Digest Prot, Block 2.2.Maret 2015

8/17/2019 Digest Prot, Block 2.2.Maret 2015

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Protein

Digestion,Absorption,

and

Transport



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



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(



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



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



"vervie of protein digestion



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



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



. 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



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



0nteropeptidase th&s &nleashes acascade of proteolytic activity, because

trypsin is the common activator of allthe pancreatic *ymogens



#. Specificity:

'ach of these enzymes has a different specificity for the amino acids R

groups ad$acent to the susceptible peptide bond



*!leavage of dietary proteins

by pancreatic proteases



Prod&cts of pancreatic digestion

Thus, the products of digestion bypancreatic en!ymes are:

*ligopeptides

+mino acids



0ndopeptidases, dipeptidases and aminopeptidasescomplete the digestion of proteins



0n*ymes responsible for protein digestion

i id di d i id



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



These a dependant symporters are located at thebrush- border membrane

This is an indirect active process



+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(



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



2ymporters for di', and tri'peptides

These are:/resent in thebrush- border

surface8- dependant



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



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



Digestion and absorption of proteins



;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



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



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.

Digest Prot, Block 2.2.Maret 2015

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