Digestive enzymes are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tracts of animals (including humans) and in the traps of carnivorous plants, where they aid in the digestion of food, as well as inside cells, especially in their lysosomes, where they function to maintain cellular survival. Digestive enzymes are diverse and are found in the saliva secreted by the salivary glands, in the stomach secreted by cells lining the stomach, in the pancreatic juice secreted by pancreatic exocrine cells, and in the intestinal (small and large) secretions, or as part of the lining of the gastrointestinal tract.
Digestive enzymes are classified based on their target substrates:
- proteases and peptidases split proteins into small peptides and amino acids.
- lipases split fat into three fatty acids and a glycerol molecule.
- carbohydrases split carbohydrates such as starch and sugars into simple sugars such as glucose.
- nucleases split nucleic acids into nucleotides.
In the human digestive system, the main sites of digestion are the oral cavity, the stomach, and the small intestine. Digestive enzymes are secreted by different exocrine glands including:
- Salivary glands
- Secretory cells in the stomach
- Secretory cells in the pancreas
- Secretory glands in the small intestine