Chemical and enzymes in small intestine

Small intestine is a long tube, with a small diameter (about 1 inch), extending from pyloric sphincter to the ileocecal valve. Small intestine can be divided into Duodenum, Jejunum, and ileum.

Food moves through it relatively slowly, over a period of hours, allowing time for the actions of digestion and absorption for which this part of the GI tract is designed. The main classes of human digestive enzymes include proteases, lipases and carbohydrases, which respectively break down the macronutrients protein, fats and carbohydrates.

Up to 3L of intestinal juice is secreted daily by cells in the walls of the duodenum, and to a lesser extent the jejunum and ileum.

Pancreatic and intestinal enzymes that finish the digestion of proteins into amino acids. Proteolytic enzymes, including trypsin and chymotrypsin, are secreted by the pancreas. Carboxypeptidase, a pancreatic brush border enzyme, splits one amino acid at a time.

Proteases - break down proteins at optimum pH 7.9-9.7
• Trypsin and chymotrypsin – break down protein polypeptides in dipeptides
• Carboxypolypeptidase – splits peptides into individual amino acids
• Aminopeptidase and dipeptidase free the end amino acid products

Lipids (fats) are degraded into fatty acids and glycerol. Lipases - break down majority of dietary fats at optimum pH 8.0
• Lipase – hydrolyses triglycerides into free fatty acids and 2-monoglyceride, with the present of bile salt.
• Phospholipase – splits the fatty acids of phospholipids
• Esterase – hydrolyses cholesterol esters

All three are serine proteases, but with different cleavage specificities. Their action is complemented by exopeptidases.

Some carbohydrates are degraded into simple sugars, or monosaccharides (e.g., glucose, galactose) and are absorbed by the small intestine. Pancreatic juice supplies a cocktail of enzymes for the digestion of nearly all major nutrients. α-Amylase is secreted in large amounts. This enzyme is different from the salivary α-amylase, which has a slightly different structure (94% amino acid identity) and is encoded by a different gene.

Pancreatic amylase breaks down some carbohydrates (notably starch) into oligosaccharides. Disaccharidases and oligosaccharidases hydrolyze sucrose and lactose, as well as the maltose, maltotriose, and α-limit dextrins that are formed by the action of α-amylase on starch. Other carbohydrates pass undigested into the large intestine, where they are digested by intestinal bacteria.

Carbohydrases - break down carbohydrates at optimum pH 6.7-7.2
• Amylase – breaks down starch, glycogen and other carbohydrates polysaccharides into disaccharides
Chemical and enzymes in small intestine

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