Template:Cs1 config Template:Short description Template:Distinguish Template:Infobox enzyme Chymosin Template:IPAc-en or rennin Template:IPAc-en is a protease found in rennet. It is an aspartic endopeptidase belonging to MEROPS A1 family. It is produced by newborn ruminant animals in the lining of the abomasum to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption. It is widely used in the production of cheese.
Historically, chymosin was obtained by extracting it from the stomachs of slaughtered calves. Today, most commercial chymosin used in cheese production is produced recombinantly in Template:Nobr, Aspergillus niger var. awamori, and Template:Nobr.Template:Citation needed
OccurrenceEdit
Chymosin is found in a wide range of tetrapods,<ref name=lineage/> although it is best known to be produced by ruminant animals in the lining of the abomasum. Chymosin is produced by gastric chief cells in newborn mammals<ref name="pmid11534329">Template:Cite journal</ref> 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,<ref name= OMIM>Staff, Online Mendelian Inheritance in Man (OMIM) Database. Last updated February 21, 1997 Chymosin pseudogene; CYMP prochymosin, included, in the OMIM</ref> and chicks.<ref name=lineage/>
One study reported finding a chymosin-like enzyme in some human infants,<ref>Template:Cite journal</ref> but others have failed to replicate this finding.<ref>Template:Cite book</ref> Humans have a pseudogene for chymosin that does not generate a protein, found on chromosome 1.<ref name= OMIM/><ref>Template:Cite book</ref> Humans have other proteins to digest milk, such as pepsin and lipase.<ref>Template:Cite book</ref>Template:Rp
In addition to the primate lineage leading up to humans, some other mammals have also lost the chymosin gene.<ref name=lineage>Template:Cite journal</ref>
Enzymatic reactionEdit
Chymosin is used to bring about the extensive precipitation and curd formation in cheese-making. The native substrate of chymosin is K-casein which is specifically cleaved at the peptide bond between amino acid residues 105 and 106, phenylalanine and methionine.<ref name="Gilliland">Template:Cite book</ref> The resultant product is calcium phosphocaseinate.Template:Citation needed When the specific linkage between the hydrophobic (para-casein) and hydrophilic (acidic glycopeptide) groups of casein is broken, the hydrophobic groups unite and form a 3D network that traps the aqueous phase of the milk.
Charge interactions between histidines on the kappa-casein and glutamates and aspartates of chymosin initiate enzyme binding to the substrate.<ref name="Gilliland"/> 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.<ref name="pmid9862200"/>
ExamplesEdit
Listed below are the ruminant Cym gene and corresponding human pseudogene:
| Template:Infobox nonhuman protein | Template:Infobox protein |
Recombinant chymosinEdit
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 certain bacteria, fungi or yeasts to make them produce chymosin during fermentation.<ref name="pmid6304731">Template:Cite journal</ref><ref name="pmid6283469">Template:Cite journal</ref> 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.<ref name="GMO Database"/> 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.<ref name="Law 2010 100–101">Template:Cite book</ref>
FPC was the first artificially produced enzyme to be registered and allowed by the US Food and Drug Administration. In 1999, about 60% of US hard cheese was made with FPC<ref name="USDA">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and it has up to 80% of the global market share for rennet.<ref name="pmid16537950">Template:Cite journal</ref>
By 2008, approximately 80% to 90% of commercially made cheeses in the US and Britain were made using FPC.<ref name="GMO Database">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The most widely used fermentation-produced chymosin is produced either using the fungus Aspergillus niger or using Kluyveromyces lactis.
FPC contains only chymosin B,<ref>Bovine chymosins A and B differ by one amino acid residue. This is probably an alleic variant, according to Uniprot:P00794. The two isoforms have identical catalytic activity, so any improvement in the product is due to the elimination of other impurities.</ref> 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.<ref name="Law 2010 100–101"/>
ReferencesEdit
Further readingEdit
External linksEdit
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