Download Digestion of Proteins

Explain the process of digestion &
absorption of proteins and
carbohydrates in the GIT.
Dr. Mohammed Vaseem
Assistant Professor
Biochemistry
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Protein
Peptide Bond
Polysaccharide
Oligosaccharide
Disaccharide
Starch
Glycogen
Sucrose
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Maltose
Lactose
Hydrolase
Digestion of Proteins
H2O
R1
R2
R1-COOH + R2-NH2
Peptide bond – Partial Double bond - Stable
Hydrolysis extremely slow (absence of catalyst 10 100 years)
In vitro – 6N HCl, 100⁰ C for 24 – 36 hrs.
Proteolytic enzymes – nucleophilic attack on
carbonyl group
Digestion of proteins
By the action of peptidases
Endo peptidases – Acts on interior peptide bonds
NH2 ------------------------------------------------ COOH
Eg: Typsin, Chymotrypsin, Rennin
Exo peptidases – Acts on terminal peptide bonds
Carboxy peptidase
NH2 ----------------------------------------------- COOH
Amino peptidase
N terminal
C - terminal
Exopeptidases
Endopeptides
Digestion of proteins
Proteins
HCl
Denaturation
Pepsin
Polypeptides
Trypsin
Chymotrypsin
Small Peptides
Amino peptidase
Carboy peptidase
Dipeptidases
Tripeptidases
Amino acids
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Gastric digestion of proteins
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Proteolytic enzymes are secreted as zymogens (inactive)
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Only Activated at specific site of action
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Gastrin – gastric mucosa – Stimulates gastric secretion
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HCl (Parietal cells) denature the dietary protein
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HCl activates pepsin (pH = 1-2)
HCl
Pepsinogen
(42KDa)
(Chief cells)
Removal of N-terminal
44 AA
Pepsin
(34KDa)
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Gastric digestion contd…
Pepsin
• Proteins
Proteoses & Peptones
• Rennin helps in the digestion of milk in infants
• Casein
Rennin
Paracaseinate
(partially digested)
Pepsin
HCl
Dietary protein
Acted on by pepsin
Denatured Protein
Peptones & Proteoses
Pancreatic digestion of Proteins
Entry of acidic food stimulate intestinal hormones
Cholecystokinin & Secretin – Intestinal hormones stimulates
secretion of pancreatic juice
Contains alkaline bicarbonate (pH 8.0) & Zymogens
Intestinal digestion of proteins
Pepsin
HCl
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
Proteolytic enzymes are highly specific in action
Enzyme
Bond hydrolysis
Pepsin
Phe, Tyr, Trp, Met
Trypsin
Arg, Lys
Chymotrypsin
Phe, Tyr, Trp, Val, Leu, Met
Elastase
Ala, Gly, Ser
Carboxypeptidase A Ala, Ile, Leu, Val
Carboxypeptidase B Arg, Lys
When the C-terminal
Amino acid is as listed
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Most of the proteolytic enzymes are serine proteases
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Catalytic traid – Histidine , Asp & Ser (Chymotrypsin, Typsin & Elastase
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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
Absorption of Amino acids
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Mainly in small intestine (duodenum and jejunum)
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Free amino acids are absorbed by active transport
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Active Transport
 Sodium dependant secondary active transport system
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Five different transport system
(neutral, basic, acidic, imno acids & beta amino acids)
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Di- and tri-peptides are absorbed by a proton linked active
transport (hydrolysed in the cytoplasm)
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Role of Tri-peptide - glutathione (Miester cycle)
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Hartnup’s disease – defect in absorption
•Intestine,
Brain, Kidney
•Mainly neutral
AA
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Largest source of calories in the average diet
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Usually constitute 40 to 45% of our caloric intake
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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.
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Amylose, the glucosyl residues form a straight chain
linked via -1,4 glycosidic bonds;
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Amylopectin, the -1,4 chains contain branches
connected via -1,6 glycosidic bonds
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The other major sugar found in fruits and vegetables
is sucrose, a disaccharide of glucose and fructose
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Sucrose and small amounts of the monosaccharides
glucose and fructose are the major natural sweeteners
found in fruit, honey, and vegetables.
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The portion of the dietary carbohydrate that
cannot be digested by human intestinal
enzymes
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It Is composed principally of plant
polysaccharides cellulose, lignins and pectins
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Contain very little carbohydrate except for
small amounts of glycogen (which has a
structure similar to amylopectin)
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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
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 the sugar,
 the glycosidic bond ( or ),
 and the number of saccharide units in the chain.
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Undigested carbohydrates enter the colon, where
they may be fermented by bacteria.
Digestion of Dietary starch and Glycogen
16
14
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DISACCHARIDES ARE CLEAVED TO MONOSACCS. BY
SPECIFIC ENZYMES
Lactose + H2O
D-glucose + D-galactose
Lactase
Sucrose + H2O
Sucrase = Invertase
D-glucose + D-fructose
Maltose + H2O
2 D-glucose
Maltase
Trehalose + H2O
2 D- glucose
Trehalase
Isomaltose + H2O
2 D- glucose
Isomaltase
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
ENZYMES ARE SPECIFIC FOR THE SUBSTRATE
• 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)
What is dietary fibre?
ABSORPTION OF CARBOHYDRATES
GLUT 2
Facilitated
Mucosal GLUT 2
cell
Facilitated
Porto - venous
Circulation
2 ⁰ Active
fructose
Most absorption happens in the
duodenal and proximal jejunal
mucosa
MALABSORBTION OF CARBOHYDRATRES
Enzyme deficiency
Primary
mutation in transporter
Proteins
Secondary
Following intestinal disease
Following disease of pancreas