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Pepsin is an endopeptidase acting on the central peptide bond in which amino group belongs to aromatic amino acids –phenylalanine, tyrosine, and tryptophan. It is secreted in an inactive form called pepsinogen. It is activated by hydrochloride acid and auto-activation into pepsin.
Digestion and absorption of protein
Keywords: Rennin, Glutathione, Tripeptides metaproteins, endopeptidase,
exopeptidase
Table of
contents
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1. |
Introduction |
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2. |
Digestion |
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3. |
Absorption |
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4. |
Protein utilization |
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5. |
Protein turnover |
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6. |
Recommended daily dose |
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7. |
Read more |
Introduction: Protein is
formed from amino acids. Amino acids are linked together in a chain by peptide
bonds and form polypeptides-dipeptide and tripeptides. They form protein. The link
is known as the polypeptide link.
Digestion
Mechanical digestion of proteins starts in the mouth and continues
in the stomach and small intestine, and chemical digestion starts in the stomach
and ends in the small intestine.
In the mouth, the mechanical digestion of protein starts. When food enters the
mouth, chewing starts and breaks large pieces of food material into small pieces. These small pieces with
saliva form a bolus that can be swallowed. There is no chemical digestion in the mouth.
Chemical digestion of protein starts in the stomach (HCach bl ) with hydrochloric acid and pepsin. The stomach releases gastric juice containing hydrochloric acid and pepsinogen, and other enzymes. Pepsinogen is converted into its active form, pepsin, by hydrochloric acid. The stomach's acidity causes the unfolding of the proteins that still retain part of their three-dimensional structure after cooking and helps break down the protein aggregates formed during cooking. Pepsin acts on the peptide chain and digests it into smaller and smaller fragments. The movements of the stomach churn the partially digested food- protein into a chyme. Eating a high-protein meal increases the emptying time, providing sufficient time for protein digestion. Protein digestion in the stomach takes longer than carbohydrate digestion but a shorter time than fat digestion. Pancreatic enzymes break polypeptides into amino acids, dipeptides, and tripeptides in the small intestines. The two major pancreatic enzymes that digest proteins are - chymotrypsin and trypsin.
In the small
intestine mucosa, proteolytic enzymes are present on the surface that finally
break the smaller protein fragments into tripeptides,
dipeptides, and amino acids. Enterocytes, tripeptides, and dipeptides are broken down into amino acids. Amino
acids are absorbed into the blood.
Gastric HCl: Denaturation of proteins converts proteins
to meta proteins. Meta proteins are easily
digestible. It causes the unfolding of the 3-D structure of protein and exposes polypeptide chains. In addition, it converts
pepsinogen to active pepsin and provides proper pH for pepsin action, i.e., 1.5 -2.2.
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| Steps of protein digestion in the stomach |
Pepsin is an endopeptidase acting on the central peptide bond in which amino group belongs to aromatic amino acids –phenylalanine, tyrosine, and tryptophan. It is secreted in an inactive form called pepsinogen. It is activated by hydrochloride acid and auto-activation into pepsin.
Rennin
is a milk clotting enzyme present in
the stomach of infants and young children. The enzyme works optimally at pH 4. It
acts on casein, converting it to soluble paracasein, which binds calcium ions forming insoluble
calcium paracasein, then digested by
pepsin.
Casein---Rennin------>
paracasein --+ca----> calcium
paracasein--- pepsin--->AA
Gelatinase
liquefies gelatine.
Protein polypeptide site of endopeptidase cleavage.
Trypsin is an
endopeptidase acting on the central peptide bond in which the carboxyl group
belongs to essential amino acids, e.g., arginine and histidine.
Trypsinogen
is secreted, which is the inactive form of
trypsin. It is activated by the enterokinase enzyme into trypsin. Once activated, trypsin causes auto-activation
and also starts other proenzymes.
