8/9/2019 Chok Biochem 2nd Shift Reviewer Protein Digestion

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DIGESTION OF PROTEINS

• Protein (CHON) AA via hydrolysis • Daily CHON Load: 70-100 g (dietary); 35-200g (endogenous) • Very efficient digestion in humans

FUNCTIONS 

• Building and maintaining body tissues

• Source of energy (4 kcal/g)

• Formation of enzymes, hormones, fluids, secretions

• Maintaining normal osmotic relations

• Acid-base balance of blood and tissues

• Transport of various substances

DIGESTIVE ENZYMES 

• Enzymes secreted in zymogen form – inactive precursors of digestive enzymes activated by cleavage of peptide chains

• CHON digestive enzymes are peptidases (a type of hydrolase) that cleave peptide bonds

o Endopeptidase – attack internal bonds liberating large peptide fragments

o Exopeptidase – cleaves off one AA at a time from the terminals

 

Enzyme ZymogenSite of Fragment Cleaved for 

 Activation

Means of activation

Specificity Notes

GASTRIC PHASE

• Optimum pH 1.5-2.5; Gastric juice (HCl) lowers pH, kills microorganisms, denatures proteins

• produces large peptides and some free AA  stimulate CCK release initiate pancreatic phase

Pepsin Pepsinogen 42-44 AA HCl, pepsin peptide bonds with C-group fromaromatics and acidics ( Phe, Tyr,Glu, Asp)

From chief cells of stomach, acidity enables pepsinogen to autoactivate, product(large peptides) stimulate CCK release to initiate pancreatic phase

Rennin ? ? ?PANCREATIC PHASE 

• OpH 7.5-8.5; Bicarbonate rich = ↑ pH

•Produces: shorter oligopeptides, free AA

TrypsinTrypsinogen Hexapeptide Enterokinase,

trypsinpeptide bonds with C-group frombasic AA (arg, lys)

Endopeptidase

ChymotrypsinChymotrypsinogen 2 dipeptides, creating

a 3 subunit enzymeTrypsin pept ide bonds with C-g roup from

aromatics, leu, metEndopeptidase

ElastaseProelastase Decapeptide? Trypsin peptide bonds with C-group from

small AA (ala, gly, ser )Endopeptidase

Carbopeptidase AProcarbopeptidase A 2 large fragments Trypsin N-side of C-terminal AA

Aliphatic / hydrophobic val, leu,ile, ala

Exopeptidase

Carbopeptidase BProcarbopeptidase B ? Trypsin N-side of C-terminal AA

Basic AA (arg, lys)Exopeptidase

INTESTINAL PHASE 

• OpH 7.5-8.5

• From luminal surface of epithelial cells

• Produces free AAs

Aminopeptidase ? ? ? Exopeptidase

Di/tripeptidase ? ? ? Dipeptides EndopeptidaseTripepditase ? ? ? Tripeptides Endopeptidase

 ABSORPTION and TRANSPORT 

Location Type of Transport Notes

Lumen to Intestinal Cell Na+ dependent co-transport Same as sugars; semi-specifig, driven by ↓intraceullar Na+ due to Na-K-ATPase on serosa; overlapping specificity for different AA

Facilitated diffusion ↑ to ↓ concentration

γ-glutamyl cycle AA passes membrane through γ-glutamyl transpeptidase Glutathione (Glu,Cys,Gly) + AA cysteinylglycine γ-glutamylAA AA + 5-

8/9/2019 Chok Biochem 2nd Shift Reviewer Protein Digestion

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Oxoproline Glutamate + Cysteine γ-glutamylcysteine + glycine Glutathione

Intestinal Cell to Plasma Facilitated transport In vivo: ↑ intestinal cell AA conc > blood; during starvation: ↑ AA conc in blood > cell = bidirectional transport

Plasma to Other cellsFacilitated diffusion Lesser extent

Na+ dependent co-transport Liver / muscle conc AA from blood