Mouth
In humans, the digestion of proteins begins in the stomach. However, it is important to note that the process of protein digestion actually starts in the mouth.

Stomach
Once the proteins are ingested, they travel down the esophagus and reach the stomach. In the stomach, the enzyme pepsinogen is secreted by the gastric glands. Pepsinogen is then activated into pepsin by the acidic environment of the stomach. Pepsin is a protease enzyme that starts breaking down proteins into smaller peptides.

Small intestine
After the proteins have been partially broken down in the stomach, they move into the small intestine. Here, the pancreas secretes enzymes such as trypsin, chymotrypsin, and carboxypeptidase that further break down the proteins into amino acids. These amino acids are then absorbed by the small intestine and transported to the bloodstream for various bodily functions.

Large intestine
The large intestine is not directly involved in the digestion of proteins. Instead, it primarily absorbs water and electrolytes from the remaining undigested food particles before excretion.
In conclusion, while the digestion of proteins begins in the mouth with the mechanical breakdown of food, the actual chemical breakdown and digestion of proteins start in the stomach with the secretion of pepsin. The small intestine plays a crucial role in further breaking down proteins into amino acids for absorption, while the large intestine focuses on water absorption and waste elimination.