Unit 3 Lecture 13 Protein Absorption PDF

Unit 3 Lecture 13 Protein Absorption Chapter 37 Lecom MMS Program Objectives 1. Describe the proteases required to digest dietary protein, and their organs of origin. 2. Describe how pancreatic proteases are activated, and explain the differences in their action on proteins. 3. Explain the transpo...

Unit 3 Lecture 13 Protein Absorption Chapter 37 Lecom MMS Program Objectives 1. Describe the proteases required to digest dietary protein, and their organs of origin. 2. Describe how pancreatic proteases are activated, and explain the differences in their action on proteins. 3. Explain the transport mechanism for amino acids on the apical and serous sides of the intestinal epithelium. 4. Explain the 2 ways that intracellular proteins are degraded. Describe the regulatory pathway for autophagy. Section Introduction p. 693 Overview of Nitrogen Metabolism I. Protein digestion A. Stomach Parietal cells – HCL: denatures proteins Chief cells – Pepsinogen Pepsinogen: inactive precursor (zymogen) (-ogen: generated from) • • • • • • Acid environment activates Pepsinogen Conformational change Cleaves itself to Pepsin: autocatalytic Resistant to HCl denaturation Broad specificity Prefers cleaving peptide bonds before aromatic or acidic amino acids B. Digestion of Proteins by Pancreatic enzymes Pancreatic Secretions: Amylase, Lipase, colipase Bicarbonate Gastric and Pancreatic Zymogen Activation Trypsinogen cleaved by: Enteropeptidase (secreted by intestinal brush border cells) Trypsin then cleaves Chymotrypsinogen Proelastase Procarboxypeptidase Specificities Serine proteases have a serine in their active site: Endopeptidases • Trypsin most specific: cleaves before Arg, Lys • Chymotrypsin favors hydrophobic amino acids • Elastase cleaves amino acids with small side chains Endopeptidases create small peptides Carboxypeptidases: exopeptidase A: cleaves hydrophobic C-terminal a.a. B. Cleaves Arg or Lys C-terminal a.a Aminopeptidases: exopeptidases • cleave from amino terminal end • Present on the intestinal brush border II. Absorption of Amino Acids Intestinal epithelial cells can absorb amino acids & small peptides (apical side) Sodium Co-transport of amino acids Down [Na] gradient (remember glucose?) Na gradient maintained by Na+ K+ ATPase 6 or more specifc Na+ dependent transporters: Neutral amino acids Proline and hydroxyproline Acidic Basic Cysteine (disulfide bonds) Facilitated transport out on the serosal side Reversible: during starvation, intestinal cells take up amino acids for fuel B. Transport of Amino Acids from Blood into Cells Most are Na+ dependent transporters Allows cells to concentrate amino acids from blood Cf. glucose where most cells take up glucose independent of Na+ Glut 2, 3, 4 Many systems Note that intestine and kidney have similar amino acid transport systems III. Protein Turnover & Replenishment of Intracellular Amino Acid Pools Sources: Dietary proteins Intracellular proteins: some have t1/2 of 5-15 minutes High turnover proteins: 1. Hemoglobin: red cell turnover 3e11/day globin is degraded in lysosomes of macrophages 2. Muscle protein (for gluconeogenesis) 3. Digestive enzymes, proteins of intestinal cells sloughed off (100 g/day) Protein cannot be stored: Excess consumption – converted to glycogen (via gluconeogenesis) or TG for storage A. Lysosomal Protein Turnover Autophagy: cytoplasmic components are surrounded by membranes: autophagylosomes Fuse with lysosomes Lysosomal proteases: Cathepsins: degrade proteins to amino acids Triggered by starvation Survival mechanism Autophagy AMP kinase phosphorylates and activates TSC1/TSC2 (Tuberous sclerosis) which then Stimulates GTPase activity of Rheb Insulin causes Akt to phosphorylate TSC1/TSC2 at a different site, inactivating it Rheb: Ras homolog enriched in brain-GTP Rheb-GTP Activates mTOR AMP kinase active Rheb-GDP cannot Activate mTOR So Autophagy is No longer blocked mTOR (Target of Rapamycin) Protein kinase: phosphorylates & inactivates a Protein crucial to initiation of phagosome formation 2016 Nobel Prize awarded to Yoshinori Ohsumi for discoveries of mechanisms for autophagy In yeast B. Ubiquitin Proteasome Pathway Ubiquitin: 76 aa protein Covalently binds to e-amino group of lys in proteins: Marks them for degradation by Proteosomes 20S: internal proteolytic sites in a cylinder 19S: regulatory ATP required to unfold & push Proteins through cylinder PEST sequences Pro-Glu-Ser-Thr Most proteins that Have PEST are ubiquitylated and Degraded by proteosomes g-Glutamyl Cycle & Glutathione Synthesis Glu-Cys-Gly is synthesized in most cells Protects against oxidation Intestine & kidney g-Glutamyl Transpeptidase in Cell membrane can Transfer an amino acid onto glutathione On inside of membrane, Cysteinylglycine & Glutamyl-amino acid are released. Amino acid cleaved from glutamyl By Oxoprolinase Study questions Name 7 enzymes involved in protein digestion. Name two types of protein degradation Explain the roles of mTOR, Rheb and TSC1/TSC2 in regulating autophagy. What happens to excess protein in the diet?