Digestion and absorption of proteins 02:14 Digestion of protein in mouth 02:25 Digestion of protein in the stomach 02:32 Gastrin 02:45 Gastric juice contains HCl …
Hello everyone, today we are going to
study about digestion and absorption of
proteins. So, you know the dietary proteins,
they are primary source of nutrition,
they are building blocks of our body and
they form structural and functional
aspect of our body. So on an average per
day, we consume 70 to 100 grams of proteins
per day, this may vary depending upon our
dietary habits. So dietary proteins are
mainly they are polypeptides or peptides
and these must be digested to their
respective amino acids and these
amino acids, we know that there are 20
naturally occurring amino acids and
these amino acids could be essential
amino acid that means these amino acids
which are required by the body for the
synthesis of enzymes hormones or any
structure and functional aspect of our
body and our body could not synthesize
these amino acids, so we in our diet
whatever protein we are taking that must
contain essential amino acids because
some amino acids can be synthesized by
the body, we call non-essential amino
acids and these dietary proteins,
actually cooking helps for the digestion
because cooking makes denaturation of
the protein, so that whatever proteolytic
enzymes released by the stomach or
pancreas or intestine can act on
internal peptide bond, so they are called
endopeptidase. So cooking helps for
digestion of proteins. So digestion of
proteins mainly take place in the stomach
and small intestine, so by the juice
released from the stomach, that is called
gastric juice, juice released from the
pancreas, that is called pancreatic juice
and also juice released from the small
the intestine is called succus
enericus, intestinal juice. So these 3 juice
gastric juice, pancreatic juice and small
intestinal juice contains various
enzymes, which help in digestion of
proteins. So now we will see the exact
steps. First digestion of protein in the mouth.
So there is no digestion of protein in
mouth, because there are no proteolytic
enzymes released from the salivary glands.
So, that means digestion of protein
begins in the stomach. So we will move on
to digestion of proteins in the stomach. So
in the stomach as soon as proteins, the
dietary proteins reach the stomach
stimulate gastric mucosal cells to
release a hormone called gastrin. So this
gastrin stimulates gastric mucosal
cells to release what is called gastric
So this gastric juice contains
hydrochloric acid and an enzyme which is
released as zymogen or proenzyme called
pepsinogen and alkaline, little bit of
alkaline mucus and especially in infants
there is one more enzyme which is
released from the gastric mucosal cell
is called rennin it is not renin,
it is rennin, it is only in
infant not in adults. So the gastric
juice contains hydrochloric acid, a
proenzyme pepsinogen and a little bit of
alkaline mucus and rennin. So what is the
role of this hydrochloric acid even
though it is not an enzyme but
definitely, it helps in digestion of
proteins because so we know that acids,
strong acids, alkalis are denaturing
agents. So this hydrochloric acid
actually helps in denaturation of
proteins, so that endopeptidase can
act easily and also they kill various
microorganisms, unwanted microorganisms
and also they provide acidic environment
for the action of enzyme pepsin, acidic
medium so hydrochloric acid is very very
important. Now coming to, and you should
know this hydrochloric acid
is released from parietal cells of
gastric mucosa.
Similarly, pepsinogen is released from
chief cells of gastric mucosal cells,
which is an inactive enzyme or
proenzyme or zymogen, so this pepsinogen is converted to active enzyme pepsin
initially by the acidic medium or
hydrogen ion, once pepsinogen is
activated to pepsin, so this pepsin can
auto catalyze conversion of pepsinogen
to pepsin. So this is called autocatalysis. Remember hydrochloric acid
provides acidic mediums, so pH will be
around 1.5 to 2.5. so these hydrogen ions
converts inactive proenzyme pepsinogen
to active pepsin.
So once pepsin is activated, it can
converts its conversion of pepsinogen to
pepsin, so this process is called autocatalysis. What is the role of this
So this pepsin it converts polypeptide
to peptide, because it is an
endopeptidase, so that means it can
attack internal peptide bonds whose
carboxyl groups are aromatic amino acids,
like phenylalanine, tyrosine, tryptophan.
So wherever there is a peptide bond, you
know peptide bond is CO-NH, so this
carboxyl group is CO is from one amino
acid and this NH is from the other
amino acid there is a peptide bond.
