Digestion (for 3 rd year students of College of Medicine, Aljouf University) (GIT block)

1. Digestion(for 3rd year students of College of Medicine, Aljouf University)(GIT block) Dr. Tarek A salem Ass. Prof. of Biochemistry

2. The following objectives will be covered • Biochemical basis of carbohydrates digestion. • Enzymes participating in digestion processes of carbohydrates and their specific role. • Biochemical basis of proteins digestion. • Enzymes participating in digestion processes of proteins and their specific role. • Biochemical basis of nucleic acids digestion. • Enzymes participating in digestion processes of nucleic acid and their specific role. • Biochemical basis of lipids digestion. • Enzymes participating in digestion processes of lipids and their specific role.

3. Digestion • It is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream.

4. Carbohydrates: overview • They are essentially hydrates of carbon (i.e. they are composed of carbon and water and have a composition of (CH2O)n. Carbohydrates used by man are: • Monosaccharides, e.g. glucose, fructose and galactose. • Disaccharides , e.g. sucrose (glucose + fructose), maltose (glucose + glucose), lactose (glucose + galactose). -Polysaccharides, e.g. starch and glycogen

5. Structures of Disaccharides

6. Starch Glycogen G G G G G G G G G G G a 1-6 link G G G G a 1-4 link G G G G

7. Digestion of carbohydrates 1- In mouth, salivary amylase, a 1-4 endoglycosidasepartially digests starch and glycogen to dextrin and few maltoses. 2- In stomach, Salivary a-amylase continues to act on polysaccharides in the stomach until it is inactivated by HCl. No further carbohydrate digestion takes place in the stomach.

8. Salivary amylase a amylase: Cannot attack a 1-4 linkage close to 1-6 branch points. G G G a Limit dextrins G G G G G G G G G G G G amylase G G G G G a 1-6 link G G G G maltotriose G G a 1-4 link G G G G G G maltose G G isomaltose

9. Digestion of carbohydrates 3- Pancreatic amylase completely digests starch, glycogen and dextrin with help of a 1-6 splitting enzyme into maltose and few glucose. 4- In small intestine, the enzymes maltase, lactase and sucrase secreted from intestinal mucosa hydrolyze the corresponding disaccharides to produce glucose, fructose and galactose.

11. PROTEINS • Proteins or polypeptide are composed of amino acids. Linked to each other by peptide bonds.

12. Protein digestion • Dietary protein are very large molecules that cannot be absorbed from the intestine. • To be absorbed, they must be digested to small simple molecules, amino acids.

13. Digestion of protein in the stomach • Protein digestion begins in the stomach by gastric juice. 1- Gastric HCl: - It causes denaturation of proteins - It activates pepsinogen into pepsin - It makes pH suitable for the action of pepsin. 2- Pepsin: - It is an endopeptidase acting on central peptide bond in which amino group belongs to aromatic amino acids e.g. phenyl alanine and tryptophan. - Its optimum pH is 1.5 to 2.2.

14. Digestion of protein in the stomach 3- Renin: - It is a milk-clotting enzyme. - It is present in stomach of infants and young animals. - It acts on casein converting it to soluble paracasein, which in turn binds Ca2+ forming insoluble Ca-paracasinate which is then digested by pepsin. 4- Gelatinase: - It liquifies gelatin. The end products of protein digestion in the stomach are proteoses (water-soluble components that are produced by hydrolysis of protein), peptones (short peptides) and large polypeptides.

15. Digestion of protein in the small intestine • Digestion of proteins is completed in the small intestine by proteolytic enzymes present in pacreatic and intestinal juice

16. Pancreatic juice 1- Trypsin: - It is an endopeptidase that hydrolyzes central peptide bond in which the carboxyl group belongs to basic amino acids e.g. arginine, lysine and histidine. - It is secreted in an inactive form called trypsinogen. - It is activated by enterokinase . - Its optimum pH is 8.

17. Pancreatic juice 2- Chymotrypsin: - It is an endopeptidase that hydrolyzes central peptide bond in which the carboxyl group belongs to aromatic amino acids. - It is secreted in an inactive form called chymotrypsinogen. - It is activated by trypsin. - Its optimum pH is 8.

18. Pancreatic juice 3- Elastase: - It is an endopeptidase that acts on peptide bonds formed by glycine, alanine and serine. - It is secreted in inactive form called proelatase. - It is activated by trypsin - It digest elastin and collagen. - Its optimum pH is 7.4.

19. Pancreatic juice 4- Carboxypeptidase: - It is an exopeptidase that hydrolyzes the terminal peptide bond at the C-terminus. - It is secreted in an inactive form called procarboxypeptidase. - It is activated by trypsin. - Its optimum pH is 7.4.

20. Intestinal juice 1- Aminopeptidase: - It is an exopeptidase that acts on the terminal peptide bond at the N-terminus. - It releases a single amino acid. 2- Tripeptidase: - It acts on tripeptides. - It releases a single amino acid & dipeptide. 3- Dipeptidase: - It acts on dipeptides. - It releases 2 amino acids. The end products of protein digestion in intestine are amino acids

21. Digestion of lipids • Lipids or fats are good source as 1 g supplies 9.1 cal. A minimal amount of fats is essential to provide our bodies with an adequate supply of essential fatty acids and of fat-soluble vitamins.

22. Digestion of triglycerides • Ingested TG are first emulsified and then undergo enzymatic hydrolysis by lipase enzymes • Emulsification is done in mouth by chewing, in stomach by peristalatic contraction, in intestine by peristalatic movement, bile salts and lysophospholipids. • Lipases are lingual lipase, gastric lipase, pancreatic lipase and intestinal lipase. The most effective is pancreatic lipase

23. Gastric lipase • It hydrolyzes TG containing short, medium and unsaturated long chain FA into FFA and 1,2 diglycerides.

24. Pancreatic lipase • It hydrolyzes the primary ester bond to produce FFA and β-monoglyceride. • Hydrolysis of β-monoglyceride requires transfer of FA from beta- to alfa- carbon of glycerol by action of ismerase, which is very slow process. • Bile salts assist emulsification by lowering surface tension and so exposing large area of TG to the action of pancreatic lipase and to dissolve FA.

25. Intestinal lipase • Acts within intestinal mucosal cells where it hydrolyze the absorbed monoglyceride to yield glycerol and FFA.

26. Digestion of phospholipids • Glycerophospholipids are hydrolyzed by pancreatic enzyme phospholipase A2 which remove FA in position 2 to form lysophospholipids. • Intestinal phospholipase may complete the hydrolysis of lysophospholipids to glycerol, FA and nitrogenous base.

27. Digestion of cholesterol esters • Cholesterol ester is hydrolyzed by cholesterol ester hydrolase (cholesterol esterase) into FA and free cholesterol.

28. Nucleic acids digestion • Nucleic acids released from ingested nucleoproteins in the small intestine are degraded to mononucleotides by pancreatic ribonuclease, deoxribonuclease and polynucleotidases. • Mononucleotides are then hydrolyzed to nucleoside and phosphates by nucleotidase (phosphatase).

29. Nucleic acids digestion • Nucleosides are either absorbed as such or are further degraded by intestinal phosphorylase to purine and pyrimidine bases, in addition to ribose-1P. • Purines and pyrimidines are further degraded into uric acid and ureidopropionate, respectively.