Digestion and absorption of proteins

Digestion and absorption of proteins


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 juice.
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 pepsin?
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 enzymes.
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 called
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
carriers 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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