Section 10 Nutrients & Their Functions 1. Acquiring nutrients Digestion & absorption of carbohydrates & proteins 1/3/06

Section 10 Acquiring Nutrients Overviewnutrients digestion, absorption glucose, amino acids, fats distribution precursors, available fuels metabolism macromolecules, energy, end products 1

Digestion: general features u general function: convert nutrients into absorbable form u larger molecules (macro)  smaller molecules u chemical reaction: hydrolysis H 2 O + RX YR'  RXOH + HYR' where X = C, P Y = O, N H 2 O + RX – YR'  RXOH + HYR' where X = C, P Y = O, N u reaction catalyzed by hydrolases major nutrientshydrolase typebond cleaved poly, oligosaccharidesglycosidaseglycosidic proteins, peptidesprotease, peptidasepeptide fats (triglycerides)esteraseester minor nutrients phospholipids, nucleic acidsphospholipase, nucleasephosphoester esters ( e.g., cholesterol, retinol )esterasesester H2OH2O 2

Activation of hydrolases u noncovalent activator binds reversibly to enzyme examples: enzymeactivator  -amylase Cl – lipasecolipase u covalent activator catalyzes hydrolysis of one or more peptide bonds of precursor activator precursor  active enzyme (proenzyme or zymogen) HCl examples:pepsinogen  pepsin HCl examples:pepsinogen  pepsin trypsin trypsin proelastase  elastase 3

Carbohydrates: digestion (poly   oligo) u only monosaccharides are absorbable u Stage I: polysaccharides  oligosaccharides enzyme:  -amylase (specificity: internal  1,4 glucosidic bonds) enzyme:  -amylase (specificity: internal  1,4 glucosidic bonds) sources: salivary glands (minor) & pancreas (major) l substrates: amylose (only  1,4 links ) products: maltose, maltotriose 4 (glc 3 )

Carbohydrates: digestion (oligo   mono) u Stage II: oligosaccharides  monosaccharides source of enzymes: mucosal cell surface (brush border) source of enzymes: mucosal cell surface (brush border)  -limit dextrins  glc 3 + glcisomaltase + maltose(  -dextrinase) α 1,6 glucosidic bonds maltose  2 glcmaltase maltose  2 glcmaltase glc 3  3 glc " glc 3  3 glc " sucrose  glc + fructosesucrase sucrose  glc + fructosesucrase lactose  glc + gallactase (inducible) lactose  glc + gallactase (inducible)  -limit dextrins: (glc) with one  1,6 linkage glc: glucose gal: galactose glc: glucose gal: galactose 5

Carbohydrates: absorption from lumen u frc & mannose facilitated diffusion (carrier-mediated) u glc & gal l transported against concentration gradient l cotransport (Na + gradient-driven symport) l aka secondary active transport u pentoses, others absorbed via simple diffusion Na + glc symport protein 6

glc-Na + symport animation u for “snapshots” of this animation, see the next slide u stop animation by clicking browser’s Stop button u resume animation by going to another slide, then return- ing to this one glc-Na + symport protein 7

glc-Na + symport mechanism 1. outer gate open, sites unoccupied 2. sites occupied, outer gate closing 3. inner gate opening 4. Na +, glc dissociating 5. inner gate closed, outer gate opening glc-Na + symport protein 7

Glucose movement: lumen  blood lumen of small intestine glucose (lower conc.) glucose (lower conc.) glucose (higher conc.) glucose (higher conc.) interstitial fluid blood Na + glc transporter (symport) GLUT2 glucose carrier (facil. diffusion) mucosal cell 8 Na +,K + ATPase Na + K+K+K+K+ K+K+K+K+ u transport into mucosal cell (enterocyte) l across luminal (apical) membrane l against concn. gradient l Na + glc symport u transport out of mucosal cell l across contraluminal (basolateral) membrane l down concn. gradient l glc transporter (GLUT2)

Proteins: digestion to peptides & amino acids u only amino acids & some small peptides absorbable u stomach: food polypeptides stimulate G-cells in pyloric region to secrete the hormone gastrin u gastrin stimulates parietal cells to secrete HCl u effects of HCl l denatures (unfolds) proteins l solubilizes iron, other metals (ions) l inactivates pathogenic organisms (antiseptic) l stimulates secretion of secretin & cholecystokinin (CCK) l activates pepsinogen l in bulimia, erodes enamel 9

