Comprehensive Overview of Protein Digestion in the Gastrointestinal Tract

 
 
 
Lecture series
Gastrointestinal tract
 
 
 
 
 
 
 
Professor Shraddha Singh,
Department of Physiology,
KGMU, Lucknow
 
Learning Objectives:
 
Understand the composition of protein
Understand the enzymes responsible for digestion of proteins
What are sites for absorption Molecular basis of protein
transportation
Learn about  diseases related to protein  digestion
 
What are Protiens ?
 
Proteins are a sequence of amino acids
One amino acids is joined to the next by a PEPTIDE bond
Provide energy substrate for metabolism (4 kcals/g).
 Protein load received by the gut is derived from two primary
sources: 70-100 g dietary protein, and
     35-200 g endogenous protein,
 
 
Amino acids
 
Of the 20 amino acids that exist, 9 are
essential amino acids, and 11 are non-
essential
AMINO ACID: Sequence
 
Dipeptide – 2 amino acids
Tripeptide – 3 amino acids
Oligopeptides – 4-10 amino acids
Polypeptide – more than 10 amino acids
Proteins in the body and diet are long polypeptides
(100s of amino acids)
AMINO ACID: Sequence
 
Dipeptide – 2 amino acids
Tripeptide – 3 amino acids
Oligopeptides – 4-10 amino acids
Polypeptide – more than 10 amino acids
Proteins in the body and diet are long polypeptides
(100s of amino acids)
 
AMINO ACIDS: Structure
 
  Consist of a central carbon
atom bonded to: a
hydrogen, a carboxylic
acid, an amino group,
and an additional side
group that is unique to
each amino acid
 
Digestion of proteins
Initial digestion of protein in stomach
 
No digestion of protein takes place in the mouth,
Hydrochloric acid denatures protein and also
converts pepsinogen to pepsin
Pepsin breaks the protein down into peptides of
various lengths and some amino acids
Pepsin act only at pH 1.6-3.2
Pepsin completes ~ 10-20% of digestion
Denaturing of Proteins
 
Acid, alkaline, heat and alcohol, can disrupt the
chemical forces that stabilize proteins and can cause
them to 
lose their shape 
(denature)
Denaturing of proteins happens during food
preparation (cooking, whipping, adding acids) or
digestion (in the stomach with hydrochloric acid)
 
Action of Proteolytic enzymes
 
Pepsin hydrolyses the bonds
between aromatic amino
acids(phenylanine or
tyrosine)  and  a second
amino acid
So the product of  pepsin
hydrolysis is  polypeptides
of diverse sizes
 
Polypeptidases
 
Trypsinogen and
chymotrypsinogen
(proenzymes)  are secreted
by pancreas in response to
protein in the small intestine
They will be activated to
trypsin and chymotrypsin
(now called proteases)
 
 
Peptidases hydrolyse proteins
 
 
 
These enzymes can either cleave
internal peptide bonds (i.e.
endopeptidases
)
exopeptidases
 cleave off one
amino acid at a time from either
the –COOH or –NH2
 
 terminal
of the polypeptide (i.e. they are
carboxypeptidases , and
aminopeptidases, respectively)
 
 
The 
endopeptidases
 cleave the large
polypeptides to smaller oligopeptides, which
can be acted upon by the 
exopeptidases
 to
produce the final products of protein digestion,
amino acids, 
di- and tripeptides
, which are then
absorbed by the enterocytes
 
 
Further hydrolysis by Peptidases
 
 By the action of endo and exopeptidases some free
amino acids are liberated in the intestinal lumen,
 
 But others are liberated at the cell surface by the
aminopeptidases, carboxypeptidases, endopeptidases,
and dipeptidases in the brush border of the mucosal
cells
.
 
Absorption of proteins
 
Transport of amino acids and polypeptides
in the enterocytes
 
The di- and tripeptides
are 
actively
 transported
into enterocytes by a
system known as peptide
transporter 1) that
requires H + instead of
Na +
 
At basolateral membrane
 
The movement of any one amino acid can occur through one
or more amino acid transporters.
 
At least 
five amino acid transporters 
are present in the
basolateral membrane.
 
