Protein digestion and absorbtion

1. Proteins digestion and absorption
• Most of the nitrogen in the diet is consumed in the form of protein,
typically amounting to 70–100 g/day in the American diet
• Proteins are generally too large to be absorbed by the intestine. They
must, therefore, be hydrolyzed to yield di- and tripeptides as well as
individual amino acids, which can be absorbed.
• Proteolytic enzymes responsible for degrading proteins are produced
by three different organs: the stomach, the pancreas, and the small
intestine

2. Proteins digestion and absorption
• The digestion and absorption of proteins is very efficient in healthy humans,
hence very little protein is lost through feces
• Proteins which we take in our diet are either from animal source or vegetable
source.
• Principal animal sources: Milk and dairy products, meat, fish, liver, eggs.
• Principal vegetable sources: Cereals, pulses, peas, beans and nuts.
• Some food materials contain enzyme inhibitors or certain enzymes which can
destroy certain vitamins.
• Egg white and soybeans contain trypsin inhibitors.
• Raw clams contain thiaminase which destroys thiamine (Vit B1),
• Such inhibitors or enzymes are also destroyed on cooking. Cooking also
destroys harmful bacteria and other pathogenic microorganisms existing in
the raw food materials.

3. • There are no proteolytic enzymes in mouth. After mastication and
chewing, food reaches stomach where it meets the gastric juice.
• —a unique solution containing hydrochloric acid and the proenzyme,
pepsinogen
DIGESTION IN STOMACH
1. Hydrochloric acid: Stomach acid (pH 2–3) to hydrolyze proteins.
2. The acid, secreted by the parietal cells, functions instead to kill some
bacteria and to denature proteins, thus making them more susceptible to
subsequent hydrolysis by proteases
2. Pepsin: It is a potent proteolytic enzyme and is present in gastric juices of
different species including the mammals.
• This acid-stable endopeptidase is secreted by the chief cells of the
stomach as an inactive zymogen (or proenzyme), pepsinogen.
• Pepsinogen, having a mol. wt. of 42,500 approx.

4. • In general, zymogens contain extra amino acids in their sequences that
prevent them from being catalytically active. [Note: Removal of these
amino acids permits the proper folding required for an active enzyme.]
• Pepsinogen is activated to pepsin (mol. wt = 34,500), either by HCl, or
autocatalytically by other pepsin molecules that have already been
activated.
• HCl maintains the gastric pH at about 1 to 2 and ensures maximum
pepsin activity. Optimum pH for pepsin is 1.6 to 2.5 and pepsin gets
denatured if the pH is greater than 5.
• Pepsin is a proteinase, a non-specific endopeptidase, and it hydrolyses
peptide bonds well inside the protein molecule and produces
proteoses and peptones, releases peptides and a few free amino acids
from dietary proteins

5. • Pepsin cannot act on proteins like keratins, Silkfibroins, mucoproteins,
mucoids and protamines (arginine-rich, nuclear proteins).
• Action on Milk: Pepsin can act on milk. It hydrolyses the soluble
phosphoprotein casein of milk to produce paracasein and a proteose, the
latter is the whey protein.

6. • Other enzymes present in Gastric Juice Are
• 3. Rennin: it is absent in adult humans, and many non-ruminants. Certain
amount of rennin activity is seen in babies, in infancy.
• In the calf, it is secreted in zymogen form as prorennin, which is activated
in the stomach to form active rennin and in the process of activation an
inactive peptide is split off.
• Optimum pH for activity is 4.0 and specificity of action is very similar to
pepsin, in that it hydrolyses peptide bonds connected with aromatic
amino acids.
• 4. Gastriscin: The enzyme is secreted in the gastric juice of humans as
inactive zymogen form, which is activated in presence of HCl. Optimum pH
is 3 to 4. It acts as a Proteinase and requires an acidic medium for its
activity.
• 5. Gelatinase: Gelatin is hydrolysed by the enzyme Gelatinase present in
gastric juice to form polypeptides. It acts in an acidic medium.

7. Digestion of proteins by pancreatic enzymes
• On entering the small intestine, large polypeptides produced in the stomach by the
action of pepsin are further cleaved to oligopeptides and amino acids by a group of
pancreatic proteases.
• A number of proteolytic enzymes are present in Pancreatic juice to act on proteins and
partly digested products. Chief enzymes are:
• Trypsin
• Chymotrypsin
• Carboxy peptidases
• Elastases
• Collagenases.
These enzymes are also secreted as zymogen
The release and activation of the pancreatic zymogens is mediated by the secretion of
cholecystokinin and secretin, two polypeptide hormones of the digestive tract
CCK acts on the exocrine cells of the pancreas (causing them to release digestive
enzymes).

