Section 10 Nutrients & Their Functions

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Section 10Nutrients Their Functions

• 1. Acquiring nutrients
• Digestion absorption of
• carbohydrates proteins

1/3/06
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Section 10Acquiring Nutrients
Overview nutrients digestion, absorption
glucose, amino acids, fats distribution precu
rsors, available fuels metabolism macromolecules
, energy, end products
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Digestion general features

• general function convert nutrients into
  absorbable form
• larger molecules (macro) ? smaller molecules
• chemical reaction hydrolysis
• H2O RX YR' ? RXOH HYR' where X C, P
  Y O, N
• reaction catalyzed by hydrolases
• major nutrients hydrolase type bond
  cleavedpoly, oligosaccharides glycosidase glycosi
  dic
• proteins, peptides protease, peptidase peptidefat
  s (triglycerides) esterase ester
• minor nutrients
• phospholipids, nucleic acids phospholipase,
  nuclease phosphoester
• esters (e.g., cholesterol, retinol) esterases este
  r

H2O
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Activation of hydrolases

• noncovalentactivator binds reversibly to enzyme
• examples enzyme activator a-amylase Cl
• lipase colipase
• covalentactivator catalyzes hydrolysis of one or
  more peptide bonds of precursor
• activator precursor ?
  active enzyme (proenzyme or zymogen)
• HClexamples pepsinogen ?
  pepsin
• trypsin
• proelastase ? elastase

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Carbohydrates digestion (poly? oligo)

• only monosaccharides are absorbable
• Stage I polysaccharides ? oligosaccharides
• enzyme a-amylase (specificity internal a1,4
  glucosidic bonds)
• sources salivary glands (minor) pancreas
  (major)
• substrates
• amylose (only a1,4 links) products maltose,
  maltotriose

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(glc3)
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Carbohydrates digestion (oligo? mono)

• Stage II oligosaccharides ? monosaccharides
• source of enzymes mucosal cell surface
  (brush border)
• a-limit dextrins ? glc3 glc isomaltase
  maltose (a-dextrinase) a1,6 glucosidic bonds
• maltose ? 2 glc maltase
• glc3 ? 3 glc "
• sucrose ? glc fructose sucrase
• lactose ? glc gal lactase (inducible)
• a-limit dextrins (glc)4-9 with one a1,6
  linkage
• glc glucose gal galactose

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Carbohydrates absorption from lumen

• frc mannose facilitated diffusion
  (carrier-mediated)
• glc gal
• transported againstconcentration gradient
• cotransport (Na gradient-driven symport)
• aka secondaryactive transport
• pentoses, othersabsorbed via simple diffusion

Na glc symportprotein
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glc-Na symport animation

• for snapshots of this animation, see the next
  slide
• stop animation by clicking browsers Stop button
• resume animation by going to another slide, then
  return-ing to this one

glc-Na symportprotein
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glc-Na symportmechanism
glc-Na symportprotein

• outer gate open, sites unoccupied
• sites occupied, outer gate closing
• inner gate opening
• Na, glc dissociating
• inner gate closed, outer gate opening

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Glucose movement lumen ? blood
lumen of small intestine
Na glctransporter(symport)
glucose(lower conc.)

• transport into mucosal cell(enterocyte)
• across luminal (apical) membrane
• against concn. gradient
• Naglc symport
• transport out of mucosal cell
• across contraluminal (basolateral) membrane
• down concn. gradient
• glc transporter (GLUT2)

Na
mucosalcell
Na
glucose(higher conc.)
Na
K
GLUT2glucose carrier(facil. diffusion)
Na,KATPase
K
glucose(lower conc.)
Na
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interstitial fluid blood
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Proteins digestion to peptides amino acids

• only amino acids some small peptides absorbable
• stomach food polypeptides stimulate G-cells in
  pyloric region to secrete the hormone gastrin
• gastrin stimulates parietal cells to secrete HCl
• effects of HCl
• denatures (unfolds) proteins
• solubilizes iron, other metals (ions)
• inactivates pathogenic organisms (antiseptic)
• stimulates secretion of secretin
  cholecystokinin (CCK)
• activates pepsinogen
• in bulimia, erodes enamel