Chymotrypsin
is an endopeptidase that hydrolyzes the central peptide bond in which the
carboxyl group belongs to aromatic amino acids. It is secreted in an inactive
form called chymotrypsinogen, activated by trypsin.
Elastase is
an endopeptidase acting on peptide bonds
formed by glycine, alanine, and serine. It digests elastin and collagen. It is
secreted in an inactive form called proelastase, activated by trypsin.
site of
exopeptidase cleavage site
of exopeptidase cleavage
Carboxypeptidase
is an exopeptidase that hydrolyzes the terminal peptide bond at the carboxyl terminus of the polypeptide
chain. It is secreted in an inactive form called procarboxypeptidase, activated by trypsin.
Intestinal
juice :
Aminopeptidase
is an exopeptidase that hydrolyzes the terminal peptide bond at the amino terminus of the polypeptide chain. It releases a single amino acid.
Tripeptidase is an enzyme that catalyzes the hydrolysis of
tripeptides, e.g., glycylglycylglycine. They
hydrolyze only tripeptides containing a
free primary or secondary α-amino group, a free terminal carboxyl group, and peptide hydrogen at the sensitive bond.
Absorption
The absorption of amino acids is rapid in the duodenum and jejunum but slow
in the ileum. Dipeptides and tripeptides can enter the small
intestinal mucosal cell directly in a trace. In the mucosal cell, they are
digested into amino acids. Amino acids are transported from the intestinal
lumen to the intestinal cell and from the cell to the blood. The movement of
individual amino acids requires special transport proteins, which is an
active process.
Usually, whole proteins are not absorbed. The
occurrence of this in man is suggested by the allergic reactions shown by some
individuals to certain foodstuffs. However, there is extensive absorption of
whole proteins from milk in newborns, including
maternal antibodies. It occurs by pinocytosis.
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| Absorption of aminoacids |
Mechanism of amino acids absorption:
There are two mechanisms :
1. Carrier proteins
transport system
2. Glutathione transport system
Carrier, the transport of the protein system, is the primary system for amino acid absorption. This is an active process. Absorption of one amino acid molecule needs one ATP molecule. There are 7 carrier proteins, one for each group of amino acids. Each carrier protein has two sites, one for amino acid and one for Na+.
Co-transports amino acids and Na+ from the intestinal lumen to the cytosol of
intestinal mucosa cells. The
absorbed amino acids enter the portal
circulation, while Na+ is
extruded from the cell in exchange with K+ by the sodium-potassium pump.
Glutathione
transport system: Glutathione is used to transport amino acids from the intestinal lumen to the cytosol of intestinal mucosa cells. This is an active process. Absorption of one amino acid molecule needs 3
ATP molecules. Glutathione reacts with amino acid in the presence of glutamyl
transpeptidase to form glutamyl amino acid. Glutamyl amino acid releases amino acid in the cytosol of intestinal mucosa
cells with the formation of 5-oxoproline that is used to regenerate glutathione to begin another cycle.
Utilization
Once the amino
acids are in the blood, they are
transported to the liver. Amino acids are synthesized in the liver deamination of amino acids, forming many proteins.
Protein turnover: All cells in the body continually break down proteins and build new
proteins. Amino acids in the cellular pool come from diet and the
destruction of cellular protein.
The
recommended daily dose of protein is 1 gm per Kg. Bodyweight per day. But it
is more in pregnant women, lactating women, growing children, and adolescents.
Internal link:
https://totalphysology20.blogspot.com/2021/02/glands-we
must know
https://totalphysology20.blogspot.com/2021/03/small intestine. We must know
https://totalphysology20.blogspot.com/2021/05 /bile- we must know
https://totalphysology20.blogspot.com/2021/05 /Gall bladder- we must know
External link:
https://en.m.wikipedia.org>wiki
https://openoregon.pressbooks.pub>
https://www.healthline.com>nutrition
https://www.nestle.in .nutrients >di
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Hashtags: Glutathione # Rennin # metaproteins # endopeptidase# exopeptidase#
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