3.3.2 Enzymes in Digestion

• Enzymes are substrate specific, meaning different enzymes digest and breakdown the different biological molecules contained in food
• Digestive enzymes are extracellular enzymes, they work outside of cells
• There are three main types of digestive enzymes – carbohydrases, proteases and lipases
• If a molecule's name ends in 'ase' it is an enzyme (not all enzymes have this suffix but many do)

Digestion of Carbohydrates by Amylases and Membrane-bound Disaccharidases

• The digestion of carbohydrates takes place in the mouth and the small intestine
• Amylase is a carbohydrase that hydrolyses (breaks down) starch into maltose
• Maltose is then hydrolysed into glucose by the enzyme maltase
• Amylase is made in the salivary glands, the pancreas and the small intestine
• Maltase is a disaccharidase which is found in cell-surface membranes of the epithelial cells lining the small intestine
• There are also other disaccharidases in the cell-surface membrane of the epithelial cells in the small intestine: sucrase and lactase, that hydrolyse sucrose and lactose respectively
  • This allows the absorption of monosaccharides into epithelial cells of the small intestine which pass them into the blood stream

• Note that the lining of the small intestine is folded and there are microvilli present.
• This increases the surface area substantially, allowing more membrane-bound disaccharidases to fit into the membrane and more absorption to take place

Image showing the digestion of starch by enzymes. Amylase is a carbohydrase enzyme while maltase is a disaccharidase enzyme.

Image showing maltase inside the cell-surface membrane of an epithelial cell

Digestion of Proteins by Proteases

• Protein digestion begins in the lumen of the stomach by protease enzymes
• A protease enzyme called an endopeptidase hydrolyses peptide bonds within proteins, creating smaller sized protein “chunks”
  • This enzyme is secreted along with hydrochloric acid, meaning the pH in the stomach is low and therefore acidic
  • The partially digested food moves from the stomach into the small intestine

• Fluid secreted by the pancreas travels to the small intestine and helps to neutralize the acidic mixture and increase the pH. This pancreatic juice contains endopeptidases and exopeptidases
  • Endopeptidases hydrolyse peptide bonds within polypeptide chains to produce dipeptides
  • Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains to produce dipeptides

• Lastly, there are dipeptidase enzymes found within the cell surface membrane of the epithelial cells in the small intestine. These enzymes hydrolyse dipeptides into amino acids which are released into the cytoplasm of the cell

Image showing the digestion of protein by several enzymes

Image showing dipeptidase inside the cell-surface membrane of an epithelial cell

Digestion of Lipids by Lipases

Emulsification

• There are several stages of breakdown that lipids go through prior to digestion
• In the stomach, solid lipids are turned into a fatty liquid consisting of fat droplets - but this is not digestion
• When the fatty liquid arrives in the small intestine, bile (containing bile salts) which has been made in the liver and stored in the gallbladder is secreted
• The bile salts bind to the fatty liquid and breaks the fatty droplets into smaller ones via emulsification
• Emulsification helps to increase the surface area of the fatty droplets for action of digestive enzymes

Digestion of Lipids

• The digestion of lipids takes place solely in the lumen of the small intestine
• Lipase enzymes break down lipids (fats) to glycerol and fatty acids
• Lipase enzymes are produced in the pancreas and secreted into the small intestine by the pancreas

Image showing the digestion of lipids by lipase enzymes in the lumen of the gut

Table of Digestive Enzymes