 Three amino acid transport processes on the basolateral
membrane mediate amino acid 
 exit from the cell into the blood
 
Two other amino acid transporters mediate 
uptake from the
blood for the purposes of 
cell nutrition.
 
Amino acid transport at basolateral
 
Individual amino acids are
transported across the
basolateral membrane without
the need for cotransport.
 Many different amino acid
transporters are located on the
basolateral membrane and
provide specificity
 
Further hydrolysis by Peptidases
 
 By the action of endo and exopeptidases some free
amino acids are liberated in the intestinal lumen,
 
 But others are liberated at the cell surface by the
aminopeptidases, carboxypeptidases, endopeptidases,
and dipeptidases in the brush border of the mucosal
cells
.
 
Absorption of proteins
 
Transport of amino acids and polypeptides
in the enterocytes
 
The di- and tripeptides
are 
actively
 transported
into enterocytes by a
system known as peptide
transporter 1) that
requires H + instead of
Na +
 
At basolateral membrane
 
The movement of any one amino acid can occur through one
or more amino acid transporters.
 
At least 
five amino acid transporters 
are present in the
basolateral membrane.
 
 Three amino acid transport processes on the basolateral
membrane mediate amino acid 
 exit from the cell into the blood
 
Two other amino acid transporters mediate 
uptake from the
blood for the purposes of 
cell nutrition.
 
Amino acid transport at basolateral
 
Individual amino acids are
transported across the
basolateral membrane without
the need for cotransport.
 Many different amino acid
transporters are located on the
basolateral membrane and
provide specificity
 
Diseases associated with absorption of proteins
 
Hartnup disease and cystinuria are hereditary disorders of amino
acid transport across the apical membrane.
 
These 
autosomal recessive disorders 
are associated with both small
intestine and renal tubule abnormalities
 
the absorption of neutral amino acids in the case of Hartnup disease
and of cationic (i.e., basic) amino acids and cystine in the case of
cystinuria.
 
References
 
Lippincott’s Illustrated Reviews: Physiology (2013)
Medical Physiology, Updated second edition (
walter F. Boron,
MD, phd
)
Berne & levy, physiology, sixth edition, updated edition
Ganong’s Review of Medical Physiology, 
26 t h e d i t i o n
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Understanding the process of protein digestion is crucial for optimal nutrient absorption and overall health. In this lecture series by Professor Shraddha Singh, delve into the composition of proteins, the role of enzymes in protein digestion, sites of absorption, molecular basis of protein transportation, and diseases related to protein digestion. Explore the structure of amino acids, the breakdown of proteins into peptides and amino acids, and the initial digestion of proteins in the stomach. Gain insights into the importance of essential and non-essential amino acids, dipeptides, tripeptides, and more in the intricate process of protein metabolism.


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  1. Lecture series Gastrointestinal tract Professor Shraddha Singh, Department of Physiology, KGMU, Lucknow

  2. Learning Objectives: Understand the composition of protein Understand the enzymes responsible for digestion of proteins What are sites for absorption Molecular basis of protein transportation Learn about diseases related to protein digestion

  3. What are Protiens ? Proteins are a sequence of amino acids One amino acids is joined to the next by a PEPTIDE bond Provide energy substrate for metabolism (4 kcals/g). Protein load received by the gut is derived from two primary sources: 70-100 g dietary protein, and 35-200 g endogenous protein,

  4. Amino acids Of the 20 amino acids that exist, 9 are essential amino acids, and 11 are non- essential

  5. AMINO ACID: Sequence Dipeptide 2 amino acids Tripeptide 3 amino acids Oligopeptides 4-10 amino acids Polypeptide more than 10 amino acids Proteins in the body and diet are long polypeptides (100s of amino acids)

  6. AMINO ACID: Sequence Dipeptide 2 amino acids Tripeptide 3 amino acids Oligopeptides 4-10 amino acids Polypeptide more than 10 amino acids Proteins in the body and diet are long polypeptides (100s of amino acids)

  7. AMINO ACIDS: Structure Consist of a central carbon atom bonded to: a hydrogen, a carboxylic acid, an amino group, and an additional side group that is unique to each amino acid

