Proteins, the primary constituents of the body.
A regular & adequate supply of protein in the diet is
essential for cell integrity & function.
Dietary proteins are the primary sources of the nitrogen.
Adult man requires 70 to 100 gm protein/day.
Dietary proteins serve three broad functions:
Their constituent AAs are used for synthesis of body
The carbon skeletons of AAs can be oxidized to yield energy
Their “C” & “N” atoms may be used to synthesize other
nitrogenous and non-nitrogenous metabolites.
Digestion is the disintegration of complex
nutrients into simple, soluble and assimilable
Proteins are too large to be absorbed.
The dietary proteins are hydrolyzed to amino
acids by proteolytic enzymes, which can be
Proteolytic enzymes responsible for
degrading proteins are produced by three
different organs; The stomach, pancreas and
the small intestine.
• Endopeptidase : acts inside the core of
protein, forms small peptide fragments
• Exopeptidase : acts from the amino terminal
or carboxyl terminal ends of protein
• Peptidases are secreted in the inactive form
• When zymogens reach the site of action they
Proteins Salivary gland:
Pancreatic juice Intestine:
No digestion of protein in mouth.
No proteolytic enzymes present in the saliva.
Function of the saliva - lubricate the food, this
helps in making food soluble for the action of
After mastication and chewing, the bolus of food
enters stomach where it is acted upon by gastric
Digestion in Mouth
Digestion of protein starts in stomach.
When proteins enters the stomach, it stimulates
the secretion of the hormone gastrin, from
gastric mucosal cells.
This gastrin, in turn, stimulates the release of
gastric juice, which contains...
Hydrochloric acid (HCL).
Rennin (in infants).
The pH of gastric juice is 1.5-2.5.
Digestion in Stomach
Digestion of protein begins in the stomach
• Denatures proteins
• Decreases pH (2-3)
• Activates pepsinogen
• Kills some bacteria
• Helps in the
Secreted by Chief
cells. Activated to
pepsin by HCl.
As the acidic stomach contents pass into duodenum,
the low pH triggers the secretion of the hormones…
Secretin – stimulate pancreas to secrete
bicarbonate. It neutralizes HCL and rises the pH
from 1.5-2.5 to 7.0
Cholecystokinin - stimulate secretion of
Endopeptidase – Trypsin, Chymotrypsin,
Exopeptidase – Carboxypeptidases,
Digestion of proteins in the Intestine By Pancreatic Enzymes
• Secreted in the zymogen form (Trypsinogen)
• activated by Enterokinase and trypsin itself.
• specific for cleaving peptide bonds contributed
by lysine, arginine (basic amino acids)
• Acts on the zymogen forms of other pancreatic
enzymes and activates them
• It has weak action on casein
Zymogen form –
chymotrypsin by trypsin
Hydrolyzes the peptide
bond formed by the
carboxyl group of
aromatic amino acids
Zymogen form –
activated to elastase by
Acts on peptide bonds
formed by the amino
acids like glycine,
Chymotrypsin and elastase
- Zinc containing
- Zymogen form is
- Activated by trypsin
- Cannot act on
acts on the Carboxy
bond connected to
acts on the carboxy
bond connected to
Chymotrypsin Elastase Carboxypeptidase A
Pro-elastase procarboxypeptidase A
Overview of Digestion of proteins
The digestion products of hydrolysis by pepsin, trypsin,
elastase, chymotrypsin & carboxypeptidase is completed
by the intestinal peptidases, secreted by the mucosa of
the small intestine.
Some of these peptidases are…
• Present on the luminal surface of
the intestinal mucosa
• Is an exopeptidase.
• Acts on the N terminal peptide bond
• Release free amino acid
• Present on the surface of the intestinal
• Act on dipeptides & release free AAs
• Enterocytes take up some di & tripeptides
• These peptides are hydrolyzed to amino
acids by intracellular dipeptidase
Di and Tri
•Absorption of most of the AAs takes
place by active transport mechanism.
•By Na+ dependent active transport
system (Na+ amino acid cotransport)
• An energy requiring process
Absorption of amino acids
ADP + Pi
Na+ - K+
Amino Acids -
• Small neutral amino
• Large neutral amino
• Basic amino acids
• Acidic amino acids
• imino acid and
• Alanine, Serine &
• Isoleucine, Leucine,
• Arg, Lys, Ornithine &
• Glu acid & Asp acid
• Proline, OH-proline &
L- Amino acid
Outer membraneγ-glutamyl transferase
γ – glutamyl cycle (Meister cycle)
intact proteins & 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.
Premature activation of trypsinogen inside the pancreas itself will
result in the autodigestion of pancreatic cells
Defects in the intestinal amino acid transport systems
are seen in inborn errors of metabolism
• Hartnup’s disease
• Cystinuria : [dibasic amino acids, ornithine, arginine, and
lysine (represented as “COAL”)
Cystinuria- Common transporter for cystine,
ornithine, arginine and lysine(COAL) is present in
gut and renal tubules. Deficiency of transporter
results in loss of these amino acids in the feces
Hartnup’s Disease- There is deficiency of
transporter for tryptophan and neutral amino
acid. no absorption of tryptophan takes place
,tryptophan deficiency produce neurological and
skin manifestation (pellagra-like rashes).
Contact no. – 07418831766
E mail – email@example.com
Dept. of Biochemistry,
Dhanalakshmi Srinivasan Medical College,