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8/17/2019 Digest Prot, Block 2.2.Maret 2015
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Protein
Digestion,Absorption,
and
Transport
8/17/2019 Digest Prot, Block 2.2.Maret 2015
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Digestion and absorption of proteins
The protein load received by the gut is derivedfrom two primary sources:
70-100 g dietary protein, and
35-00 g endogenous protein,
the latter either as secreted en!ymes and proteinsin the gut or from intestinal epithelial cell turnover
"n healthy adults, only 1- g nitrogen, e#uivalent to
$-1g protein, are lost in the feces on a daily basis
Thus, the digestion and absorption of protein isextremely efficient
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Proteins, li%e other dietarymacromolecules, are bro%en down by
hydrolysis of specific peptide bonds andhence the en!ymes involved are termed
‘peptidases’
These en!ymes can either cleave internapeptide bonds (i.e. endopeptidases) or
cleave off one amino acid at a time from
either the !""# or $#% terminal of thpolypeptide &i'e' they are exopeptidases
subclassified into carboxypeptidases , anaminopeptidases, respectively(
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The endopeptidases cleave the large polypeptides
to smaller oligopeptides, which can be acted &ponby the exopeptidases to produce the finalprod&cts of protein digestion, amino acids, di' andtripeptides, which are then absorbed by the
enterocytes)epending on the so&rce of the peptidases, theprotein digestive process can be divided intogastric, pancreatic and intestinal phases
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A. Digestion of proteins by gastric secretion
The digestion of proteins begins in the stomach, which secretegastric juice, a unique solution containing hydrochloric acid
and the proenzyme pepsinogen:
1. Hydrochloric acid: Stomach acid is too dilute (pH 2-3 to hydrolyze proteins!
howe"er, the acid functions to kill some bacteria and to
denature proteins, ma#ing them more susceptible to
subsequent hydrolysis by proteases
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"vervie of protein digestion
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Pepsin
This acid'stable endopeptidase is secreted by thesero&s cells of the stomach as an inactive *ymogen &or
proen*yme(, pepsinogen
"n general, *ymogens contain extra amino acids in theirse#uences, which prevent them from being catalyticallyactive
$ote +emoval of these amino acids permits theproper folding re&ired for an active en*yme
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Pepsinogen is activated to pepsin either by #!-, or a&tocatalyticallyby other pepsin molec&les that have already been activated
Pepsin releases peptides and a fe free amino acids from dietaryproteins
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. Digestion of proteins by pancreaticen*ymes
*n entering the small intestine, large polypeptides produced in the stomach by the action of pepsin are
further cleaved to oligopeptides and amino acidsby a gro&p of pancreatic proteases
These en*ymes, li/e pepsin, are synthesi*ed and
secreted as inactive *ymogens
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1. Release of zymogens:
The release and activation of the pancreatic zymogens is
mediated $y the secretion of cholecystokinin and secretin,
two polypeptide hormones of the digesti"e tract
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0nteropeptidase th&s &nleashes acascade of proteolytic activity, because
trypsin is the common activator of allthe pancreatic *ymogens
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#. Specificity:
'ach of these enzymes has a different specificity for the amino acids R
groups ad$acent to the susceptible peptide bond
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*!leavage of dietary proteins
by pancreatic proteases
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Prod&cts of pancreatic digestion
Thus, the products of digestion bypancreatic en!ymes are:
*ligopeptides
+mino acids
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0ndopeptidases, dipeptidases and aminopeptidasescomplete the digestion of proteins
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0n*ymes responsible for protein digestion
i id di d i id
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Absorption of amino acids di', and tri'peptides
1ree amino acids, di' and tripeptideare absorbed across the enterocytemembrane by specific carrier'mediated transport
Amino acids are transported by
specific active transporters shoingmechanisms hich are similar toones active in gl&cose transport
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These a dependant symporters are located at thebrush- border membrane
This is an indirect active process
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+t least si specific symporter systems have beenidentified as follows:
• eutral amino acid symporter for a'a' with shortor polar side- chains&.er, Thr, +la(
• eutral amino acid symporter for aromatic or
hydrophobic side chains &/he, Tyr,Tryp,et,al,2eu,"leu(• "mino acid symporter &/ro, *-/ro(• 4asic amino acid symporter &2ys, +rg, ys(•
+cidic amino acid symporter & +sp, 6lu(• - amino acid symporter & - +la, Tau(
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These transporter systems arealso present in the renal tubulesand defects in their constituent
protein structure can lead todisease& e'g' artnup disease(
/athologies can be produced inboth the %idney and intestine
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2ymporters for di', and tri'peptides
These are:/resent in thebrush- border
surface8- dependant
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Di' and tripeptides
are furtherhydroly*ed to theirconstituent aminoacids inside the
enterocyte
The final transferis9 therefore9 of
free amino acidsacross the
contraluminalplasma membrane
into the portalblood system
$a' independenttransporters are
present in thecontral&minal
s&rface, allowinga'a' facilitatedtransport to the
portal vein
lumen of
ll
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small
intestine
amino acids
amino acids
amino acids
interstitial fluid blood
Na+ aasymport
amino acid carriers
(facil. diffusion)
mucosal
cell
)
Na ATP
Na+ H+
Na+
Na+
K+
K+
* analogous to glcmovement
* absorption via symports*aa: a8 gradient-driven
*peptides: 8 gradient-driven
* in cytosol:peptides → amino acids
* aas cross contraluminalmembrane via facilitated
diffusion carriers
Na+small
peptidesH+
small
peptides
peptidases
Amino acids, peptides movement l&men 3 blood
H + peptsympo
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Digestion and absorption of proteins
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;or a n&mber of reasons, protein absorption mightbe incomplete
2ome of the proteins, because of their physical orchemical str&ct&re, are resistant to proteolytic
attac% and therefore pass through the smallintestine relatively &nmodified
;urthermore, the absorption of free amino acids
and peptides may be less than 5667, particularly ifg&t f&nction is impaired
Defects in protein digestion and absorption
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This occ&rs in a number of clinicalconditions, such as intestinalinfection or in4&ry, and when
certain ‘antin&tritional’ factors such as lectins or trypsin inhibitorproteins are present in the diet
This &nabsorbed protein or aminoacid then passes through into the
colon
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8etabolism by the colonic microflorathen occ&rs, b&t the amino acids areno longer available to the body, and
are excreted in the faeces, mainly inthe form of bacterial protein.