Digestion & absorption of protein

1. PROTEIN
METABOLISM
SNEHITA PRASAD
LECTURER
Department of biochemistry
Narsinhbhai patel dental college &
hospital, SPU

2. Digestion of protein
 The dietary proteins are denatured on cooking and therefore more
easily to digested by a digestive enzymes.
 All these enzymes are hydrolases in nature.
 Proteolytic enzymes are secreted as inactive zymogens which are
converted to their active form in the intestinal lumen.
 Dietary proteins are the primary sources of the nitrogen that is
metabolized by the body.
 Adult man requires 70 to 100 gm protein per day.

5. The proteolytic enzymes include:
They act on peptide
bonds inside the
protein molecule, so
that the protein
becomes successively
smaller and smaller
units.
This group includes
pepsin, trypsin,
chymotrypsin, and
elastase
This group acts at the
peptide bond only at the
end region of the chain.
This includes
carboxypeptidase acting
on the peptide only at
the carboxyl terminal
end on the chain and
aminopeptidase, which
acts on the peptide bond
only at the amino
terminal end of the
chain.

6. DIGESTION IN STOMACH
• In the stomach, hydrochloric acid is secreted. It makes the pH optimum for
the action of pepsin and also activates pepsin. The acid also denatures the
proteins.
• Thus in the stomach, HCl alone will not able to digest proteins; it needs
enzymes.

8. RENNIN
• Rennin otherwise called chymosin, is active in infants and is involved in the curdling
milk.
• It is absent in adults.
• Milk protein, casein is converted to paracasein by the action of rennin.
• The denatured protein is easily digested further by pepsin.

9. PEPSIN
• It is secreted by the chief cells of stomach as inactive pepsinogen.
• The conversion of pepsinogen to pepsin brought about by the
hydrochloric acid.
• The optimum pH for activity of pepsin is around 2.
• Pepsin is an endopeptidase, by the action of pepsin, proteins are
broken into proteoses.

10. DIGESTION IN PANCREAS
• The optimum pH for the activity of pancreatic enzyme (pH 8) is provided by the alkaline
bile and pancreatic juice.
• The secretion of pancreatic juice is stimulated the peptide hormones, cholecystokinin
and pancreozymin.
• Pancreatic juice contains the important endopeptidases, namely
1) Trypsin
2) Chymotrypsin
3) Elastase
4) Carboxypeptidase.

11. 1) TRYPSIN
• Trypsinogen is activated by enterokinase present on the intestinal microvillus
membranes. Once activated, the trypsin activates other enzyme molecules.
• Trypsin catalyzes hydrolysis of the bonds formed by carboxyl groups of Arg and Lys.
• Acute pancreatitis: Premature activation of trypsinogen inside the pancreas itself
will result in the auto digestion of pancreatic cells. The result is acute pancreatitis.
• It is a life-threatening condition.

12. 2) Chymotrypsin:
Trypsin will act on chymotrypsinogen, so that the active site is formed. Thus, produces the
catalytic site. selective proteolysis
3) Carboxypeptidases:
• Trypsin and chymotrypsin degrade the proteins into small peptides; these are further
hydrolyzed into dipeptides and tripeptides by carboxypeptidases present in the
pancreatic juice.
• They are metallo-enzymes requiring zinc.

13. DIGESTION IN INTESTINE
• Complete digestion of the small peptides to the level of amino acids is brought about by enzymes
present in intestinal juice (succus entericus). Enzymes secreted by intestine is aminopeptidases and
dipeptidases.
• The hydrolysis of most proteins is thus completed to their constituent amino acids which
are then ready for absorption into the blood.
Aminopeptidases
• exopeptidase, hydrolyze
peptide bonds next to N-
terminal amino acids of the
short peptides
Dipeptidases
• complete digestion of
dipeptides to free amino acids.
• Then finally convert all
ingested protein into free
amino acids. It require cobalt
or manganese ions for their
activity.

14. ABSORPTION OF AMINO ACIDS
• The absorption of amino acids occurs mainly in the small intestine.
• The absorption of most amino acids involves an active transport mechanism It is an
energy requiring process in the intestinal mucosal cells.
• Many transporters have Na+ dependent mechanisms, coupled with Na+ K+ pump,
similar to those described for glucose absorption
• Several Na+ independent transport proteins are found in the brush-border
membrane that are not specific for each amino acid but rather for the groups of
structurally similar amino acids
• All are specific for only L-amino acid. D-amino acids are transported by passive
diffusion.
• • Thus, amino acids, released by digestion, pass from the gut through hepatic portal
vein to the liver.

16. • THESE ARE FIVE DIFFERENT CARRIERS FOR
DIFFERENT ACIDS
• There are 5 different carriers for amino acids:
• 1. Neutral amino acids (Alanine, Valine, Leucine, Methionine, Phenylalanine, Tyrosine, Isoleucine)
• 2. Basic amino acids (Lys, Arg) and Cysteine
• 3. Imino acids and Glycine
• 4. Acidic amino acids (Asp, Glu)
• 5. Beta amino acids (beta alanine).
• Alton Meister proposed that glutathione (γ-glutamyl cysteinylglycine) participates in absorption of
amino acids in intestine, kidneys and brain and the cycle is called gammaglutamyl cycle or Meister
cycle.