8. • Enteropeptidase (formerly called
enterokinase)—an enzyme synthesized by and
present on the luminal surface of intestinal
mucosal cells of the brush border
membrane—converts the pancreatic
trypsinogen to trypsin by removal of a
hexapeptide from the N-terminus of
trypsinogen.
• Trypsin subsequently converts other
trypsinogen molecules to trypsin by cleaving a
limited number of specific peptide bonds in
the zymogen.
• Trypsin, in turn, activates other trypsinogen
molecules (autocatalysis) Further, trypsin is the
common activator of all other pancreatic
zymogens to produce the active proteases,
namely chymotrypsin, elastase and
carboxypeptidases (A and B)

9. • Enteropeptidase thus unleashes a cascade of proteolytic activity, because
trypsin is the common activator of all the pancreatic zymogens
• Specificity and action of pancreatic protease: Trypsin, chymotrypsin and
elastase are endopeptidases active at neutral pH.
• Gastric HCI is neutralized by pancreatic HCO3 in the intestine and this
creates favorable pH for the action of proteases
• secretin, in response to the low pH of the chyme entering the intestine.
Secretin causes the pancreas and the liver to release a solution rich in
bicarbonate that helps neutralize the pH of the intestinal contents, bringing
them to the appropriate pH for digestive activity by pancreatic enzymes

10. • Specificity: Each of these enzymes has a different specificity for the amino
acid R-groups adjacent to the susceptible peptide bond.
• Trypsin
• For example, trypsin cleaves only when the carbonyl group of the peptide
bond is contributed by arginine or lysine.
• Chymotrypsin
• During its activation two inactive peptides are liberated
• It hydrolyses peptide bonds which are connected with carbonyl groups of
aromatic amino acids. like Tryp, Tyr, and Phe.
• To some extent, it can also attack peptide bonds connected with Met, His,
Leu and asparagine residues.

11. • Action of carboxypeptidases: The pancreatic carboxypeptidases (A and
B) are metalloenzymes that are dependent on Zn2+ for their catalytic
activity, hence they are sometimes called Zn-proteases.
• They also possess certain degree of substrate specificity in their action.
For example, carboxypeptidase A is an exopeptidase cannot act on
peptide bonds well inside the protein molecule. The enzyme hydrolyses
the terminal peptide bond
• Carboxypeptidase B is also an “exopeptidase”. Also hydrolyses terminal
peptide bonds, which are connected with “basic” amino acids e.g., Arg,
Lysine
• Similarities of both carboxy peptidases A and B:
• • Both are exopeptidases.
• • Optimum pH = 7.5 for both.

12. Elastase and Collagenase
• Elastase: A serine protease, secreted as inactive zymogen
proelastase, activated by trypsin to active elastase.
• The enzyme has maximum activity on peptide bonds connected to
carbonyl groups of neutral aliphatic amino acid
• Collagenase: An enzyme which can act on proteins present in
collagen.

13. Absorption of amino acids and dipeptides
• Mechanism of Absorption L-Amino Acids
• Ion gradient hypothesis: L-amino acids are absorbed from small intestine by sodium
(Na+) dependant, carrier mediated process.
• This transport is energy dependant and energy is provided by ATP (similar to
absorption of glucose and galactose).
• • L-amino acids and Na+ combine with a common “carrier protein” molecule present
on the outer or mucosal surface of the microvillous membrane to form a “Carrier-
a.a.-Na+” complex.
• The complex passes to the inner or cytoplasmic surface of the same membrane.
There it dissociates to liberate free a.a and Na+.
• • Na+ is actively carried out through the cell membrane by a “Sodium pump”
mechanism, with the help of transport ATP-ase, so that intracellular Na+
concentration is always maintained low.

14. • • Carrier-protein molecule comes back to the brush border again.
• • The amino acid ultimately passes out through the membrane of the
cell by diffusion down an outward concentration gradient of a.a and
taken by portal blood to liver.
• Note: Different classes of L-amino acids viz., diaminoacids, small
neutral a.a, imino acids, and large neutral a.a are believed to be
absorbed by different “carrier” protein molecules present in the
microvillus membrane of intestinal cells

15. • Once the amino acids are in the blood, they are transported to the liver.
As with other macronutrients, the liver is the checkpoint for amino acid
distribution and any further breakdown of amino acids, which is very
minimal.
• Recall that amino acids contain nitrogen, so further catabolism of
amino acids releases nitrogen-containing ammonia. Because ammonia
is toxic, the liver transforms it into urea, which is then transported to
the kidney and excreted in the urine

16. • Urea is a molecule that contains two nitrogens and is highly soluble in
water. This makes it a good choice for transporting excess nitrogen out
of the body.
• Very little protein makes it to the large intestine if you are not eating
excessive amounts.
• If you have smelly flatulence, this may be a sign you are eating too much
protein because the excess is making it to the colon where you gut
microbes are digesting it and producing smelly gas.