  8. Digestion of proteins

  9. Initial digestion of protein in stomach No digestion of protein takes place in the mouth, Hydrochloric acid denatures protein and also converts pepsinogen to pepsin Pepsin breaks the protein down into peptides of various lengths and some amino acids Pepsin act only at pH 1.6-3.2 Pepsin completes ~ 10-20% of digestion

  10. Denaturing of Proteins Acid, alkaline, heat and alcohol, can disrupt the chemical forces that stabilize proteins and can cause them to lose their shape (denature) Denaturing of proteins happens during food preparation (cooking, whipping, adding acids) or digestion (in the stomach with hydrochloric acid)

  11. Action of Proteolytic enzymes Pepsin hydrolyses the bonds between aromatic amino acids(phenylanine or tyrosine) and a second amino acid So the product of pepsin hydrolysis is polypeptides of diverse sizes

  12. Polypeptidases Trypsinogen and chymotrypsinogen (proenzymes) are secreted by pancreas in response to protein in the small intestine They will be activated to trypsin and chymotrypsin (now called proteases)

  13. Peptidases hydrolyse proteins These enzymes can either cleave internal peptide bonds (i.e. endopeptidases) exopeptidases cleave off one amino acid at a time from either the COOH or NH2 terminal of the polypeptide (i.e. they are carboxypeptidases , and aminopeptidases, respectively)

  14. The endopeptidases cleave the large polypeptides to smaller oligopeptides, which can be acted upon by the exopeptidases to produce the final products of protein digestion, amino acids, di- and tripeptides, which are then absorbed by the enterocytes

  15. Further hydrolysis by Peptidases By the action of endo and exopeptidases some free amino acids are liberated in the intestinal lumen, But others are liberated at the cell surface by the aminopeptidases, carboxypeptidases, endopeptidases, and dipeptidases in the brush border of the mucosal cells.

  16. Absorption of proteins

  17. Transport of amino acids and polypeptides in the enterocytes The di- and tripeptides are actively transported into enterocytes by a system known as peptide transporter 1) that requires H + instead of Na +

  18. At basolateral membrane The movement of any one amino acid can occur through one or more amino acid transporters. At least five amino acid transporters are present in the basolateral membrane. Three amino acid transport processes on the basolateral membrane mediate amino acid exit from the cell into the blood Two other amino acid transporters mediate uptake from the blood for the purposes of cell nutrition.

  19. Amino acid transport at basolateral Individual amino acids are transported across the basolateral membrane without the need for cotransport. Many different amino acid transporters are located on the basolateral membrane and provide specificity

  20. Further hydrolysis by Peptidases By the action of endo and exopeptidases some free amino acids are liberated in the intestinal lumen, But others are liberated at the cell surface by the aminopeptidases, carboxypeptidases, endopeptidases, and dipeptidases in the brush border of the mucosal cells.

  21. Absorption of proteins

  22. Transport of amino acids and polypeptides in the enterocytes The di- and tripeptides are actively transported into enterocytes by a system known as peptide transporter 1) that requires H + instead of Na +

  23. At basolateral membrane The movement of any one amino acid can occur through one or more amino acid transporters. At least five amino acid transporters are present in the basolateral membrane. Three amino acid transport processes on the basolateral membrane mediate amino acid exit from the cell into the blood Two other amino acid transporters mediate uptake from the blood for the purposes of cell nutrition.

  24. Amino acid transport at basolateral Individual amino acids are transported across the basolateral membrane without the need for cotransport. Many different amino acid transporters are located on the basolateral membrane and provide specificity

  25. Diseases associated with absorption of proteins Hartnup disease and cystinuria are hereditary disorders of amino acid transport across the apical membrane. These autosomal recessive disorders are associated with both small intestine and renal tubule abnormalities the absorption of neutral amino acids in the case of Hartnup disease and of cationic (i.e., basic) amino acids and cystine in the case of cystinuria.

  26. References Lippincott s Illustrated Reviews: Physiology (2013) Medical Physiology, Updated second edition (walter F. Boron, MD, phd) Berne & levy, physiology, sixth edition, updated edition Ganong s Review of Medical Physiology, 26 t h e d i t i o n

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