Chymosin

Chymosin (or rennin) is an aspartic acid protease enzyme found in rennet. It is produced by the cow of its lining of the abomasum (its fourth 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
It causes cleavage of a specific linkage, the peptide bond between phenylalanine and methionine. 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.