digestion (for 3 rd year students of college of medicine, aljouf university) (git block)

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

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Digestion (for 3 rd year students of College of Medicine, Aljouf University) (GIT block). Dr. Tarek A salem Ass. Prof. of Biochemistry. The following objectives will be covered. Biochemical basis of carbohydrates digestion. - PowerPoint PPT Presentation

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Page 1: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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

(GIT block)

Dr. Tarek A salemAss. Prof. of Biochemistry

Page 2: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

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

Digestion

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

Page 4: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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

Page 5: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Structures of Disaccharides

Page 6: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

GG

GG

G

GG

Ga 1-4 linkG

G

G

GG a 1-6 link

GG

G

GG

G

Starch Glycogen

Page 7: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Digestion of carbohydrates1- In mouth, salivary amylase, a 1-4 endoglycosidase partially

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.

Page 8: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Salivary amylase

a amylase: Cannot attack a 1-4 linkage close to 1-6 branch points.

GG

GG

G

GG

Ga 1-4 linkG

G

GG a 1-6 link

GG

G

GGG G G G

GG

G

G G

G

maltose

G

GG

isomaltose

amylase

maltotriose

G

G

G

G

a Limit dextrins

Page 9: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 10: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)
Page 11: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

PROTEINS

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

Page 12: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 13: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 14: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Digestion of protein in the stomach3- 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.

Page 15: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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

Page 16: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 17: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 18: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 19: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 20: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Intestinal juice1- 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

Page 21: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 22: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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

Page 23: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Gastric lipase

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

Page 24: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 25: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Intestinal lipase

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

Page 26: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.

Page 27: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

Digestion of cholesterol esters

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

Page 28: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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).

Page 29: Digestion (for 3 rd  year students of College of Medicine,  Aljouf  University) (GIT block)

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.