structure of proteins definition of Digestion sources of Proteins --> EXOGENEOUS SOURCES 50-100g/day and ENDOGENEOUS SOURCES 30-100g/day Proteins DEGRADED BY --> HYDROLASES specifically PEPTIDASES(ENDOPEPTIDASES & EXOPEPTIDASES) 1. Gastric Digestion of Proteins 2. Pancreatic Digestion of Proteins 3. Digestion of Proteins by Small Intestine Enzymes Absorption of Amino ACids by Na+Dependent, Na+ Independent, Meister Cycle or gama-glutamyl cycle

1. DIGESTION AND ABSORPTION OF
PROTEINS
BY
DR SHRADDHA BHARATH
PG STUDENT
ESIC-MC & PGIMSR
BANGLORE-10
DEPARTMENT OF BIOCHEMISTRY

2.  All proteins polymers of L- a amino acids
 Proteins  AAs  Peptide bonds
 Total dry body weight 3/4th proteins

3.  Digestion  hydrolysis of large & complex organic
molecules of foodstuffs  smaller and preferably water-
soluble molecules which can be easily absorbed by the
GIT for utilization by the organism.
 Digestion as well as absorption  complicated process
 in GIT.

4. SOURCES OF PROTEINS
Two sources
Exogenous Endogenous
Range- 30-100g/day
- digestive enzymes
- worn out cells of the
digestive tract.
Dietary source
Range-50-100g/day
Animal source
MILK,EGGS,
MEAT,FISH,LIVER,
Vegetable source
CEREALS, PULSES,
PEAS, BEANS &
NUTS

5.  About 5-10g/day Lost through feces.
 Dietary Proteins  Denatured on cooking Easily
Digested.
 Proteins are degraded by a class of enzymes namely
Hydrolases.
Specifically cleaves the peptide bonds Peptidases

6. Peptidases
Endopeptidases Exopeptidases

7.  The enzymes responsible for the digestion of
proteins are produced by the
 Proteolytic enzymes Inactive
ZymogensActive form.
 Proteins  Not digested  Mouth  Absence
of Proteases in saliva.
Stomach
Pancreas
Small Intestine

8. I. GASTRIC DIGESTION OF PROTEINS
 Stomach  chemical digestion of proteins
 Gastric Juice produced by the stomach contains
HCL and PEPSINOGEN

10.  HCL
 pH of the stomach is <2.
 Acid
Denaturation of proteins, more susceptible to
proteases for digestion.
Killing of certain Micro-organisms.

11.  PEPSIN (GREEK :PEPSIS DIGESTION)
 Secreted by the chief cells/serous cells of the
stomach as PEPSINOGEN.
 Pepsinogen Pepsin
 Optimum pH  around 2
 Pepsin  ACID STABLE ENDOPEPTIDASE
HCL

12. Digestion of proteins by PEPSIN
PEPTIDES AMINO ACIDS
Hormones( CCK(Cholecystokinin) & SECRETIN)
(DUODENUM)
PANCREATIC JUICE (Enzymes)

13. II. PANCREATIC DIGESTION OF PROTEINS
 The optimum pH for the activity of pancreatic enzymes 
pH 8  alkaline bile and pancreatic enzymes.
 The secretion of pancreatic juice is stimulated by the
hormones, CCK & SECRETIN in intestine
 Pancreatic juice TRYPSIN
contains important CHYMOTRYPSIN
endopeptidases namely ELASTASE
CARBOXYPEPTIDASE

14. TRYPSIN ,CHYMOTRYPSIN, ELASTASE, CARBOXYPEPTIDASE
These enzymes are also secreted as Zymogens
(Trypsinogen, Chymotrypsinogen, Proelastase,
Procarboxypeptidase)
 These are also c/a SERINE PROTEASES

15.  Proteins large polypeptides Small Intestine
Trypsinogen Trypsin  hydrolysis of Peptide bond
Enteropeptidase/
Enterokinase
Ca2+

16.  3 Reasons for Big deal of Trypsinogen:
1) Automatically stimulates the conversion of more trypsinogen to
more trypsin during the digestion
2) Chymotrypsinogen Chymotrypsin(active form)
(inactive form)
hydrolysis of internal peptide bond
3) Procarboxypeptidase Carboxypeptidase(active)
(A & B)
hydrolysis of peptide bond from the
carboxyl end

17. CLINICAL CONDITION:
 Acute Pancreatitis:
premature activation of trypsinogen  autodigestion
Clinical features: mild to severe epigastric pain, with radiation
to the flank, the back or both. Presents with nausea and
vomiting also.
Diagnosis:  Blood tests :- Serum Amylase, Serum Lipase,
Serum trypsin / elastase, Hepatic transaminase levels.
 Radiological :- ultrasonograpy, CT

18. CHYMOTRYPSIN:
 secreted as inactive form zymogen
Chymotrypsinogen
p- chymotrypsin
d-chymotrypsin
a-chymotrypsin
Proteins, peptones smaller peptides & AA
& peptides
TRYPSIN
TRYPSIN

