Physiology - GI - DIGESTION AND ABSORPTION - PROTEINS AND AMINO ACIDS

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  1. Protein is derived from ________.
    diet (50%), digestive secretions (25%, enzyme degraded by intestinal bacteria), and sloughed epithelial cells (25%, epithelial turnover)
  2. Protein digestion begins in ______ through the action of ________.

    The ________ delivered to ______ are further degraded by ________ and ________.

    Specific carriers transport _____ and ________ across _______. _______ further degrade _______ to _______, which are extruded from the cell into the blood by a variety of _________.
    • stomach
    • pepsins and HCl

    • polypeptides
    • the small intestine
    • pancreatic proteases
    • brush border enzymes

    • amino acids
    • small peptides of 2-4 amino acids
    • the brush border
    • Cytosolic peptidases
    • the small peptides
    • amino acids
    • basolateral transporters
  3. Because of _________, protein and amino acid deficiencies seldom result in malnutrition.
    multiple transport pathways
  4. Compare lumenal, mucosal and cytosolic protein digestion. Describe the function of trypsin inhibitor. Discuss the importance of enterokinase and trypsin in protein digestion.
    • lumenal: products: oligopeptide 70%; di/tri-peptides 30%, simple AAs
    • mucosal: jejunum, 4-8 AAs oligopeptides -> di/tri-peptides
    • cytosolic: mainly digest di-/tri-peptides into single AA; 90-99%

    trpsin inhibtor prevents premature conversion of trpsinogen into trpsin in acinar cells; protective.

    • enterokinase convert trpsinogen into trypsin inside duodenum and jejunum
    • Trypsin autocatalyse more trypsinogen into trypsin, and convert other proenzymes into their active forms.
  5. Identify the primary intestinal sites for amino acid and small peptide absorption. Describe and compare the brush border mechanisms responsible for absorption of amino acids and small peptides.
    • AA-> ileum
    • small peptide-> jejunum

    • small peptide:
    • pepT-1 PAT
    • H-gradient
    • L-AA
    • 2-3 AAs

    • AAs:
    • 5-7 systems overlapping affinities
    • L-AA
    • mainly Na-dependent, can be Na-independent too
    • may have facilitated diffusion and channels
  6. Discuss the role of cytosolic peptidases and the pathways for transport of amino acids across the basolateral membrane.
    cytosolic endo- or exo- peptidases convert 2-3AA peptide to simple AAs

    • mainly Na-dependent
    • bidirectional
    • can be used during starvation to get AAs from blood

    • minority Na-indepent
    • net absorption of AA into blood
  7. Discuss the causes and primary symptoms of Hartnup’s Disease and the effect of disorders of amino acid transport on nutritional state.
    lack transporter for neutral AA (phe, leu)

    • non-lifethreatening
    • neurodisorder: ataxia, psychiatric abnormality, photo sensitivity

    • non-fatal
    • multiple carrier/pathway exist
  8. Protein Function
    tissue repair, maintenance and growth
  9. Protein Digestion Types
    • - luminal
    • - mechanical (motility)
    • - chemical (enzymes)
  10. Digestive Sites
    • Mouth
    • 1) luminal; 2) mechanical (chewing)

    • Stomach, Intestine - Two Types
    • 1) mechanical (motility) - luminal
    • 2) chemical - HCl less important
    • - proteases – pancreatic acinar cells - essential
    •     - protease subtypes - activated in gut
    •          - exopeptidases - cleave single amino acid from C- terminal peptide
    •          - endopeptidases - cleave interior peptide bonds adjacent to AAs
  11. endopeptidases
    • - cleave interior peptide bonds adjacent to amino acids                  
    • - enter lumen in inactive form then converted to active form
    • - produce small peptides - 2-6 amino acids
    • - do not release individual acids
  12. Pepsin
    • - Gastric Endopeptidase
    • - secreted as proenzyme, pepsinogen by chief cells
    •     - activated by H+
    •     - pH optimum 1-3
    •     - activity terminated in small intestine
    • - limited specificity - reduces about 15% into small peptides and amino acids (digesting collagen in meat)
    • - HCl - lesser importance in direct gastric digestion
  13. Digestion in Small Intestine
    - sites
    - types in lumen
    - proteases source and types
    - Lumenal, Brush Border, Cytosolic

    • - mechanical and chemical
    •      - 50% of total digestion

    • - Acinar cell pancreatic proteases
    • - endopeptidases
    • - exopeptidases
  14. When ______ are secreted into the duodenal lumen, enterokinase, a _____ released from the _______ by ______, converts ______ to ______, which is ______, i.e. activates its own conversion from _______, as well as ________.
    • inactive proteases
    • hydrolase
    • brush border
    • bile salts

