Question

Rennin acts on
A. Proteins in stomach
B. Milk, changing casein into paracaseinate at 7.2- 8.2 pH
C. Fat in intestine
D. Milk, changing casein into calcium paracaseinate at 1-3 pH

Answer 1

Stomach secretes gastric juices (pH 1-3), which contains prorenin secreted by zymogen cells. Inactive proteins converted into rennin by hydrochloric acid. Rennin acts on casein, and changes it into calcium paracaseinate; this process is known as curdling of milk.

Complete answer:
Rennin is found in gastric juice of infants which causes coagulation of milk and is the main component for the digestive process of infants. Coagulation of milk is salient for infants because it stops the rapid passage of milk from the stomach.
In the presence of calcium, this enzyme converts casein of milk into paracasein which is then treated by pepsin. Hence, rennin acts on milk proteins and converts casein into paracasein.
Fat is separated from other food substances in the stomach. Fats are emulsified by bile and digested by enzyme and then fat is absorbed by intestinal cells.
Rennin has an optimum pH of 3 for proteolysis.

Hence, the correct answer is option (D).

Additional information:
Rennin is also known as chymosin. Coagulation of milk consists of two main phases; primary and secondary phases. In the primary phase, peptides are released from casein by the hydrolytic action. In the secondary phase, cations in milk cause casein micelles to coagulate.
Typically, rennin cleaves at the peptide bond between amino acid residues 105 and 106 (phenylalanine and methionine).
In the absence of rennin, milk is coagulated by the action of enzyme pepsin. Rennet is a commercial form of rennin which is used for the manufacturing of junket and cheese.

Note: Enzymes are valuable for industrial processes such as fermenting of wine, cheese curdling, brewing etc and in medical processes involving killing of disease causing organisms, promoting wound healing, and diagnosing certain diseases.