Dr. Mohammed Vaseem Assistant Professor Biochemistry Explain the process of digestion & absorption of proteins and carbohydrates in the GIT.

 Protein  Peptide Bond  Polysaccharide  Oligosaccharide  Disaccharide  Starch  Glycogen  Sucrose  Maltose  Lactose  Hydrolase

Digestion of Proteins R1 R2 R1-COOH + R2-NH2 Peptide bond – Partial Double bond - Stable Hydrolysis extremely slow (absence of catalyst years) In vitro – 6N HCl, 100 ⁰ C for 24 – 36 hrs. Proteolytic enzymes – nucleophilic attack on carbonyl group H2O

Digestion of proteins By the action of peptidases Endo peptidases – Acts on interior peptide bonds NH COOH Eg: Typsin, Chymotrypsin, Rennin Exo peptidases – Acts on terminal peptide bonds NH COOH Amino peptidase Carboxy peptidase

N terminal C - terminal Endopeptides Exopeptidases

Proteins Denaturation HCl Polypeptides Small Peptides Amino acids Pepsin Trypsin Chymotrypsin Amino peptidase Carboy peptidase Dipeptidases Tripeptidases Digestion of proteins

 Gastric digestion of proteins  Proteolytic enzymes are secreted as zymogens (inactive)  Only Activated at specific site of action  Gastrin – gastric mucosa – Stimulates gastric secretion  HCl (Parietal cells) denature the dietary protein  HCl activates pepsin Pepsinogen (42KDa) (Chief cells) Pepsin (34KDa) (pH = 1-2) HCl Removal of N-terminal 44 AA

 Gastric digestion contd… Proteins Proteoses & Peptones Rennin helps in the digestion of milk in infants Casein Paracaseinate (partially digested) Pepsin Rennin Acted on by pepsin HCl Pepsin Dietary protein Denatured Protein Peptones & Proteoses

Cholecystokinin & Secretin – Intestinal hormones stimulates secretion of pancreatic juice Contains alkaline bicarbonate (pH 8.0) & Zymogens Pancreatic digestion of Proteins Entry of acidic food stimulate intestinal hormones Auto Activation

HCl Pepsin Dietary protein Denatured Protein Trypsin Chymotrypsin Aminoacids By Tri & dipeptidases Carboxy and aminopeptidase Complete digestion of proteins in to amino acids Intestinal brush border enzymes Intestinal digestion of proteins

Proteolytic enzymes are highly specific in action Enzyme Bond hydrolysis Pepsin Trypsin Chymotrypsin Elastase Carboxypeptidase A Carboxypeptidase B Phe, Tyr, Trp, Met Arg, Lys Phe, Tyr, Trp, Val, Leu, Met Ala, Gly, Ser Ala, Ile, Leu, Val Arg, Lys When the C-terminal Amino acid is as listed When the C-terminal Amino acid is as listed

 Most of the proteolytic enzymes are serine proteases  Catalytic traid – Histidine, Asp & Ser (Chymotrypsin, Typsin & Elastase  Other proteases 1) Cysteine proteases – Papain 2) Aspartyl proteases – Renin 3) Metallo proteases (mostly Zn) - Carboxy peptidase A Protease inhibitors Natural Pancreatic trypsin inhibitor – inhibits trypsin α1 - Anti –trypsin – Inhibits elastase

 Mainly in small intestine (duodenum and jejunum)  Free amino acids are absorbed by active transport  Active Transport  Sodium dependant secondary active transport system  Five different transport system (neutral, basic, acidic, imno acids & beta amino acids)  Di- and tri-peptides are absorbed by a proton linked active transport (hydrolysed in the cytoplasm)  Role of Tri-peptide - glutathione (Miester cycle)  Hartnup’s disease – defect in absorption Absorption of Amino acids

Intestine, Brain, Kidney Mainly neutral AA

 Largest source of calories in the average diet  Usually constitute 40 to 45% of our caloric intake  The plant starches amylopectin and amylose, which are present in grains, tubers, and vegetables, constitute approximately 50 to 60% of the carbohydrate calories consumed.  These starches are polysaccharides, containing 10,000 to 1 million glucosyl units.

 Amylose, the glucosyl residues form a straight chain linked via -1,4 glycosidic bonds;  Amylopectin, the -1,4 chains contain branches connected via -1,6 glycosidic bonds  The other major sugar found in fruits and vegetables is sucrose, a disaccharide of glucose and fructose  Sucrose and small amounts of the monosaccharides glucose and fructose are the major natural sweeteners found in fruit, honey, and vegetables.

 The portion of the dietary carbohydrate that cannot be digested by human intestinal enzymes  It Is composed principally of plant polysaccharides cellulose, lignins and pectins

 Contain very little carbohydrate except for small amounts of glycogen (which has a structure similar to amylopectin)  The major dietary carbohydrate of animal origin is lactose, a disaccharide composed of glucose and galactose found exclusively in milk and milk products

Carbohydrate digestion begins in the mouth No Enzymatic Hydrolysis occurs in the Stomach There is participation of Pancreas Final steps are catalyzed by small intestinal enzymes

 Dietary polysaccharides and disaccharides are converted to monosaccharides by glycosidases  Enzymes that hydrolyze the glycosidic bonds between the sugars.  All of these enzymes exhibit some specificity for  the sugar,  the glycosidic bond ( or ),  and the number of saccharide units in the chain.  Undigested carbohydrates enter the colon, where they may be fermented by bacteria.

1414 1616 Digestion of Dietary starch and Glycogen

Lactose + H2O D-glucose + D-galactose Lactase Sucrose + H2O D-glucose + D-fructose Sucrase = Invertase Maltose + H2O 2 D-glucose Maltase Trehalose + H2O 2 D- glucose Trehalase Isomaltose + H2O 2 D- glucose Isomaltase DISACCHARIDES ARE CLEAVED TO MONOSACCS. BY SPECIFIC ENZYMES Glucoamylase: exoglucosidase that is specific for the –1,4 bonds between glucosyl residues begins at the nonreducing end of a polysaccharide or limit dextrin, and sequentially hydrolyzes the bonds to release glucose monosaccharides

Amylase Acts on  1  4 glycosidic linkage Maltase Acts on maltose and maltotriose (  1  4 ) Iso-maltase Acts on  -1  6 glycosidic linkage Lactase Acts on  -1  4 glycosidic linkage Sucrase Acts on  1  2 glycosidic linkage Trehalase Acts on  1  1 glycosidic linkage β - GLYCOSIDASE COMPLEX (LACTASE-GLUCOSYLCERAMIDASE) ENZYMES ARE SPECIFIC FOR THE SUBSTRATE What is dietary fibre?

Mucosal cell fructose Facilitated 2 ⁰ Active Facilitated GLUT 2 Circulation Porto - venous ABSORPTION OF CARBOHYDRATES Most absorption happens in the duodenal and proximal jejunal mucosa

Enzyme deficiency Primary mutation in transporter Proteins Secondary Following intestinal disease Following disease of pancreas MALABSORBTION OF CARBOHYDRATRES