BIOC Dr. Tischler Lecture 21 – February 14, 2006 METABOLISM: PROTEIN DIGESTION & ABSORPTION

Table 1. Phases of Digestion and Absorption of Protein and its Degradative Products Phase of Digestion LocationAgentsOutcome 5. Cleavage of di-/tripeptides transport to capillaries epithelial cell – cytoplasm contraluminal membrane dipeptidases tripeptidases facilitated diffusion free amino acids from di-/tripeptides; amino acids transported into capillaries 4. Absorptionintestinal epithelial cell brush border membrane transport systemsuptake into epithelial cell 3. Brush Border Surface brush border surface of intestine endopeptidases and aminopeptidases free amino acids and di-/ tripeptides 2. Pancreatic Proteases lumen of small Intestine trypsin, chymotrypsin, elastase, and carboxypeptidases free amino acids and oligopeptides – 2 to 8 amino acids 1. Gastric Digestion stomachstomach acid pepsin denaturation large peptide fragments + some free amino acids

Gastric Parietal Cell Plasma Lumen of the Stomach CO 2 HCO 3 - H+H+ ATP ADP + P i K+K+ H+H+ CO 2 + H 2 O carbonic anhydrase H 2 CO 3 HCO 3 - Cl - Figure 1. Production of gastric acid and its secretion H +,K + -ATPase

Phase 1- Gastric digestion Dietary Protein denaturation by stomach acid Figure 2. Gastric digestion of dietary protein. large peptide fragments free amino acids hydrolysis by pepsin Pyloric sphincter Duodenum Gastric Chief Cells Acid from parietal cells denatures protein to be more susceptible to pepsin cleavage. Pepsinogen activated to pepsin by autoactivation and autocatalysis by pepsin. Large peptide fragments/some amino acids pass through the pyloric sphincter to the duodenum Pepsinogen autocatalysis Pepsin autoactivation (intramolecular cleavage) aa

Pancreatic Acinar Cell Duodenal Endocrine Cell Blood- stream Mucosal Epithelial Cells Entero- peptidase Duodenal Endocrine Cell Phase 2- Digestion by pancreatic proteases Trypsinogen Trypsin free amino acids from gastric digestion CCK-PZ (hydrolysis) Figure 3. Secretion, activation and action of pancreatic proteases and brush border endopeptidases and aminopeptidases

Duodenal Endocrine Cell CCK-PZ Pancreatic Acinar Cell Blood- stream Mucosal Epithelial Cells Entero- peptidase (hydrolysis) Duodenal Endocrine Cell CCK-PZ Phase 2- Digestion by pancreatic proteases Trypsinogen Trypsin free amino acids from gastric digestion HCO 3 - neutralizes acid Secretin autocatalysis Figure 3. Secretion, activation and action of pancreatic proteases and brush border endopeptidases and aminopeptidases

Duodenal Endocrine Cell CCK-PZ Secretin Pancreatic Acinar Cell Blood- stream Mucosal Epithelial Cells Entero- peptidase (hydrolysis) Duodenal Endocrine Cell CCK-PZ Phase 2- Digestion by pancreatic proteases Trypsinogen Trypsin autocatalysis Chymotrypsinogen Proelastase Procarboxypeptidases Chymotrypsin Elastase Carboxypeptidases catalysis free amino acids from gastric digestion HCO 3 - neutralizes acid Figure 3. Secretion, activation and action of pancreatic proteases and brush border endopeptidases and aminopeptidases

Duodenal Endocrine Cell CCK-PZ Secretin Pancreatic Acinar Cell Blood- stream Mucosal Epithelial Cells Entero- peptidase (hydrolysis) Duodenal Endocrine Cell CCK-PZ Phase 2- Digestion by pancreatic proteases Phase 3- Digestion at the brush border Trypsinogen Trypsin autocatalysis Chymotrypsinogen Proelastase Procarboxypeptidases Chymotrypsin Elastase Carboxypeptidases catalysis amino acids dipeptides tripeptides free amino acids from gastric digestion HCO 3 - neutralizes acid brush border endo-/aminopeptidases hydrolyze products; amino acids, di-/tripeptides absorbed by epithelial cells

Table 2. Summary of the gastric and pancreatic digestive proteases ProteaseSourceProtease family ProenzymeActivationSpecificity Trypsin (endo-)pancreasserinetrypsinogenenteropeptidase trypsin basic (arg, lys) Chymotrypsin (endo-) pancreasserinechymo- trypsinogen trypsinbulky aromatic (trp, phe, tyr, met) Elastase (endo-) pancreasserineproelastasetrypsinsmall neutral R groups (gly, ser, ala) Carboxypeptidase A (exo-) pancreaszincprocarboxy- peptidase A trypsinaromatic (tyr, phe, trp) hydrophobic (val, leu, ile) Carboxypeptidase B (exo-) pancreaszincprocarboxy- peptidase B trypsinbasic (arg, lys) Pepsin (endo-)stomachaspartatepepsinogenautoactivation/ H + ; pepsin aromatic (tyr,phe, trp) acidic (glu)

LUMEN OF INTESTINE Intestinal Epithelium Amino acids Di-, tri- peptides Na + = Na + -dependent co-transport contraluminal membrane Brush border Amino acids Dipeptides, tripeptides Phase 4 - Absorption Figure 4. Absorption of amino acids and di- and tripeptides from the intestinal lumen 3Na + 2K + ATP ADP + Pi = Na +,K + -ATPase 3Na + 2K + Na + Dipeptidases, tripeptidases

a) neutral amino acids (uncharged aliphatic and aromatic) b) basic amino acids and cystine (Cys-Cys) c) acidic amino acids (Asp, Glu) d) imino acids (Pro) e) dipeptides and tripeptides BRUSH BORDER TRANSPORT SYSTEMS

Figure 4. Absorption of amino acids and di- and tripeptides from the intestinal lumen LUMEN OF INTESTINE Intestinal Epithelium Amino acids Di-, tri- peptides Na + = facilitated diffusion = Na + -dependent co-transport contraluminal membrane Amino acids Brush border Dipeptides, tripeptides Dipeptidases, tripeptidases Phase 4 - Absorption Phase 5 capillaries Phase 5 3Na + 2K + ATP ADP + Pi = Na +,K + -ATPase 3Na + 2K +      

CLINICAL CORRELATE - CYSTINURIA cystine is a homodimer of cysteine with two cysteine molecules connected by a disulfide linkage that usually arises from proteins absorbed via intestinal basic amino acid transporter reabsorbed via similar kidney transporter defective transporter leads to cystine kidney stone formation due to low solubility excessive excretion of other dibasic amino acids (lys, orn, arg) is diagnostic NH 2 -CH-CH 2 -SH | COOH HS-CH 2 -CH-NH 2 | COOH NH 2 -CH-CH 2 -S-S-CH 2 -CH-NH 2 | | COOH COOH NH 2 -CH-CH 2 -SH | COOH HS-CH 2 -CH-NH 2 | COOH NH 2 -CH-CH 2 -SH | COOH HS-CH 2 -CH-NH 2 | COOH

CLINICAL CORRELATE – CYSTIC FIBROSIS (CF) defective transport of chloride and sodium by epithelial cells leads to “thickening” of mucous from exocrine glands; e.g., pancreatic ducts blockage allows activation of digestive proteases in duct causing pancreatic destruction 85% of CF patients show pancreatic failure lose ability to secrete digestive enzymes causing malabsorption of nutrients and vitamins fat malabsorption causes loss of fat-soluble vitamins CF-malnutrition includes: reduced energy intake (protein-caloric malnutrition) increased energy loss increased energy expenditures