Final answer:
The enzyme secreted by the gastric mucosa that coagulates milk proteins in a newborn is rennin. It is specific to infants for milk digestion, while pepsin and trypsin are the main enzymes for protein digestion in general.
Step-by-step explanation:
The enzyme secreted by the gastric mucosa of a newborn that coagulates milk proteins is rennin (also known as chymosin). While the protein digestion begins in the stomach with pepsin, rennin is uniquely produced in infants to help them digest the proteins found in milk. Unlike trypsin or chymotrypsin, rennin is not involved in protein digestion past the infant stage. In adults, pepsin is the primary enzyme for breaking down proteins in the stomach, and trypsin carries on this process in the small intestine. The hormone gastrin is responsible for stimulating the secretion of the gastric acid necessary for pepsinogen activation, while cholecystokinin plays a role in the digestion of proteins and fats in the duodenum by stimulating the release of digestive enzymes from the pancreas.