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The proteins subjected to digestion and absorption are obtained from two sources.
1.Exogenous2.Endogenous
The Fate of Dietary ProteinThe intake of dietary protein is in the range of
50-100g/day.Digestion and absorption .Maintenance of body protein stores.Net protein synthesis. Synthesis of non-protein compoundsOxidative deamination
PROTEINS in the BODYAmino Acid Pool – amino acids that are
available throughout the body (tissues and fluids) for use when needed.
Protein Turnover – of the ~ 300 grams of protein synthesized by the body each day, 200 grams are made from recycled amino acids.
Protein DigestionWhole proteins are not absorbed.
Too large to pass through cell membranes intact.
Digestive enzymes.HydrolasesBreak peptide bonds
Secreted as inactive pre-enzymes.Prevents self-digestion.
H3N+ C
HC
R
O
NH
CH
CO
RNH
CH
C
R
O
O–
Protein DigestionInitiated in stomach
HCl from parietal cellsStomach pH 1.6 to 3.2Denatures 40, 30, and 20 structures
Pepsinogen from chief cells
Cleaves only when carbonyl group of the peptide bond is contributed by Aromatic amino acids.
Protein leaves stomach as mix of insoluble protein, soluble protein, peptides and amino acids
Pepsinogen HCl Pepsin
Protein Digestion – Small IntestinePancreatic enzymes secreted
TrypsinogenChymotrypsinogenProcarboxypeptidaseProelastaseCollagenase
Zymogens
The release of pancreatic zymogens is mediated by the secreation of Cholecystokinin and secretin,two polypeptide hormones of digestive tract.
Digestion inSmall Intestine
Zymogens must be converted to active formTrypsinogen Trypsin
EndopeptidaseCleaves on carbonyl side of Lys & Arg
Chymotrypsinogen ChymotrypsinEndopeptidase
Cleaves carboxy terminal Phe, Tyr and TrpProcarboxypeptidase Carboxypeptidase
ExopeptidaseRemoves carboxy terminal residues
Enteropeptidase/Trypsin
Trypsin
Trypsin
Trypsin InhibitorsSmall proteins or peptidesPresent in plants, organs, and fluids
Soybeans, peas, beans, wheat Pancreas, colostrum
Block digestion of specific proteinsInactivated by heat
Protein DigestionProteins are broken down to
TripeptidesDipeptidesFree amino acids
Peptide Absorption
Form in which the majority of protein is absorbed
More rapid than absorption of free amino acids
Active transportEnergy required
Metabolized into free amino acids in enterocyte
Only free amino acids absorbed into blood
Free Amino Acid AbsorptionFree amino acids
Carrier systems Neutral AA Basic AA Acidic AA Imino acids
Entrance of some AA is via active transport Requires energy
Na+ Na+
Protein DigestionSmall intestine (brush border)
Aminopeptidases Cleave at N-terminal AA
Dipeptidases Cleave dipeptides into Aas.
(Enterokinase or enteropeptidase) Trypsinogen trypsin Trypsin then activates all the other enzymes
In the Enterocytes…First cells that can use
the amino acids Transport into portal
bloodProtein synthesis
Digestive enzymesStructure and
growthEnergy
Groff & Gropper, 2000
*Whole proteins are nutritionally insignificant...
Basolateral MembraneTransport of
free amino acids only*Peptides are
hydrolyzed within the enterocyte
Transport mainly by diffusion and Na-independent carriers
Absorption of Intact ProteinsNewborns
First 24 hours after birthImmunoglobulins
Passive immunityAdults
Paracellular routesTight junctions between cells
Intracellular routesEndocytosisPinocytosis
Of little nutritional significance...Affects health (allergies and passive immunity)
Abnormalities in the protein digestion and amino acid absorption.Defect in the pancreatic secreation.
Cystic fibrosis,incomplete digestion of fat and protein,results in abnormal appearance of lipids (steatorrhea) and proteins in feces.
Defective carrier system
HARTNUP’S DISEASEInability of itestinal and epithelial cells
to absorb neutral amino acids.Tryptophan absorption is severely effected resulting in pellagra.
CYSTINUREA.