Proteins: gastric digestion u peptidases (proteases, proteolytic enzymes) l exopeptidases: peptide bonds next to N- or C-terminus l endopeptidases: bonds not next to N- or C-terminus u pepsin l endopeptidase l secreted as pepsinogen by chief cells l activation: catalyzed by HCl, pepsin (self-activation) l specificity: bonds next to aromatic side chains (phe, tyr, trp) l an aspartate protease (2 asp at active site; slide 14) u main result of passage through stomach: polypeptides unfolded & converted to large peptides + OOOO + OOOO H 3 NCC–NCC–NCC–NCC–NCCOO – exoendoendoexo (stimulus: gastrin) 10

Gastric secretions & digestion u HCl secretion by parietal cells u pepsinogen secretion by chief cells u pepsinogen activation by l HCl l pepsin autocatalysis: product activation of precursor Sherwood, Fig HCl, pepsin pepsin pepsinogen 11 Shrwd5_16-11.jpg

Proteins: intestinal secretions & digestion u HCl, peptides in chyme stimulate duodenum to secrete l CCK: stimulates pancreas to secrete enzyme - rich juice l secretin: stimulates pancreas to secrete HCO 3 – - rich juice l gastric inhibitory peptide: inhibits antral contractions u HCO 3 – raises pH of chyme to 7- 8 u pancreatic proteases: trypsin ( an endopeptidase ) l secreted as trypsinogen l trypsinogen activated by enteropeptidase* (mucosal cells) & by trypsin (self-activation) l important as activator of many other digestive enzymes l specificity: bonds next to cationic side chains (lys, arg) l a serine protease (contains a catalytic triad; slide 14) * aka enterokinase 12

Intestinal digestion by peptidases u pancreas also secretes trypsin inhibitor u other pancreatic peptidases peptidasetypeactivatorside chain specificity chymotrypsinendotrypsinnonpolar, aromatic elastase " "small, aliphatic carboxypep- tidase Aexo "nonpolar, aromatic " B " "cationic " B " "cationic u mucosal cell luminal surface peptidases l aminopeptidase (peptide bond next to N-terminus) l dipeptidase (dipeptides) 13

Protease classes or families u all proteases belong to 1 of 4 classes u classification based on main catalytic site feature protease class examples serine (section 4) trypsin, chymotrypsin, elastase, thrombin, plasmin thrombin, plasmin zinc "carboxypeptidases, thermolysin thiol (cysteine)papain, cathepsins (lysosomes) aspartatepepsin, renin, HIV-1 protease u within a class, substrate specificity due to structure of part of the active site: the specificity site 14

Serine proteases: specificity pocket u binding pocket structured to favor side chains on substrate that are: l nonpolar/aromatic chymotrypsin l cationic trypsin l small elastase gly 216 gly 226 gly 216 thr 216 gly 226 val 226 asp 189 ser 189 chymotrypsin trypsin elastase + –+ – N N N 15

Absorption of peptides & amino acids from GI lumen u amino acids l Na + gradient-driven symports (cotransport) mechanism same as glucose–Na + symport l at least 6 kinds of symport proteins l specificities by amino acid types examples: small, polar side chains (ala, ser, thr) cationic side chains (lys, arg) anionic side chains (asp, glu) 16

Absorption of peptides & amino acids u some small peptides are absorbed l mostly di- & tripeptides l H + gradient-driven symport l absorbed peptides hydrolyzed to amino acids by cytosolic peptidases u amino acids reach portal blood via facilitated diffusion across contraluminal membrane of mucosal cells l mechanism same as monosaccharide systems l specific transmembrane carrier proteins 17

lumen of small intestine amino acids interstitial fluid blood Na + aa symport amino acid carriers (facil. diffusion) mucosal cell 18 Na +,K + ATPase Na + H+H+H+H+ K+K+K+K+ K+K+K+K+ u analogous to glc movement (slide 8) u absorption via symports l aa: Na + gradient-driven l peptides: H + gradient-driven  in cytosol: peptides  amino acids u aa’s cross contraluminal membrane via facilitated diffusion carriers Na + small peptides H+H+H+H+ peptidases Amino acids, peptides movement: lumen  blood H + peptide symport

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

Next time: Digestion & absorption of lipids