digestion and absorption of protein dr.ashok kumar j
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Dr. Ashok Kumar J; IMC; MSU 1
Digestion and Absorption of ProteinsDr. Ashok Kumar J
International School of Medicine Management and Science University
Malaysia
Proteins
• Polymers of amino acids• Macromolecules• Contain about 16% nitrogen in addition to Carbon,
Hydrogen and Oxygen
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Digestion of proteins and absorption of amino acids
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• Intake of dietary protein is in the range of 50-100 g/day • About 30-100 g/day of endogenous protein is derived form
the digestive enzymes and worn out cells of the digestive tract
• The digestion and absorption of proteins is very efficient in healthy humans, hence very little protein (about 5-10 g/day) is lost through feces
Amino acids are linked to each other by peptide linkage
Amino acid
Amino acid
Amino acid
Amino acid
Amino acid
Peptide bonds
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The peptide bonds of the proteins must be hydrolyzed to give their constituent amino acids
(Digestion of protein)
Proteases (peptidases)
Amino acid
Amino acid
Amino acid
Amino acid
Amino acid
Peptide bonds
Amino acid
Amino acid
Amino acid
Amino acid
Amino acid
5Dr. Ashok Kumar J; IMC; MSU
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Peptidases(Hydrolases; Class 3)
• Endopeptidase : acts inside the core of protein, forms small peptide fragments
• Exopeptidase : acts from the amino terminal or carboxyl terminal ends of protein• Aminopeptidase - e.g : amino
peptidase• Carboxypeptidase - e,g :
carboxypeptidase
Proenzymes (zymogens)
• Peptidases are secreted in the inactive form
• When zymogens reach the site of action they are activated
Proteolytic enzymes of GIT are secreted by three different organs
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STOMACH
PepsinRenin
Pancreas
TrypsinChymotrypsinElastaseCarboxypeptidase ACarboxypeptidase B
Intestine
AminopeptidaseDipeptidase
Digestion of protein begins in the stomach
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Stomach secrets
• Strong mineral acid HCl
• Proenzyme pepsinogen
• Denatures proteins • Decreases the pH
(pH 2 to 3)• Activates pepsinogen• Kills some bacteria• Helps in the absorption
of Vitamin B12
Hydorchloric acid (HCl) Pepsinogen
Activated to pepsin by HCl Acid Stable endopeptidase Pepsinogen
Pepsinogen
Pepsin Autocatalysis
Pepsin
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Pepsin acts on
Peptide bond formed by carboxyl group of
Phenylalanine, Tyrosine, Tryptophan, methionine
Converts proteins into proteoses and peptones
RENNIN
Also called chymosinActive in infantscurdling of milk
Absent in adult stomach
Converts Casein of milk into paracasein
It combines with calcium to form calcium paracaseinate
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Activity of pepsin reduced
Pancreatic juice secretion stimulated by hormone
1. Cholecystokinin 2. Pancreozymin
Large polypeptides further cleaved to oligopeptides by
pancreatic peptidases
Digestion of proteins in the Intestine
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Secreted as pro-enzymes
Optimum pH is 8
Pancreatic Peptidases
Carboxypeptidase ACarboxypeptidase B
(Exopeptidase)
TrypsinChymotrypsin
Elastase(Endopeptidases)(Serine proteases)
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Trypsin(endopeptidase)
Trypsin
TrypsinogenENTEROPEPTIDASEPresent on the brush border surface
Trypsinogen (Zymogen)
• Hydrolyzes the peptide bond formed by the Basic amino acids (Arginine and Lysine )
• Acts on the zymogen forms of other pancreatic enzymes and activates them
[autocatalysis]
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CHYMOTRYPSIN(Endopeptidase)
• Zymogen form – Chymotrypsinogen
• activated to chymotrypsin by trypsin
• Hydrolyzes the peptide bond formed by the carboxyl group of aromatic amino acids
Elastase(Endopeptidase)
• Zymogen form – Proelastase
• activated to elastase by trypsin
• Acts on peptide bonds formed by the amino acids like glycine, alanine, serine
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Carboxypeptidase- Zinc containing - Exopeptidase- Zymogen form is procarboxypeptidase- Activated by trypsin- Cannot act on dipeptides
Carboxypeptidase A
acts on the Carboxy terminal peptide
bond connected to Tyrosine,
Phenylalanine or tryptophan
Carboxypeptidase B
acts on the carboxy terminal peptide
bond connected to Arginine, lysine
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• Present on the luminal surface of the intestinal mucosa • Is an exopeptidase. • Acts on the amino terminal peptide bond • Release free amino acidAmino
peptidase
• Present on the surface of the intestinal mucosal They act on dipeptides and release free amino acids
• Enterocytes take up some di and tripeptides• These peptides are hydrolyzed to amino acids by intracellular
dipeptidase
Di and Tripeptidases
Absorption of amino acids
• By Na+ dependent active transport system (Na+ amino acid cotransport)• An energy requiring process
•Transporter for acidic amino acids•Transporter for basic amino acids•Transporter for neutral amino acids•Transporter for imino acid •Transporter for β- amino acids
There are 5 different carriers for amino acids
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• Intestines, kidney tubules and brain
• Absorption of neutral amino acids
• Also called Miester cycle• Requires the tripeptide
glutathione (GSH) and energy
• Catalysed by gamma glutamyl transferase
γ - glutamyl cycle
5-oxoprolinase deficiency leads to oxoprolinuria
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Absorption of intact proteins and polypeptides
• Short period, immediately after birth, the small intestine of infants can absorb intact proteins and polypeptide by endocytosis or pinocytosis
• Intact proteins and polypeptides are not absorbed by the adult intestine
• Macromolecular absorption in certain individuals appears to be responsible for antibody formation that often causes food allergy.
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Defects in the intestinal amino acid transport systems are seen in inborn errors of metabolism
• Hartnup’s disease• Iminoglycinuria• Cystinuria : [dibasic amino acids, ornithine, arginine, and lysine
(represented as “COAL”)]• Oasthouse syndrome
Acute pancreatitis: • Premature activation of trypsinogen inside the pancreas itself will
result in the autodigestion of pancreatic cells
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• Celiac disease (celiac sprue) is a disease of malabsorption resulting from immune-mediated
damage to the small intestine in response to ingestion of gluten (or gliadin produced from gluten), a protein found in wheat and barley
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Thank you
Ref:
Dr. Ashok Kumar J; IMC; MSU 22
Female flower of the hop plant dried as a part of brewing process in oast house; Characteristic smell resembles the smell of the urine in Oasthouse syndrome