Chymosin (or rennin, not to be confused with renin, a kidney hormone) is an aspartic acid protease enzyme found in rennet. It is produced by the cow, in the lining of the abomasum (the fourth, final, chamber of its stomach). Bovine chymosin is produced nowadays recombinantly in E. coli, Aspergillus niger var awamori, and K. lactis as alternative resource to the one from the cows. The gene is found in humans (on chromosome 1), but it is not in a functioning form. Rennin is produced by gastric chief cells in infants to curdle the milk they ingest, allowing a longer residence in the bowels and better absorption.
Enzymatic reaction
Chymosin causes cleavage of a specific linkage — the peptide bond between phenylalanine and methionine in the K-Casein. If this reaction applies to milk, the specific linkage between the hydrophobic (para-casein) and hydrophilic (acidic glycopeptide) group of casein inside milk would be broken, since they are joined by phenylalanine and methionine. The hydrophobic group would unite together and would form a 3D network to trap the aqueous phase of the milk. The resultant product is calcium phosphocaseinate. Due to this reaction, rennin is used to bring about the extensive precipitation and curd formation in cheese making.
Examples
Listed below are the cow Cym gene and corresponding human pseudogene:
References
- Foltmann B (1966). "A review on prorennin and rennin". C R Trav Lab Carlsberg 35 (8): 143–231. PMID 5330666.
- Harris TJ, Lowe PA, Lyons A, Thomas PG, Eaton MA, Millican TA, Patel TP, Bose CC, Carey NH, Doel MT (April 1982). "Molecular cloning and nucleotide sequence of cDNA coding for calf preprochymosin". Nucleic Acids Res. 10 (7): 2177–87. PMID 6283469.
- Visser S, Slangen CJ, van Rooijen PJ (June 1987). "Peptide substrates for chymosin (rennin). Interaction sites in kappa-casein-related sequences located outside the (103-108)-hexapeptide region that fits into the enzyme's active-site cleft". Biochem. J. 244 (3): 553–8. PMID 3128264.
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