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Proteins gastric digestion

• peptidases (proteases, proteolytic enzymes)
• exopeptidases peptide bonds next to N- or
  C-terminus
• endopeptidases bonds not next to N- or
  C-terminus
• pepsin
• endopeptidase
• secreted as pepsinogen by chief cells
• activation catalyzed by HCl, pepsin
  (self-activation)
• specificity bonds next to aromatic side chains
  (phe, tyr, trp)
• an aspartate protease (2 asp at active site
  slide 14)
• main result of passage through stomach
• polypeptides unfolded converted to large
  peptides

O O O O H3NCCNCCNCCNCCNCCOO exo endo
endo exo
(stimulus gastrin)
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Gastric secretions digestion

• HCl secretion by parietal cells
• pepsinogen secretionby chief cells
• pepsinogen activation by
• HCl
• pepsin autocatalysis product activation of
  precursor

Shrwd5_16-11.jpg
pepsinogen
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360
pepsin
HCl,pepsin
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Sherwood, Fig. 16-11
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Proteins intestinal secretions digestion

• HCl, peptides in chyme stimulate duodenum to
  secrete
• CCK stimulates pancreas to secrete enzyme - rich
  juice
• secretin stimulates pancreas to secrete HCO3 -
  rich juice
• gastric inhibitory peptide inhibits antral
  contractions
• HCO3 raises pH of chyme to 7- 8
• pancreatic proteases trypsin (an endopeptidase)
• secreted as trypsinogen
• trypsinogen activated by enteropeptidase
  (mucosal cells) by trypsin (self-activation)
• important as activator of many other digestive
  enzymes
• specificity bonds next to cationic side chains
  (lys, arg)
• a serine protease (contains a catalytic triad
  slide 14)

aka enterokinase
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Intestinal digestion by peptidases

• pancreas also secretes trypsin inhibitor
• other pancreatic peptidases
• peptidase type activator side chain specificity
• chymotrypsin endo trypsin nonpolar, aromatic
• elastase " " small, aliphatic
• carboxypep-tidase A exo " nonpolar, aromatic
• " B " " cationic
• mucosal cell luminal surface peptidases
• aminopeptidase (peptide bond next to N-terminus)
• dipeptidase (dipeptides)

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Protease classes or families

• all proteases belong to 1 of 4 classes
• classification based on main catalytic site
  feature
• protease class examples
• serine (section 4) trypsin, chymotrypsin,
  elastase,
• thrombin, plasmin
• zinc " carboxypeptidases, thermolysin
• thiol (cysteine) papain, cathepsins (lysosomes)
• aspartate pepsin, renin, HIV-1 protease
• within a class, substrate specificity due to
  structure of part of the active site the
  specificity site

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Serine proteases specificity pocket

• binding pocket structured to favor side chains on
  substrate that are
• nonpolar/aromatic chymotrypsin
• cationic trypsin
• small elastase

N
gly216
thr216
gly216
N
gly226
N

gly226
val226
asp189
ser189
chymotrypsin
elastase
trypsin
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Absorption of peptides amino acids from GI lumen

• amino acids
• Na gradient-driven symports (cotransport)
• mechanism same as glucoseNa symport
• at least 6 kinds of symport proteins
• specificities by amino acid types
• examples small, polar side chains (ala, ser,
  thr)
• cationic side chains (lys, arg)
• anionic side chains (asp, glu)

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Absorption of peptides amino acids

• some small peptides are absorbed
• mostly di- tripeptides
• H gradient-driven symport
• absorbed peptides hydrolyzed to amino acids by
  cytosolic peptidases
• amino acids reach portal blood via facilitated
  diffusion across contraluminal membrane of
  mucosal cells
• mechanism same as monosaccharide systems
• specific transmembrane carrier proteins

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Amino acids, peptides movement lumen blood
lumen of small intestine
H
smallpeptides
Na
H peptidesymport
amino acids

• analogous to glcmovement (slide 8)
• absorption via symports
• aa Na gradient-driven
• peptides H gradient-driven
• in cytosol peptides amino acids
• aas cross contraluminalmembrane via
  facilitateddiffusion carriers

Na aasymport
mucosalcell
H
Na
peptidases
smallpeptides
amino acids
Na
K
Na,KATPase
amino acid carriers(facil. diffusion)
K
amino acids
Na
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interstitial fluid blood
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Study questions

• Describe step-by-step the processes of digestion,
  absorption, and distribution of proteins.
  Include the roles of enzymes, activators,
  hormones, and membrane transport.
• Do the same for carbohydrates.

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Next timeDigestion absorption of lipids