Digestion & Absorption of Proteins
Lecture by:
Dr. Samia Naz
(APWMO)
Biochemistry Department, KEMU
Introduction
• Proteins are the primary constituents of body.
• These are genetically designed sequences of
amino acids linked together by peptide bonds.
• Amino acids are used for synthesis of body
proteins and to yield energy 4.1kcal/gm by
oxidation.
• Proteins are nitrogenous macromolecules.
Digestion of Dietary Proteins
• Process of breaking down ingested food
material into easily absorbed and assimilated
substances by mechanical and enzymatic
action in alimentary canal. Dietary proteins
are hydrolyzed to amino acids by proteolytic
enzymes which can be easily absorbed.
• Proteolytic enzymes, degrading the proteins
are present in stomach, pancreas and small
intestine.
Proteolytic enzymes in GIT
• Gastric enzymes
• Pancreatic enzymes
•
pepsin
renin
Endopeptidases
-Trypsin
- Chymotrypsin
- Elastase
Exopeptidases
- Carboxypeptidase
- Collagenase
• Enzymes of epithelial cells
• Enzymes within enterocytes
A
B
Exopeptidases
- Aminopolypeptidase
- Tripeptidase
- Prolinase
Intracellular peptidases
Digestion in mouth
No digestion in mouth, no proteolytic enzymes,
after mastication and chewing bolus is formed
which enters the stomach.
Digestion in stomach
• Digestion of proteins start in stomach.
• When proteins enters the stomach it stimulates the
secretion of the hormone gastrin, from gastric mucosal
cell.
• This gastrin, in turn, stimulates the release of gastric
juice, which contains
• Hydrochloric acid:- denatures proteins, decreases
pH(pH 2-3), activates pepsinogen
• Pepsinogen: Zymogen secreted by chief cells, activated to pepsin
by HCl and autocatalysis.
• Pepsinogen
pepsin
Pepsin breaks the peptide bonds of
a)
b)
c)
aromatic amino acids (phenylalanine, tyrosine, tryptophan)
acidic amino acids (glutamic acid, aspartic acid)
pepsin hydrolysis proteins into polypeptides and free amino acids
Rennin in infants
• Active in infants
• Causes curdling of milk, converts casein of milk into paracasein,
combines with calcium to form calcium paracaseinate
• Absent in adult stomach
Pancreatic enzymes
The bolus of food after leaving the stomach
reaches duodenum and is acted upon by the
pancreatic juice. All the enzymes are active
only in alkaline medium and alkalinity is
avoided by bile and bicarbonate present in
pancreatic juice. The release of pancreatic
zymogens is mediated by the secretion of
cholecystokinin and secretin, two polypeptide
hormones of digestive tract.
Endopeptidases
1. Trypsin occurs as trypsinogen
Trypsinogen
Enteropeptidase
Trypsin
2. Chymotrypsin occurs as chymotrypsinogen
Chymotrypsinogen
trypsin
chymotrypsin
– Both enzymes act on centrally situated peptide bonds at pH 8
– Protein molecules are hydrolyzed to polypeptides of various length.
3. Elastase occurs as Pro elastase
Pro elastase
trypsin
elastase
Exopeptidase
1. Carboxypeptidases A :• Acts on carboxyl end peptide bond
Procarboxypeptidase A
trypsin
carboxypeptidases A
2. Carboxypeptidase B:
• Acts on carboxyl end peptide bond
Procarboxypeptidase B
trypsin
3. Collagenase
pro collagen
collagenase
collagen
carboxypeptidases B
Digestion at the mucosal surface of epithelial
cells of small intestine(Enterocyte)
• Exopeptidases :
a) Amino polypeptide acts on the
peptide bond on amino terminal of
amino acids .
b) Tripeptidases and Dipeptidases.
hydrolyse tripeptides and dipeptides
to their individual amino acids.
c) Prolinase hydrolyses small peptides
having proline at terminal amino
acids. These enzymes act on their
substrates which have entered brush
border of enterocytes.
Digestion within Enterocytes
• Intra-cellular peptidases hydrolyze dipeptides,
tripeptides and oligopeptides within brush
borders of enterocytes which are completely
absorbed.
• Sodium-Amino acid carrier system it
transports proteins with Na ions and amino
acids by Na-K by ATPase activity
Basolateral Membrane
• Transport of free amino
acids into portal system.
 Peptides are hydrolyzed
within the enterocyte
• Transport mainly by
diffusion and active
transport
Absorption of Intact Proteins
• Newborns
 First 24 hours after birth
 Immunoglobulins
 Passive immunity
• Adults
 Paracellular routes
• Tight junctions between cells
 Intracellular roites
• Endocytosis
• Pinocytosis
• Of little nutritional significance…
 Affects health (allergies and passive immunity)