Endopeptidases: These enzymes hydrolyse peptide bonds within the central region of a polypeptide chain. By breaking the large polymer into several smaller 'chunks', they significantly increase the number of terminal ends available for other enzymes to act upon.
Exopeptidases: These enzymes act on the terminal peptide bonds at the ends of polypeptide chains to release individual amino acids. Their efficiency is greatly enhanced by the prior action of endopeptidases, which create more 'ends' for them to target.
Dipeptidases: Often found as membrane-bound enzymes in the ileum, dipeptidases specifically hydrolyse the bond between two amino acids (a dipeptide). This final step ensures that proteins are completely reduced to single amino acids before they enter the epithelial cells.
Role of Bile Salts: Produced in the liver and stored in the gallbladder, bile salts are not enzymes but are crucial for lipid digestion. They perform emulsification, breaking large fat globules into tiny droplets called micelles, which vastly increases the surface area for lipase to work.
Lipase Action: Lipase enzymes, primarily produced in the pancreas, hydrolyse the ester bonds in triglycerides. This reaction produces a mixture of fatty acids, glycerol, and monoglycerides.
Micelle Formation: After hydrolysis, the products associate with bile salts to form micelles. These structures transport the poorly soluble lipid products to the surface of the intestinal epithelium for absorption.
| Enzyme Type | Substrate | Primary Product | Key Location |
|---|---|---|---|
| Amylase | Starch | Maltose | Mouth / Duodenum |
| Maltase | Maltose | Glucose | Small Intestine Wall |
| Endopeptidase | Polypeptides | Smaller Peptides | Stomach / Duodenum |
| Lipase | Triglycerides | Fatty Acids / Glycerol | Duodenum |
Chemical vs. Physical Digestion: It is vital to distinguish between the chemical action of enzymes (hydrolysis) and the physical action of bile salts (emulsification). Bile salts do not break chemical bonds; they only change the physical state of the lipids to facilitate faster enzymatic action.
Secreted vs. Membrane-Bound: While many enzymes are secreted into the gut lumen (like amylase and pepsin), others are fixed to the cell membranes (like maltase and dipeptidases). This ensures the final products of digestion are released exactly where absorption occurs.
pH Sensitivity: Always check the environment mentioned in a scenario. Enzymes like pepsin in the stomach require a low (around 2), while pancreatic enzymes in the small intestine require a neutral to slightly alkaline (around 7-8) provided by bile and pancreatic juices.
Surface Area Logic: When explaining lipid digestion, always link bile salts to 'increased surface area' and 'increased rate of lipase activity'. Failing to mention surface area is a common way to lose marks in descriptive questions.
The 'Bile is an Enzyme' Trap: A very frequent mistake is describing bile as an enzyme. Remember that bile contains salts that act physically, not chemically, and it does not have an active site that binds to substrates in a catalytic manner.
