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Work Sheet 4
Digestion & Absorption of Proteins & Carbohydrates
1. Delineate the roles of gastric acid during digestion in the stomach.
1. Gastric HCl is responsible for the low pH <2 of gastric juice.
2. Gastric acid kills microorganisms and denatures dietary proteins
preparing them for hydrolysis by proteases.
3. Gastric juices contain the acid stable proteases of the pepsin family,
which produce large peptide fragments and some free amino acids.
4. Protein digestion at this stage is partial, as the amino acids and small
proteins enter the duodenum, they trigger the release of cholectystokininpancreozymin (CCK-PZ) into the bloodstream.
2
List and distinguish the proteases involved in gastric, pancreatic, brush border
surface and intracellular cleavage of peptide bonds; define their specificity in
general terms.
Gastric (Stomach) Digestion:
Gastric juices contain the acid stable proteases of the pepsin family,
which produce large peptide fragments and some free amino acids.
Protein digestion at this stage is partial, as the amino acids and small
proteins enter the duodenum, they trigger the release of cholectystokininpancreozymin (CCK-PZ) into the bloodstream.
This release initiates the secretion of protease zymogens from the pancreas and release
of enteropeptidase in the gut.
Pancreatic Proteases:
1. The pancreatic juice is rich in the proenzymes of endopeptidase and
the carboxypeptidases.
2. Enteropeptidase converts pancreatic trypsinogen to trypsin.
3. Trypsin autocatalytically activates more trypsinogen and other
proenzymes, liberating chymotrypsin, elastase and the
carboxypeptidases A and B.
Digestion at the Brush Border (surface of intestinal epithelial cells):
1. Since pancreatic juice does not contain appreciable aminopeptidase
activity, final digestion of di- and small peptides depends on brush border
enzymes.
2. The surface of intestinal epithelial cells is rich in endopeptidases and
aminopeptidases.
3. The end products of cell surface digestion are free amino acids and diand tripeptides.
3.
Describe the absorption of proteins and the five transport systems.
Absorption:
1. Following digestion, amino acids and small peptides are co-absorbed
w/ sodium via group specific amino acids or peptide transport systems.
2. These processes are carrier mediated, they discriminate between
natural L amino acids and D-amino acids, they require energy (from the
Na+ gradient, Na-K ATPase) and physiologic temperatures.
At least five brush border transport systems exist for:
1. neutral amino acids (uncharged aliphatic and aromatic)
2. basic amino acids (Lys, Arg, Cys, Cys-Cys)
3. acidic amino acids (Asp, Glu)
4. imino acids (Pro, Hydroxyproline)
5. di- and tripeptides
4.
Explain the physiology of celiac sprue, cystinuria and Hartnup disease
Clinical Correlates:
1. Hartnup Disease:
Genetic defect in the neutral amino acid transporter.
Symptoms: dermatitis due to tryptophan malabsorption ("niacin" flush)
Consequences: not serious di- and tripeptide absorption supply minimal
amounts of dietarily essential neutral amino acids.
2. Cystinuria:
Precursor to kidney stones
Symptoms: painful kidney stone formation due to malabsoprtion of
cystine (two disulfide linked cysteines)
3. Sprue:
Destruction and flattening of the intestinal villi resulting in generalized
malabsorption.
Causes: bacterial infection or gluten (contained in certain grains such as
wheat and barley) sensitivity.
5.
Compare and contrast the two polysaccharides of starch, amylose and amylopectin.
SKIP
6.
SKIP
List the products of starch digestion by salivary and pancreatic -amylase.
7.
Describe carbohydrate digestion and -glucosidases.
General Information:
1. Carbohydrates provide a major component of the daily caloric
requirement, ~40%.
2. Monosaccharides- do not need hydrolysis prior to absorption.
Disaccharides- require brush border enzymes.
Polysaccharides- require brush border enzymes, as well as, pancreatic
amylase and salivary amylase for digestion.
Enzymes
Lactase
Substrate
Lactose
Sucrase
Sucrose
Amylase
Amylose
Maltase
Maltose
Maltotriose
-Limit Dextrins
Isomaltose
Isomaltase
8.
Product
Galactose
Glucose
Glucose
Fructose
Glucose
Amylose (n-1)
Glucose
Maltose
Glucose
Distinguish between digestible and non-digestible carbohydrates.
Undigested Carbohydrates:
1. Di-, oligo- and polysaccharides that are not hydrolyzed by -amylase and/or brush
border enzymes cannot be absorbed.
2. These carbohydrates reach the lower tract of the intestine which contains bacteria.
3. The bacteria utilize many of the remaining carbohydrates, metabolizing them and
producing by- products such as: hydrogen gas, methane and carbon dioxide.
9. Draw the products resulting from the sequential action of these peptidases:
(1) Pepsin
(2) Carboxypeptidase A
(3) Elastase
on the following recently ingested protein: H-Asp-Tyr-Gly-Glu-Phe-Lys-Asp-Val-OH
Clearly indicate which fragments result from the action of each peptidase. The first one
has been done for you. (6 pts)
OH
NH3 +
CO2H
CO2H
O
+H 3N
C
H
C
H 3C
CO2H
CH2
H
N
C
H
O
H
H
N
C
O
C
H
CH2
O
H
N
C
C
H
C
H
N
C
H
O
C
O
H
N
C
H
CH3
O
H
N
C
C
H
O
H
N
C
C
H
C
(1) Pepsin: Endopeptidase, cleaves on the amino side of Tyr and Phe producing one free
amino acid and two smaller peptides.
OH
NH3 +
CO2H
CH2
O
+H 3N
C
H
C
H 3C
CO2H
CO2H
H
N
C
H
O
C
H
H
N
C
H
O
C
CH2
O
N
H
C
H
C
H
N
C
H
O
C
O
H
N
C
H
C
CH3
O
O
H
N
C
H
C
H
N
Now draw the products resulting from digestion with Carboxypeptidase A on the above
fragments.
C
H
C
O-
O-
Source
Name
Pancreas Trypsin
Activator
Enteropeptidase
Trypsin
Pancreas Chymotrypsin Trypsin
Pancreas Elastase
Trypsin
Pancreas CP-A
Trypsin
Pancreas CP-B
Trypsin
Stomach Pepsin A
H+,
autoactivation
Pepsin
Trypsinogen
Carboxy “R” Group
vs.
Specificity
Amino
C
Arg,Lys
Chymotrypsinogen
Proelastase
Pro-CP-A
Pro-CP-B
Pepsinogen A
C
C
A
A
A
Proenzyme
Tyr,Phe,Trp
Gly,Ser,Ala
Val,Leu,Ile
Arg,Lys
Tyr,Phe