| Chymosin | |||||||||
|---|---|---|---|---|---|---|---|---|---|
 Crystal structure of bovine chymosin complex with the inhibitor CP-113972.[1] | |||||||||
| Identifiers | |||||||||
| EC no. | 3.4.23.4 | ||||||||
| CAS no. | 9001-98-3 | ||||||||
| Databases | |||||||||
| IntEnz | IntEnz view | ||||||||
| BRENDA | BRENDA entry | ||||||||
| ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDBPDBePDBsum | ||||||||
| Gene Ontology | AmiGO /QuickGO | ||||||||
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Chymosin/ˈkaɪməsɪn/ orrennin/ˈrɛnɪn/ is aprotease found inrennet. It is anaspartic endopeptidase belonging to MEROPS A1 family. It is produced by newbornruminant animals in the lining of theabomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production ofcheese.
Historically, chymosin was obtained by extracting it from the stomachs of slaughtered calves. Today, most commercial chymosin used in cheese production is producedrecombinantly inEscherichia coli,Aspergillus niger var.awamori, andKluyveromyces lactis.[citation needed]
Chymosin is found in a wide range oftetrapods,[2] although it is best known to be produced byruminant animals in the lining of theabomasum. Chymosin is produced bygastric chief cells in newborn mammals[3] to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. Non-ruminant species that produce chymosin include pigs, cats, seals,[4] andchicks.[2]
One study reported finding a chymosin-like enzyme in some human infants,[5] but others have failed to replicate this finding.[6] Humans have apseudogene for chymosin that does not generate a protein, found onchromosome 1.[4][7] Humans have other proteins to digest milk, such aspepsin andlipase.[8]: 262
In addition to the primate lineage leading up to humans, some other mammals have also lost the chymosin gene.[2]
Chymosin is used to bring about the extensiveprecipitation andcurd formation incheese-making. The native substrate of chymosin isK-casein which is specificallycleaved at thepeptide bond between amino acid residues 105 and 106,phenylalanine andmethionine.[9] The resultant product iscalcium phosphocaseinate.[citation needed] When the specific linkage between thehydrophobic (para-casein) andhydrophilic (acidicglycopeptide) groups ofcasein is broken, the hydrophobic groups unite and form a3D network that traps the aqueous phase of the milk.
Charge interactions betweenhistidines on the kappa-casein andglutamates andaspartates of chymosin initiate enzyme binding to the substrate.[9] When chymosin is not binding substrate, a beta-hairpin, sometimes referred to as "the flap," can hydrogen bond with the active site, therefore covering it and not allowing further binding of substrate.[1]
Listed below are the ruminantCym gene and corresponding human pseudogene:
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Because of the imperfections and scarcity of microbial and animal rennets, producers sought replacements. With the development of genetic engineering, it became possible to extract rennet-producing genes from animal stomach and insert them into certainbacteria,fungi oryeasts to make them produce chymosin during fermentation.[11][12] The genetically modified microorganism is killed after fermentation and chymosin is isolated from the fermentation broth, so that the fermentation-produced chymosin (FPC) used by cheese producers does not contain any GM component or ingredient.[13] FPC contains the identical chymosin as the animal source, but produced in a more efficient way. FPC products have been on the market since 1990 and are considered the ideal milk-clotting enzyme.[14]
FPC was the first artificially produced enzyme to be registered and allowed by theUS Food and Drug Administration. In 1999, about 60% of UShard cheese was made with FPC[15] and it has up to 80% of the global market share for rennet.[16]
By 2008, approximately 80% to 90% of commercially made cheeses in the US and Britain were made using FPC.[13] The most widely used fermentation-produced chymosin is produced either using the fungusAspergillus niger or usingKluyveromyces lactis.
FPC contains only chymosin B,[17] achieving a higher degree of purity compared with animal rennet. FPC can deliver several benefits to the cheese producer compared with animal or microbial rennet, such as higher production yield, better curd texture and reduced bitterness.[14]
