Question

Milk casein $\xrightarrow{Y}$ Paracasein $\xrightarrow{Z}$ Calcium paracaseinate (Curdling of milk). in it, letters ‘Y’
and ‘Z’ denote.

A. Rennin and $Ca^{++}$
B. $Ca^{++}$ and rennin
C. Renin, Hcl and $Ca^{++}$
D. Renin and $Ca^{++}$

Answer 1

Casein is a type of protein found in mammalian milk. Casein is also known as milk protein. Milk protein is broken down or digested in our body by the activity of an enzyme known as renin. This is the main component of the above reaction.

Complete answer:
Rennin is a protein degrading/digesting enzyme found in gastric juice of infants. Renin is responsible for coagulation of milk and plays an important role in the milk protein digestive process of infants. In the presence of calcium, rennin changes irreversibly the casein of milk to paracasein which is further degraded by enzyme pepsin. Hence, Rennin acts on milk proteins and converts casein to paracasein whereas by the action of calcium the paracasein is converted into calcium paracaseinate which finally leads to curdling of milk.

Milk casein $\xrightarrow{{\operatorname{Rennin}}}$ Paracasein $\xrightarrow{\text{Calcium}}$ Calcium paracaseinate (Curdling of milk)

Pepsinogen is activated by HCl to become active pepsin that starts hydrolysis of protein. Further the pepsin changes proteins into proteoses, peptones and polypeptides.

Hence, the correct answer is option (A).
Rennin and $Ca^{++}$ . Chymosin is another name of rennin which is a proteolytic enzyme which is synthesized by chief cells in the stomach. In animals that lack rennin, milk is coagulated by the action of pepsin.

Note: Curdling of milk is a complex process where the milk is partially digested to form curd by action of various enzymes and microorganisms. The milk protein known as casein is partially digested by enzyme renin present in the digestive juice of young ones.