enzymatic preparation that clots milk.