Digestion is a process by which food is broken into simpler, smaller molecules that can be easily absorbed and assimilated by the body. Proteins are defined as the group of complex organic macromolecules. These are synthesized from amino acids, which are often referred to as the building blocks of the human body. Proteins are composed of one or more amino acid chains, and contain carbon, oxygen, hydrogen, nitrogen, and sulfur. They are required for making enzymes, hormones, and antibodies. The digestion of proteins takes place in the stomach and small intestine. The following sections explain this process.

Digestion of Protein in Stomach

In general, the process of digestion starts in the mouth. However, it is within the stomach and the small intestine that proteins are broken down into amino acids. The walls of the stomach are composed of strong muscles. After the food is turned into bolus by teeth, it is moved from the esophagus into the stomach by peristalsis, which refers to the wave-like muscular contractions. These contractions also help churn the food. The lining of the stomach contains glands that secrete gastric juice, which is colorless and acidic, with a pH ranging between 1 and 3. The main components of gastric juice are digestive enzymes, hydrochloric acid, and mucus.

Hydrochloric acid is produced by parietal cells, which are present in the lining of the stomach. Hydrochloric acid is very concentrated, and can easily digest the stomach itself. However, epithelial cells secrete a bicarbonate-rich solution that coats the mucosa, and neutralizes the acid secreted by the parietal cells. The cells of the stomach lining that are destroyed by hydrochloric acid are replaced by newer cells. The lining of the stomach gets completely replaced every third day.

The digestion of proteins in the stomach occurs mainly due to the action of hydrochloric acid (HCl) and an enzyme called pepsin. Pepsin forms in the stomach when pepsinogen reacts with HCl. Pepsin acts on the protein, thereby breaking peptide bonds into chains of amino acids linked together. These are referred to as polypeptides.

The enzymes needed for digesting proteins are called proteinases and proteases. These enzymes break down the molecules of proteins into amino acids by a depolymerisation process called hydrolysis. It is described as a chemical reaction wherein a water molecule breaks down into hydrogen cations and hydroxide anions.

The rate of action of these digestive enzymes is influenced by factors such as concentration and quantity of the enzyme, amount of protein that needs to be digested, temperature of the food, acidity of the food, acidity of the stomach, and the presence of antacids or other inhibitors of digestion. The task of enzymes is to break down protein molecules into simpler structures called peptones and proteose. They leave the stomach and enter the small intestine with the help of peristalsis. This entire process takes about 4 hours.

Digestion of Protein in Small Intestine

The partially-digested food, which is also called chyme, enters duodenum, which is a part of small intestine. Chyme is acidic in nature, but it becomes neutral after mixing with an alkaline secretion. Pancreas secrete digestive enzymes called trypsin and chymotrypsin. These enzymes reach the duodenum. They break the polypeptides or complex protein molecules into smaller peptides. These enzymes separate the specific amino acids from the short peptide chains, eventually breaking the peptides into individual amino acids through the process of hydrolysis.

Intestinal villi, which are finger-like projections found on the walls of the small intestine, increase the surface area of the small intestine by about 600 times. Each villus contains a network of blood capillaries and lymph vessels. The amino acids pass through the capillary walls, and are carried by the blood flowing through the network. In this manner, the amino acids thus produced get absorbed and are used by the body to make proteins. These proteins are then used by the body for making antibodies, building and repairing muscles, etc.

Since the stomach and small intestine play a vital role in digesting proteins, problems can arise, if these organs are affected by a medical condition or injury.