Transcript Digestion of Proteins and Amino Acids

Digestion of Proteins and
Amino Acids
Ezinne Akudinobi
John Butler
Hannah Casey Lim
Melody Adeuyan
Physiology 3
November 14, 2016
Review: Amino Acids
400-plus naturally occurring amino acids
20 standard set of amino acids
http://www.biology.arizona.edu/biochemistry/problem_sets/
aa/Graphics/ChemBasicLabelled.gif
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
Review : Protein Structure
http://virtuallaboratory.colorado.edu/BioFunSupport/AllGraphics/peptideBond.gif
Metabolism
"Connections of carbohydrate, protein, and lipid
metabolic pathways," Mikael Häggström
Function of Proteins
Antibodies
Enzymes
Messengers
Receptors
Structural Components
Transport and Storage
©www.sigma-aldrich.com
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
http://digestionsystem.weebly.com/mouth.html
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
Formation of hydrochloric acid in stomach
Remember that parietal cells secrete HCl. How?
Enterochromaffin-like
← cell releases histamine
←
Proton-potassium pump
HCl
← requires ATP
Bicarbonate-chloride
pump
High [Cl-]
Low [K+]
High [K+]
G cell releases
gastrin to blood
Low [Cl-]
Parasympathetic: rest and digest
Vagus Nerve
releases ACh.
ACh receptors on
parietal cells and
enterochromaffin-like
cells trigger HCl
production in
stomach
Too much HCl in stomach?
X
Enterochromaffin-like
cell releases histamine
Too much!!!!
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
X
G cell releases
gastrin to blood
X
D cells release hormone
somatostatin to inhibit all HCl
production/promotion pathways
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...
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
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
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
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
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.
Key Zymogens Affecting Protein Digestion
•Trypsinogen
•Chymotrypsinogen
•Proelastase
•Procarboxypeptidase
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)
Trypsin is key enzyme in pancreatic juice. Hydrolysis reaction cleaves peptide
chains primarily at the carboxyl side of the amino acids lysine and arginine.
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
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
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
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
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+
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
Questions?
References
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pancreas/exocrine.html
http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pancreas/anatomy.html
https://ghr.nlm.nih.gov/about/reviewed-published-dates
http://www.sigmaaldrich.com/life-science/biochemicals/biochemical-products.html?TablePage=21735648
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3312700/
https://medlineplus.gov/ency/article/002222.htm