|MadSci Network: General Biology|
Hannah – Rennin is also known as an enzyme called chymosin. As you may well know, enzymes are proteins that catalyze a reaction. In the case of chymosin, the reaction involves the breakdown of casein in milk resulting in clot formation in the presence of calcium ions – this is the basis for the production of many varieties of cheeses. Chymosin belongs to a family of enzymes called the aspartic proteinases. The reason that it belongs to this family is because it possesses an aspartic acid residue at the active site of the enzyme. Calf chymosin is secreted from the fourth stomach of the calf and while its exact function is not known, there are currently some ideas about what function it does serve for the calf. As stated earlier, chymosin breaks down casein resulting in clot formation (in the presence of calcium ions). More specifically, chymosin preferentially cleaves one "type" of casein, kappa-casein; again, the reason for this is unknown. One hypothesis states that the clot formation may retard the passage of food from the stomach to the intestine allowing the calf to extract nutrients from the clot. Also, clot formation may also stimulate further digestive reactions that will be essential as the calf matures. Since chymosin is a gastic enzyme, the pH, which it is most active, is quite low; its optimum pH is approximately 3.5. The optimum temperature is around 37 degrees C – which is the approximate body temperature of the calf. The demand for calf chymosin for use in cheesemaking is greater that the supply of the enzyme; therefore, alternative milk clotting enzymes have been sought out for use. These include bovine chymosin, porcine (pig) chymosin, fungal extracts, bacterial extracts, and recombinant chymosin (that is, the gene encoding calf chymosin has been genetically engineered into E. coli to produce chymosin). I hope that this has provided some more insight about rennin (chymosin). Best of luck :-) A couple of references that may be of use: Foltmann, B. 1970. Prochymosin and chymosin. Methods Enzymol. 19:421-436. Foltmann, B. 1987. General and molecular aspects of rennets. In: Fox, P. (ed.) Cheese: Chemistry, Physics and Microbiology, vol. 1. Elsevier Applied Science Publishers. pg. 33-61. Glynis K. Rutgers University, Food Science
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