So wherever there is carboxyl group
provided by phenylalanine, tyrosine and
tryptophan, especially aromatic amino
acid, in the internal portion of the
polypeptide, so it can cleave randomly so
that it can make smaller peptides. So
digestion begins in the stomach by
pepsin even though hydrochloric acid
denatures, it helps actually for the
action of pepsin. So what is the role of
this alkaline mucus, so this alkaline
mucus is very important because it coats
gastric mucosa,
otherwise this pepsin can attack or
hydrolyzed proteins present in the
gastric mucosal cells, so it prevents
autocatalysis of proteins from the
gastric mucosal cells. So it actually
coats the gastric mucosal cells, so that
pepsin will not attack or hydrolyze
proteins present in the gastric mucosal
cells and in infant there is an enzyme
called rennin it is also called chymosin or also called
rennet. So you know the infant newborn
baby predominant food is mothers milk
and milk contains a protein called
casein, so milk protein is casein, so by
the action of this rennin this casein
will be converted to paracasein and the
calcium present in infant diet combines
with this paracasein and it will make
calcium paracaseinate. So what is the
purpose of this? This calcium paracaseinate
is little bit solid when compared to
milk, it's nothing but a solid curd.
So curdling of the milk in infant is mainly
by rennin. Why this curdling takes place?
So milk is a liquid, so it prevents early
passage of the milk, so it delays gastric
emptying so that milk stays in the
stomach was sometime. So rennin,
especially in infants helps in curdling
of the milk so that it delays passage of
the milk so that proteins are absorbed
easily. So this is digestion of proteins
in stomach. Now we move on to digestion
of proteins in small intestine. So we
know that the acidic chyme enters
duodenum stimulates duodenal mucosal
cells to release two hormone one is
secretin another one is CCK or
cholecystokinin. So this secretin actually
stimulates pancreas to release
bicarbonate rich juice to the intestine,
so that pH will bring back to alkaline
or towards neutral
and the cholecystokinin stimulates
pancreas to release pancreatic enzymes,
proteolytic enzymes. So what are the
proteolytic enzymes or proteases
released from the pancreas? So number one
is they are released as inactive enzymes
or proenzymes or zymogen.t The first and
foremost is trypsinogen, so these are
pancreatic enzymes for the digestion of
proteins. So they are endopeptidases, they
can attack internal peptide bond, another
important enzyme is chymotrypsinogen and
the other enzyme is proelastase, so
these three enzymes are endopeptidase
and they are inactive or proenzymes or
that means they can attack or hydrolyze
internal peptide bonds. One more enzyme
released from the pancreas .which is
actually an exopeptidase, it is called
procarboxypeptidase. So how these
enzymes are activated? As I said earlier
so these enzymes are released as their
inactive form. So how these enzymes
are activated? That is activation of
pancreatic enzymes, for their activation
from the intestinal juice, there is a
protease which is released from the
intestinal brush border membrane, the
name of that enzyme is called enterokinase
it is also known as enteropeptidase. Remember this is not released
from the pancreas, which is released from
the intestinal juice or intestinal brush
border membrane cells. So this enterokinase initially converts this inactive
trypsinogen to active enzyme called
trypsin, so like pepsinogen or pepsin
once this trypsin is converted by
the action of enterokinase, it can convert
or it can catalyze
conversion of trypsinogen back to
trypsin. So this is again a very good
example for autocatalysis.
Now once trypsin is formed it can make all
other inactive enzymes to their active
So this trypsin acts on chymotrypsinogen
to active enzyme chymotrypsin. So same
trypsin acts on proelastase and proelastase
will be converted to active
enzyme called
elastase and the same trypsin acts on
procarboxypeptidase to form active
enzyme carboxypeptidase. Remember in the small intestine initially the proenzyme
trypsinogen is converted to active
trypsin by an intestinal enzyme
enterokinase. Once trypsin is activated it can convert autocatalysis of trypsinogen to
trypsin, not only autocatalysis, trypsin
helps in conversion of chymotrypsinogen,
proelastase, procarboxypeptidase to
chymotrypsin, elastase, carboxypeptidase.
