Final answer:
The enzyme secreted by the gastric mucosa of a newborn that coagulates milk proteins is rennin. Rennin targets the milk protein casein, aiding in its digestion by causing coagulation. This is distinct from pepsin, which is involved in general protein digestion in the stomach.
Step-by-step explanation:
The enzyme secreted by the gastric mucosa of a newborn that coagulates milk proteins is known as rennin. This enzyme facilitates the digestion of milk by working on the milk protein casein, causing it to coagulate or curdle. This coagulation process makes it easier for the infant's digestive system to further break down the proteins. While pepsin is also important in protein digestion by hydrolyzing protein to peptides in the stomach, rennin is specifically focused on milk proteins in newborns. Other enzymes mentioned, such as trypsin and cholecystokinin, are involved in the digestion process, but they act in the small intestine and are not responsible for milk protein coagulation.