19. CARBOXYPEPTIDASES
 The pancreatic Carboxypeptidase (A&B)  requires
Zn2+
catalytic activity
 Zinc-proteases
 Trypsin & chymotrypsin small peptides
Di-peptides tri-peptides AA
CARBOXYPEPTIDASES

20.  Exopeptidase
 Terminal peptide bond
 end Aromatic AA
 Eg: Tyrosine,
Phenylalanine or
Tryptophan
 Liberates end AA as
“FREE” form
 Exopeptidase
 Terminal peptide bond
 basic AA
 Eg: Arginine, Lysine
bearing free –COOH
group
CARBOXYPEPTIDASE-A CARBOXYPEPTIDASE-B

21. III. DIGESTION OF PROTEINS BY SMALL INTESTINE
ENZYMES
 Proteolytic enzymes Amino peptidases
present in the di & tripeptidases
intestinal juice
 On top of the intestinal cells
presence of Special enzymes c/a BRUSH BORDER ENZYMES
Specifically hydrolyse peptide bonds

23. ABSORPTION OF AMINO ACIDS
 Site of absorption
Amino acids absorbed di & tripeptides
ileum & distal jejunum duodenum & proximal
jejunum
 Energy requiring process
 Transport systems  carrier mediated & ATP Sodium
dependent symport system
 The free Amino acids, dipepties & some extent of
tripeptides intestinal epithelial cells.

24. THE SMALL INTESTINE POSSESSES AN EFFICIENT SYSTEM TO
ABSORB FREE AMINO ACIDS
L-AMINO ACIDS
 More rapidly absorbed
 Active process
D- AMINO ACIDS
 Simple diffusion

25. MECHANISM OF AMINO ACID ABSORPTION
 These Tri-peptides & Di-peptides :

26.  AMINO ACID absorption has different mechanism:
1. It is basically a Na+ - dependent active process
linked with the transport of Na+
 Energy is supplied indirectly by ATP

27. 2. Na+ - Independent system of amino acid
3. g- GLUTAMYL CYCLE or MEISTER CYCLE
 Tripeptide Glutathione( g-glutamyl-cysteinely-glycine)
 3 ATP are utilised  single amino acid

29. Immunogenic antibody reaction  FOOD ALLERGY

30. CLINICAL DISORDERS
1. The deficiency of the enzyme  5-oxoprolinase
OXOPROLINURIA (Pyroglutamic Aciduria)
2. The allergy to certain foods(peanuts, sea foods)due to
partially digested proteins
3. Partial gastrectomy
Pancreatitis
Ca. of pancreas
Cystic fibrosis
4. Protein losing Enteropathy
Affects the digestion &
absorption of proteins

31. 5. Transport systems  Inborn errors of metabolism such as:
a. Hartnup’s disease
b. Iminoglycinuria
c. Cystinuria
d. Lysinuric protein intolerance
e. Oasthouse syndrome

32. HARTNUP’S DISEASE
 Inheritated autosomal recessive disease.
 Absorption of neutral amino acids in intestine & reabsorption in renal
tubules  defective  neutral AA are excreted in urine
 Pellagra like symptoms: DERMATITIS & CEREBELLAR ATAXIA

33. LYSINURIC PROTEIN INTOLERANCE
 Hyperdibasic aminoaciduria, cationic
aminoaciduria, familial protein
intolerance
 An autosomal recessive metabolic
disorder affecting the AA transport
 Lysine poorly absorbed in intestine 
urinary excretion of this AA is increases

34. OASTHOUSE SYNDROME
 Oasthouse (building
designed for drying hops)
 Methionine malabsorption
syndrome
 An autosomal recessive condition
 Symptoms includes :
mental retardation, diarrhea,
convulsions after methionine loading,
 oasthouse odour

36. QUESTIONS:
1. Brief about digestion and absorption of proteins ?
2. Give examples of proteolytic enzymes and its specificity
3. Role of hydrochloric acid in protein digestion
4. Name two Endopeptidases with their specifications
5. How is pepsinogen activated. What is the function of
pepsin?

37.  Activation of Procarboxypeptidase-A
3 subunits III, II & I
Subunit III subunit II is changed to
inactive proteinase
subunit I active
carboxypeptidase A
TRYPSIN

38. o HARTNUP DISEASE
o Diagnosis  Aminoaciduria 
increased excretion of indole
compound Obermeyer test
o High protein diet
Supplemetation of Niacin
Minimum exposure to sunlight
o Neuropsychiatric  Saditic and
bizarre behaviour of emperors like
Nero & caligula

39. LYSINURIC PROTEIN INTOLERANCE
 Hyperdibasic aminoaciduria, cationic
aminoaciduria, familial protein
intolerance
 An autosomal recessive metabolic
disorder affecting the AA transport
 Lysine poorly absorbed in intestine 
urinary excretion of this AA is increased
 C/F: skeletal & iminological
abnormalities
 Diagnosis : biochemical findings

40. OASTHOUSE SYNDROME
 Oasthouse (building
designed for drying hops)
 Methionine malabsorption
syndrome
 An autosomal recessive condition
 Symptoms includes :
mental retardation, diarrhea,
convulsions after methionine loading,
 oasthouse odour