Digestive System
Digestive enzymes are enzymes in the alimentary canal that break down food so that the organism can absorb it. The main sites of action are the oral cavity, the stomach, the duodenum and the jejunum. They are secreted by different glands: the salivary glands, the glands in the stomach, the pancreas, and the glands in the small intestines.
Oral Cavity
In the oral cavity, salivary glands secrete ptyalin. It is a type of α-amylase that digests starch into small segments of multiple sugars and into the individual soluble sugars. Secreted by small and large salivary glands. Salivary glands also secrete lysozyme, which kills bacteria but is not classified as a digestive enzyme.
Stomach
The enzymes that get secreted in the stomach are called gastric enzymes. These are the following:
- Pepsin is the main gastric enzyme. As it breaks proteins into smaller peptide fragments, it is a peptidase.
- Gelatinase, degrades type I and type V gelatin and type IV and V collagen, which are proteoglycans in meat.
- Gastric amylase degrades starch, but is of minor significance.
- Gastric lipase is a tributyrase by its biochemical activity, as it acts almost exclusively on tributyrin, a butter fat enzyme.
Small Intestine and Pancreatic Enzymes
The pancreas is the main digestive gland in our body. It secretes the enzymes:
- Trypsin, is a peptidase, that breaks down peptides in the small intestine.
- Chymotrypsin, also a peptidase.
- Steapsin, degrades triglycerides into fatty acids and glycerol
- Carboxypeptidase, splits peptide fragments into individual amino acids. It is a protease
- Several elastases that degrade the protein elastin and some other proteins
- Several nucleases that degrade nucleic acids, like DNAase and RNAase
- Pancreatic amylase that, besides starch, glycogen and cellulose, degrades most other carbohydrates
- Bile from the liver, which emulsifies fat, allowing more efficient use of lipases in the duodenum; in converting lipids to their component fatty acid and glycerol molecules
- Several peptidases.
The jejunum and ileum secretes a juice called succus entericus, which contains the following six types of enzymes that degrade disaccharides into monosaccharides:
- Sucrase, which breaks down sucrose into glucose and fructose
- Maltase, which breaks down maltose into glucose
- Isomaltase, which breaks down maltose and isomaltose
- Lactase, which breaks down lactose into glucose and galactose
- Intestinal lipase, which breaks down fatty acids
The small intestine receives lipase, trypsin and amylase from the pancreas. They are transported from the pancreas to the duodenum through the pancreatic duct. Protein, fats and starch are broken down into smaller molecules. However, they are not fully broken down yet. This causes the enzymes of the small intestine to act upon them. These enzymes include peptidase, which breaks down peptides into amino acids and the enzyme maltase acts upon maltose, which produces glucose. These molecules are absorbed by the villi in the small intestine and according to the molecule they are either absorbed by the lacteal or blood capillaries.
