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Protein and AAs: Digestion & Absorption

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Overview

  • dietary proteins need to be hydrolyzed into free AAs and small peptides before absorption

  • starts in the stomach

  • major site is the small intestine

  • goes to the liver via portal vein after being absorbed

    • capillaries --> portal vein --> liver --> other cells​

  • unabsorbed proteins are lost in feces

    • can be used by bacteria in colon​

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Digestion: Gastric Phase

  • minor role in protein digestion

  • mainly to prepare protein for small intestine

  • partial denaturation by HCl

  • partial hydrolysis by pepsin

    • secreted as pepsinogen​, activated simultaneously when pH <5

    • a portion on the N-terminal is removed --> activated

  • autocatalysis: pepsin activates other pepsinogen

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Digestion: Small Intestine

Pancreas secretes 3 enzymes

  • all are in inactive form so they don't digest the pancreas tissue and the critical proteins of the small intestine

    • pancreatic juice also has a trypsin inhibitor​

  • these 3 enzymes are sufficient in the lumen 

    • not specific for individual proteins​, but for specific aspects of protein structure

  • activated after being released into the lumen

  1. partially hydrolyzed proteins enter duodenum

  2. bicarb is released from pancreas to increase pH

  3. CCK is released and stimulate the pancreas to secrete precursors of digestives enzymes: zymogens

  4. to activate these enzymes, a small peptide portion of each is cleaved off, which exposes active site​​

  • enteropeptidase at brush border

  • highly specific for trypsinogen: between C of lysyl and N of isoleusyl

  • cuts octopeptide from the amino terminus from trypsinogen

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We now have 3 active enzymes in the lumen

  • trypsin and chymotrypsin

    • endopeptidases​

    • go after certain peptide bonds WITHIN the protein or polypeptide

  • carboxypeptidase

    • exopeptidase: one AA at a time from the C-terminal​

    • carboxypeptidase A cleaves off most except for lysine, arginine, and proline

    • carboxypeptidase B cleaves off lysine or arginine 

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Small intestinal mucosal phase at the brush border

  • result is a mixture of peptides and a few free AA

  • go to the brush border

    • peptidases bound to brush border (similar to disaccharases) ​

    • free amino acids must be carried into the cell across the membrane

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Absorption

dipeptides and tripeptides can be transported into absorptive cells

  • transporter: PEPT1

    • does not compete with AA transporters​

  • more efficient than a free AA mixture

  • particularly digested proteins to patients with pancreatic insufficiency

  • can be absorbed along the entire small intestine

    • mainly upper​

  • individual amino acids are water soluble

    • need carrier for lipid membrane​

  • same as monosaccharides

    • active transport​

      • carrier + energy​

      • uphill

    • facilitated diffusion

      • carrier + no energy​

      • downhill

  • many AA carriers - not specific to each AA, but for groups of them (mostly sodium dependant)

    • acidic - have 2 COO​

    • basic - have 2 NH3

    • large neutral

  • different AAs compete for the same transporter

    • concentration difference --> imbalanced absoprtion​

    • BCAAs, neutral, and essential AAs absorbed faster

  • not all AAs leave the mucosal cell

    • many are used by the cells for protein synthesis​

    • some are used as energy for the mucosal cells

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©2023 by Syracuse University Dr.Margaret Voss

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