Digestion absorption of proteins, Protein digestion abnormalities, simplified notes.pdf

Digestion absorption of proteins simplified notes. with flow charts and important enzymes and reactions. This is important 5 marks question which saves time in the exam

1. Digestion and absorption proteins
(Simplified Notes)
P.SantoshKumar

2. Digestionandabsorptionproteins
Sources:
Exogenous: dietary intake of proteins –50 to 100 g/day
Endogenous: digestive enzymes and worn-out cells of digestive tract –
30 to 100 g/day
•Theproteinspresentindietaredenaturedbycookingcausesunfoldingof
proteinandpeptidebondsbecomemoreaccessiblefortheenzymeaction
thereforemoreeasilydigested.
•Proteinsdegradedbyclassofenzymescalledashydrolases(class3
enzymes)cleavespeptidebondshencecalledaspeptidases.
•Peptidasesaresecretedasinactivezymogenswhichareconvertedtotheir
activeformintheintestinallumen.Thiswouldpreventautodigestionofthe
secretoryacini.

3. Peptidases/proteases/proteolyticenzymesinclude:
1.Endopeptidases(proteases):Theyactonpeptidebondsinsidetheprotein
molecule,andreleasepeptidesfragments.Ex:Pepsin,Trypsin,
ChymotrypsinandElastase.
2.Exopeptidases:Whichactonterminalpeptidebondsofthepolypeptide
chain.Thisgroupincludes:
a.Carboxypeptidase; acts on the peptide bond at the carboxy terminal
end (C-terminal end) of the chain.
b.Aminopeptidase;acts on the peptide bond at the amino terminal end
(N-terminal end) of the chain.

4. Proteindigestioncanbedividedinto
Gastricphase
Pancreaticphase
Intestinalphases
Thereisnodigestionofproteinsinthemouth,onlymechanical
breakdownofproteinsoccurs.

5. Gastric phase of protein digestion
Small peptides and amino acidsCasein
Rennin /Chymosin
Pepsin
Calcium paracaseinate
(curdling of milk)
Proteins enters into stomach
Hormone Gastrin
Gastric juice contain
HCl, zymogens – pepsinogen and rennin (Secreted from infant stomach)
Pepsinogen (zymogen)
HCl + pepsin by autocatalysis
Proteins Proteoses and peptones, amino acids
Pepsin (active endopeptidase)
Stimulates the secretion
Stimulates secretion
•HCl – maintain pH <2; kills microorganism and denatures proteins thereby accessible
to hydrolysis by peptidases.

6. Pancreatic phase of protein digestion

7. Partially digested Proteins; peptides, amino acids enters into small intestine
Hormones Cholecystokinin (Pancreozymin) & secretin
Pancreatic juice: contain
HCO
3, endopeptidases as zymogens, – trypsinogen,
chymotrypsinogen, proelastase and ProCarboxypeptidase A and B
Stimulates the secretion
Stimulates secretion
Pancreatic phase of protein digestion

8. The optimum pH for the activity of pancreatic enzymes (pH 8) is provided by
the alkaline bile HCO
3 and pancreatic juice.
Alkaline pH
Oligo peptides, Tripeptides
and Dipeptides
Procarboxypeptidase A, B
Trypsin
Carboxypeptidase A, B
Small digested peptides by
Trypsin, chymotrypsin
Zn+
Cleaves one amino acid at time from
carboxyterminal end
Alkaline pH
Alkaline pH
Alkaline pH

9. Intestinal phase: of protein digestion
Digested products of gastric and pancreatic phase enter into intestinal phase
of digestion acted upon by
Aminopeptidase is an exopeptidase that cleaves one amino acid at a time
from amino terminals of peptides.
Dipeptidases and tripeptidases are endopeptidases.

10. Absorption of Proteins
Absorption of amino acids occurs mainly in the small intestine and released in the
blood by different transport mechanisms
•Simple Passive diffusion; L-amino acids
•An energy requiring system amino acids are actively transported by Na
+

dependent amino acid transport
•Five different carriers system of amino acids

11. Absorption of proteins by Meister cycle
γ-Glutamyl cycle (Meister cycle) – Occurs in the intestine, kidney tubules
and brain
•Amino acid reacts with glutathione in the cell membrane forming
γ-glutamyl amino acid
•Amino acid is carried across the cell membrane attached to theγglutamate
and released into the cytosol
•γ-Glutamyl and dipeptide cysteinyl-glycine moiety is reused in the
synthesis of glutathione
•Transport of one molecule of amino acid and regeneration of glutathione
requires 3 ATP molecules

12. Meister’s Cycle/Gamma Glutamyl Cycle

13. Abnormalities of protein digestion and absorption
•Acute pancreatitis: Premature activation of trypsinogen inside the
pancreas itself, will result in the autodigestion of pancreatic cells. The
result is acute pancreatitis. It is a life-threatening condition.
•Hartnup’s disease (Neutral amino aciduria) – inability of intestinal and
renal epithelium cells to absorb neutral amino acids, tryptophan absorption
is severally affected leading to symptoms of pellagra
•Allergic to certain foods proteins- in milk and fish is believed to results
from absorption of partial digested proteins.
•Partial gastrectomy, pancreatitis, Cystic fibrosis or surgical removal of
pancreas leads to defect in the pancreatic secretion and action and impairs
protein digestion and leads to loss of protein in feaces
•Protein loosing enteropathy: excessive loss of proteins through the
gastro intestinal tract.

14. References:
1.Vasudevan DM, Sreekumari S, Vaidyanathan K. Textbook of
biochemistry for medical students. JP Medical Ltd; 2013.
2.Satyanarayana U. Biochemistry. Elsevier Health Sciences; 2013 Jun 15.
3.Krishnananda Prabhu. Jeevan K Shetty. Quick Review of Biochemistry
for Undergraduates. Jaypee Brothers Medical; 2014.