So now what are the action, specific
action of these enzymes? So trypsinogen,
it is an endopeptidase, it can attack
whose carboxyl groups are arginine or
lysine, so in the internal peptide bond
if the carboxylic group of that
particular peptide bond is made of
either arginine or lysine
so this trypsin hydrolyzes that
particular peptide bond. Similarly
chymotrypsin is also endopeptidase it
can hydrolyze peptide bond whose
carboxyl group, especially internal
peptide bond whose carboxyl group is
made up of aromatic amino acids like
pepsin, phenylalanine, tyrosine, tryptophan
and also valine and leucine. This is the specificity of chymotrypsin. Elastase it
can hydrolyze carboxyl group made up of,
the peptide bonds whose carboxyl group
is made up of alanine, glycine, and serine
and also other small nonpolar amino
acids, all this enzyme has got their own
specificity, they will not cleave random
internal peptide bond, they have a
specificity. If the carboxyl group of
internal peptide bond is arginine and
lysine then trypsin can attack that valine
and leucine and aromatic amino acids
chymotrypsin can hydrolyze that internal
peptide bond. So internal peptide bond
carboxyl group if it is, alanine glycine,
serine or small nonpolar amino acids then
elastase can easily break that
particular peptide bond and coming to
the exopeptidase, carboxypeptidase
actually, there are two carboxypeptidase
that is carboxypeptidase A and
carboxypeptidase B. Carboxypeptidase
since it is an exopeptidase, it will
cleave that C-terminal amino acid, so
carboxypeptidase A removes if the last
amino acid or c-terminal amino acid is
hydrophobic amino acids. Carboxypeptidase B
hydrolyzes or removes c-terminal
amino acid if they are basic amino acids.
That is activation of pancreatic enzymes
and their group specificity. Now we will
see digestion of protein by intestinal
enzymes, so intestinal juice it is also
called succus entericus or
intestinal juice, small intestinal juice.
It is released from the intestinal
mucosal cells, membrane cells or villi.
So this juice contains some proteolytic
enzymes and the first is called
aminopeptidase which is an again exopeptidase and another enzyme is
dipeptidase so this aminopeptidase as
I said it is and exopeptidase
similar to carboxypeptidase. The only
difference carboxypeptidase removes
amino acid
from the c-terminal and whereas aminopeptidase removes amino acid from the
n-terminal and again they are specific
for leucine, we call leucine amino
peptidase, for proline. we call proline
aminopeptidase. There are different
aminopeptidase which is present in
the succus enetricus or intestinal
juice. So they remove amino acid from the
n-terminal or amino-terminal end, whereas
dipeptidase as the name suggests there
are many many dipeptidases present in
our intestinal juice, so they convert
dipeptide to amino acids. So finally
whatever proteins are present in our
diet they are converted to amino
acids, that is the goal or purpose of
digestion of protein. So digestion of
protein begins in the stomach then in
the intestine by gastric juice,
pancreatic juice and intestinal juice. Now
we converted all the protein to their
respective amino acids. Now we will move
on to see the absorption of amino acids.
So now we have intestinal lumen, so this
is the small intestinal lumen, mucosal
cell, intestinal mucosal cell and this
is the circulation, blood. Now we have
amino acids in the intestinal lumen by
the digestion of proteolytic enzymes
endopeptidase and exopeptidases and dipeptidases, so now we have only amino
acids. How these amino acids are absorbed to the intestinal mucosal cell, again for
their absorption we require a
transporter. So they are called carriers
or transporters. Remember each amino
acid does not have separate transporter
rather the group the structurally or
functionally similar amino acids they
have their specific
transporters for example acidic amino
acids they have their own transporters,
basic amino acids they have their own
specific transporters or carriers even
neutral amino acids
they have their own transporters. So
these amino acid absorptions to the
intestinal mucosal cell is dependent on
the energy not directly, indirectly. So
whenever there is energy required for
the transport, we call it as active
transport and this energy is not derived
directly, this energy is derived by
hydrolysis of ATP to ADP and inorganic
phosphate in the basolateral surface of
the mucosal cells. So since the energy
which is released in the hydrolysis of this
particular reaction helps in the
absorption of amino acid even though it
is not directly involved, so this is
called secondary active transport and it
also requires sodium. so this is sodium
dependent secondary active transport.
Amino acids are absorbed along with the
sodium, so this is a co-transport.
Remember individual amino acids does not
have their separate transporter, group of
or structurally or functionally similar
amino acids they have their own specific
carriers are transporters. This transport
is called sodium-dependent secondary
active transport. Once amino acids
are absorbed they will be transported to
circulation by just facilitated
transportation that means they require
carrier but energy is not required, so
this is called facilitated
transportation. So this sodium which is
absorbed along with amino acid will
reach portal circulation, so in order to
maintain electrical neutrality potassium
will be coming inside the intestinal
mucosal cell in exchange with sodium.
This is absorption of amino acids. So the
absorption is almost similar to glucose
absorption, glucose absorption also
requires sodium-dependent and it is
secondary active transport, here one
point you need to remember amino acid
absorption requires transporters are
and there are many transporters which
are present in our intestinal mucous
membrane cells brush border cells and
they are groups specific. So this is with
respect to digestion and absorption of
proteins. Thanks for watching.

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