    • trypsinogen
    • trypsin
    • autocatylic
    • trypsinogen
    • convert other proenzymes
  15. Trypsin
    • - Pancreatic Endopeptidases
    • - secreted as trypsinogen (proenzyme)
    • - activated by enterokinase (enteropeptidase)
    •      - hydrolase from brush border, released by bile salts
    •           - highly concentrated @ duodenal & jejunal mucosa
    •           - 41% carbohydrate, not readily degradable by proteolytic enzyme

    •                       enterokinase
    • Trypsinogen ---------------------> trypsin

    • - autocatalytic
    • - converts other proteases
  16. Chymotrypsin and Elastase
    • - Pancreatic Endopeptidases
    • - secreted as proenzymes
    • - activated by trypsin
    • - cleve proteins at neutral amino acids
  17. Pancreatic Exopeptidases
    • - remove amino acid from C-terminal
    • - secreted as proenzymes
    • - activated by trypsin

    •                                          Trypsin
    • Procarboxypeptidase (A/B) --------------> carboxypeptidase (A/B)
  18. Final Protease (lumenal) Digestion Products
    • - oligopeptides (70%; >= 4 AAs)
    • - di- or tri-peptides (30%)
    • - simple amino acids
  19. As digestion progresses, the pancreatic enzymes ________
    Trypsin _________
    • get very rapidly inactivated
    • digests itself, shuts down proteolysis, to prevent mucosal damage
  20. Brush Border Digestion
    • - Oligopeptides
    • - enzyme preference - 4-8 amino acids
    • - jejunum – highest activity

    • - Final Products
    • - primarily di- and tri- peptides
    •     - not too much osmotic gradient
    •     - small enough for mucosal transport
    • - single amino acids
    • - limited tetrapeptides
  21. Apical Small Peptides Absorption
    - where
    - transporter: energy, specificity
    - advantage of the system
    • primarily jejunum
    • PepT-1 PAT (protein-dependent AA transporter) - primary
    • - energy - H gradient
    • - L-AAs - broad specificity (none for D-AAs)
    • - highest affinity - 2-3 amino acids
    • - tetrapeptides low affinity

    • kinetic advantage
    • - transport is "faster" than single AA
  22. Apical Amino Acids Absorption
    - where
    - what
    - specificify
    - dependence
    - mechanism
    • - ileum - major site
    • - 5-7 systems with overlapping affinities - difficult to separate
    • - L - amino acids - highest affinity
    • - Na - dependent - majority
    • - Na - independent (minority)
    • - facilitated diffusion and channels postulated, in addition to active Na-cotransport
  23. Cytosolic Digestion of small peptides
    - who
    - specificity
    - outcome
    - special outcome and consequences
    Endo or exo peptidases

    di and tri linkages

    90-99% amino acids

    • few resistant peptides pass to circulation
    • - produce anaphylaxis or hypersensitive responses
  24. Small intestine - Basolateral Transport - Amino Acids
    - specificity
    - dependence
    broad specificity - at least 5

    • Na-dependent
    • - majority
    • - bidirectional transport
    • - important during starvation
    •     - can extract AAs from blood for cell maintenance

    • Na-independent - minority
    • - net secretion - absorption into blood
  25. Protein Deficiencies
    • malnutrition - infrequent
    • - multiple carriers and peptide absorption
    • - passive - hydrophilic absorption
    • - hydrophilic

    child deficiencies -> jaw and tooth malformations
  26. Pancreatitis
    autodigestion of pancreas

    variable causes

    • acute cases
    • - biliary stones, alcohol
    • - activates cascades of other proteases

    • congenital
    • - failure of trypsin inhibitor
    •     - trypsin not inactivated

    • - Pancreatic acinar cells store proteases in inactive form in zymogen granules
    • - cytosol contains trypsin inhibitor that discourages premature protease activation
  27. Hartnup’s Disease
    • hereditary absence of neutral amino acid transporter in intestine and kidney
    • - (B or NBB transporters) - transport phenylalanine, leucine
    • - non-life-threatening
    •     - neurological disorders (ataxia), photosensitivity, psychiatric abnormalities

    • PepT-1 and basolateral transporters – normal
    •     - neutral amino acids absorbed in small peptides
  28. Cystinuria
    • Absence of transporter for basic amino acids (cysteine) in intestine and kidney
    •     - cystine in urine
    •         - causes kidney stones
Author:
akhan
ID:
315911
Card Set:
Physiology - GI - DIGESTION AND ABSORPTION - PROTEINS AND AMINO ACIDS
Updated:
2016-02-15 02:18:05
Tags:
physiology gi
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Description:
Physiology - GI - DIGESTION AND ABSORPTION - PROTEINS AND AMINO ACIDS
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