1. Digestion of Proteins and Amino Acids
Ezinne Akudinobi
John Butler
Hannah Casey Lim
Melody Adeuyan
Physiology 3
November 14, 2016

2. Review: Amino Acids 400-plus naturally occurring amino acids
20 standard set of amino acids
9 essential amino acids
Obtained through diet
Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine
4 nonessential amino acids
Produced in the body
Alanine, asparagine, aspartic acid and glutamic acid
7 conditional amino acids
Needed in times of illness and stress
Arginine, cysteine, glutamine,glycine, proline, serine and tyrosine

4. Review : Protein Structure

5. Metabolism
"Connections of carbohydrate, protein, and lipid metabolic pathways," Mikael Häggström

6. Function of Proteins Antibodies Enzymes Messengers Receptors
Structural Components
Transport and Storage ©

7. Digestion: Mouth to Stomach
Mechanical digestion begins in the mouth
Trigeminal Nerve (Mandibular Division) innervates muscles of mastication
Teeth cut and grind proteins into smaller components
Saliva lubricates and binds food to form a bolus
Bolus travels through the esophagus to the stomach

8. Stomach Pepsinogen from chief cells HCl from parietal cells
Histamine from enterochromaffin-like cells → promote HCl release from parietal cells
Gastrin from G cells → promote HCl release from parietal cells
Somatostatin from D cells → inhibits parietal cells, chief cells and enterochromaffin-like cells to slow the HCl release pathway
With all these chemicals, how does it all work? What does physiology have to do with it?
Stomach

9. Formation of hydrochloric acid in stomach
Remember that parietal cells secrete HCl. How?
Enterochromaffin-like cell releases histamine ← ←
← requires ATP
Proton-potassium pump
HCl
Bicarbonate-chloride pump
High [Cl-]
Low [K+]
High [K+]
G cell releases gastrin to blood
Low [Cl-]

10. Parasympathetic: rest and digest
Vagus Nerve releases ACh.
ACh receptors on parietal cells and enterochromaffin-like cells trigger HCl production in stomach
Parasympathetic: rest and digest

11. X X X Too much HCl in stomach?
Enterochromaffin-like cell releases histamine X
HCl
G cell releases gastrin to blood
Too much!!!! X
D cells release hormone somatostatin to inhibit all HCl production/promotion pathways

12. Pepsin to pepsinogen Chief cells
Secrete pepsinogen (inactive) which is activated by the HCl of parietal cell
HCl
Pepsin hydrolyses proteins, breaking them down into peptides ready to enter duodenum...

13. Protein Digestion: Role of Pancreas
•Organ lies medial and superior to duodenum
•Both endocrine and exocrine function
•Pancreatic juice and bile are secreted into the duodenum through pancreatic duct.
•Pancreatic duct (1/16 inch in diameter) joins the common bile duct to empty secretions through duodenal papilla
•Release is controlled by sphincter of ampulla

14. Protein Breakdown Continues
•Arrival of chyme into duodenum prompts reflex secretion of pancreatic juice and bile
•Stimulated by neural reflexes in duodenum and by secretion of cholecystokinin (CCK) and secretin (hormones in the duodenum)
•Secretion of pancreatic enzymes stimulated by ACh released by Vagus nerve and CCK (secreted by duodenum)
•Partially digested fat and protein content of chyme stimulate CCK

15. Exocrine Function: Ductal Cells
Secrete a solution rich in bicarbonate into lumen
Pancreatic ductal cells put hydrogen ions into blood
Neutralizes effect of gastric acid secretion’s alkaline tide

16. Exocrine Function: Acinar Cells
• Common bile duct passes through head of pancreas
• Joins pancreatic duct and forms ampulla of Vater
• Empties into duodenum
• Acinar cells surround lumen of pancreatic duct, which receives contents of secreted pancreatic juice

17. Exocrine Function: Acinar Cells
•Within lobules, exocrine secretory units are called acini (Latin for “berry”)
•Each acinus has single layer of acinar epithelial cells
•To minimize risk of self-digestion within pancreas, most pancreatic enzymes are produced as inactive molecules called Zymogens. Stored in Zymogen granules.

18. Key Zymogens Affecting Protein Digestion
•Trypsinogen
•Chymotrypsinogen
•Proelastase
•Procarboxypeptidase

19. Functions of Protein-Specific Zymogens on Activation
Endopeptidases: Cleave internal peptide bonds
•Trypsinogen to Trypsin (activated by enterokinase)
•Chymotrypsinogen to Chymotrypsin (activated by trypsin)
•Proelastase to Elastase (activated by trypsin)
Exopeptidases: Cleave last amino acid from carboxyl-terminal end of polypeptide
•Procarboxypeptidase to Carboxypeptidase (activated by trypsin)
•Aminopeptidase (NOT in pancreatic juice, but a brush border enzyme)

20. Trypsin is key enzyme in pancreatic juice
Trypsin is key enzyme in pancreatic juice. Hydrolysis reaction cleaves peptide chains primarily at the carboxyl side of the amino acids lysine and arginine.

21. Digestion of Polypeptides
Free Amino Acids
•Absorbed by secondary active transport carriers across brush border membrane
Dipeptides and Tripeptides
•Enter epithelium by single membrane carrier using a hydrogen gradient
•Hydrolyzed into free amino acids
•Move across basolateral membrane to interstitial fluid, then capillary blood

22. Proteins in the Small Intestine
Digestion of proteins is completed in the S.I. and mostly occurs in the duodenum
Absorption occurs in the jejunum
Villi: increase surface area to allow for increased nutrient absorption

23. Protein Digestion in the Small Intestine
Zymogens released by the pancreas begin the process
Trypsinogen, Procarboxypeptidase and Chymotrypsinogen
Activated by the enzyme Enterokinase on the surface of the S.I. cells
Trypsinogen → Trypsin
Procarboxypeptidase → Carboxypeptidase
Chymotrypsinogen → Chymotrypsin

24. Protein Digestion in the Small Intestine
Trypsin hydrolyzes peptide bonds between the amino acids
Trypsin is the most important of the protein-specific Zymogens because it:
stimulates the conversion of more Trypsin
helps convert Chymotrypsinogen to Chymotrypsin
helps convert Procarboxypeptidase to Carboxypeptidase
Carboxypeptidase also hydrolyzes peptide bonds
Chymotrypsin hydrolyzes peptide bonds at the carboxyl ends
After these enzymes do their jobs, the proteins are now smaller polypeptide fragments but digestion in the small intestine is not complete

25. Brush Border Enzymes Dipeptidases: hydrolyze dipeptides
Aminopeptidases: hydrolyze peptide bonds from the amino terminal
Break down into tripeptides, dipeptides, and single amino acids
Tripeptides → absorbed through a cotransporter with H+ ion into SI cell
Intracellular peptidases break it down into amino acids
Dipeptides → absorbed through a cotransporter with H+ ion into SI cell
Na+ ions antiport into the cell while the H+ ions are pumped back into the lumen

26. Single Amino Acids
Single Amino Acids → absorbed with one Na+ ion into cell
Once at the amino acid level, they are transported into the blood capillary to travel the liver
Net absorption of Na+ ions causes water to also be absorbed
Using the Na+/K+-ATPase pump, sodium is pumped into extracellular matrix in exchange for potassium
In the liver, amino acids are stored or used to synthesize new proteins

27. Questions?

28. References