Metabolic fuels and Dietary components Lecture - 5 By Dr.
Abdulrahman Al-Ajlan 1
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The Michaelis-Menten equation The rate (V) of many
enzyme-catalyzed reactions can be described by the Michaelis Menten
equation. The general scheme for an enzyme reaction is E + S Es E +
P 2 K 1 K- 1 K 2
Slide 4
During a reaction, an enz-sub-coplex is formed that may either
dissociate to re-form the free enzyme and the substrate or react
(to release the product and regenerate the free enzyme. 3
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Where E is the enzyme, S the substrate, ES the enzyme-substrate
complex, P the product, and K 1 and K 2 and K 3 are rate constants.
From this concept, the Michaelis- Menten equation was derived
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Where K m = V max is the maxium velocity V max and K m are
characteristics of a given enzyme. V max is the velocity when the
enzyme is saturated with substrate S. K m is the substrate
concentration [S] at V max /2. When [S] = K m When the velocity is
plotted versus [s], a hyperbolic curve is produced. 5
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The lineweaver-Burk equation Because of difficulty in
determining V max from a hyperbolic curve, the Michaelis-Menten
equation was transformed by lineweaver and Burk into the equation.
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Inhibitor of enzyme: Inhibitors decrease Clinical Bio 1 Monthly
exam Q1:- Suppose that the data shown in the margin are obtained
for an enzyme-catalyzed reaction. a-from a Lineweaver-Burk equation
plot of the data, determine K M and V max 9
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[S](mM) 0.1 0.2 0.5 0.8 1.0 2.0 V (mmol ml -1 min -1 ) 3.33
5.00 7.14 8.00 8.33 9.09 V (mmol ml -1 min -1 )[S](mM) 3.330.1
5.000.2 7.140.5 8.000.8 8.331.0 9.092.0 11
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Dependence of velocity on [E], [S], [T] and pH. the velocity of
reaction, V, increases with enzyme concentration, if the substrate
con, [S] is constant. Q2:- Discuss briefly the dependence of enzyme
activity on pH. 12
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Protein Digestion The digestion of proteins begins in the
stomach, where pepsin is the major proteolytic enzyme in stomach.
Pepsin is produced and secreted by the chief cells of stomach as
the inactive zymogen (pepsinogen). HCl causes pepsinogen to be
cleaved to pepsin. The acid (pH 2-3) functions to kill some
bacteria and to denature proteins, making it more susceptible to
proteolysis. 13
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Pepsin cleaves proteins to smaller polypeptides. Pepsin also
catalyzes the cleavage of pepsinogen to pepsin (autocatalysis).
Pepsin cleaves the carbonyl group of the peptide bond is
contributed by the aromatic amino acids or by leucine. Pepsin
releases peptides and a few free amino acids from dietary proteins.
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Poly peptides produced by the action of pepsin in the stomach
enter the small intestine where they are cleaved to amino acids in
a series of steps that require the combined action of a number of
peptidases. Digestion of proteins by pancreatic enzymes 15
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In the intestine, the partially digested material from the
stomach encounters the pancreatic secretions that include
bicarbonate and a group of proteolytic enzymes. The bicarbonate
neutralizes the stomach acid, raising the pH of the contents of the
intestinal lumen into the optimal range for the digestive enzymes
to act. 16
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Endopeptidases enzymes Pancreatic endopeptidases cleave poly
peptides to oligopeptides and amino acids 1.Trypsin is secreted as
the inactive zymogen, trypsinogen. Trypsinogen is cleaved to form
trypsin by the enzyme enteropeptidase (enterokinase), which is
synthesized by the intestinal cells. 17
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Continue. 1.(From previous1) Inactive zymogens are activated in
the intestine by trypsin. Trypsin cleaves only when the carbonyl
group of the peptide bond is contributed by arg or lys
2.Chymotrypsin, secreted as chymotrypsinogen which is converted to
chymotrypsin by trypsin. Chymotrypsin cleaves peptide bond in which
the carbonyl group is contributed by the aromatic amino acids or by
leucine 18
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3.Elestase cleaves the carbonyl end of amino acids residues
with small, uncharged side chains such as ala, gly, ser. This
ezymogen, proelastase is cleaved to elastase by trypsin 19
Slide 21
Exopeptidases from the pancreas; The carboxypeptidases are
produced as procarboxypeptidases, which are cleaved to the active
form by trypsin 1.Carboxypeptidase A cleaves aromatic amino acids
from the C- terminal end of peptides. 2.Carboxypeptidase B cleaves
the basic amino acids, lys and arg, from the C-terminal end of
peptides 20
Slide 22
Proteases Associated with the intestinal Epithelial Cell
Enzymes produced by intestinal epithelial cells complete the
conversion of dietary proteins to amino acids. A minopeptidases are
exopeptidases produced by intestinal cells that cleave on amino
acid at a time from the N- terminal end of peptides. 21
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Absorption of amino acids: A mino acids are absorbed by
entestinal epithelial cells and released into the bloodstream by
transport systems. Sodium amino acid carrier system. 22
