Metabolic fuels and Dietary components Lecture - 5 By Dr. Abdulrahman Al-Ajlan 1

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

 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

 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 4

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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

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. 6

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) V (mmol ml -1 min -1 ) V (mmol ml -1 min -1 )[S](mM)

 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

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

 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. 14

 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

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

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

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

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

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

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

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

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