Enzyme Inhibitors Seminar -

MEDICINAL CHEMISTRY (I) : 

Seminar on Approaches to rational design of Enzyme Inhibitors Presented by Mr. ABDUL RAZZAQ M. Pharm. Department -: Pharmaceutical Chemistry Luqman College of Pharmacy Gulbarga. 1 MEDICINAL CHEMISTRY (I)

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Topics going to discus are, Introduction Enzyme Inhibitors in Medicine Enzyme Inhibitors in Basic Research Study of the rates of Enzyme- catalyzed reactions

Enzyme : 

Enzyme Definition Description Classification Enzymes are soluble, colloidal, organic catalysts, formed by living cells specific in action, protein in nature, inactive at 00c and destroyed by moist heat at 1000c. Enzymes are the specialized proteins which catalyze various biochemical reactions. The status of mental & physical health of a person depends on the rates of on-going biochemical reactions.

International Classification of Enzyme : 

International Classification of Enzyme Oxido-reductases – Enzymes catalyzing oxidoreductions between two substrates. e.g. A & B Areduced + Boxidized = Aoxidized + Breduced These enzymes act on , , , Functional groups.

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2. Transferases – They catalyze the transfer of some group or radical ,R, from one molecule A ,to another B. A.R + B A + B.R Hydrolases – These enzymes have the ability to induce hydrolysis of ester , acid anhydrides, glycosidic bonds, peptide bonds & other C-N bonds.

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Lyases – These enzymes catalyze such reactions characterized by addition to double bonds in molecules containing , & linkages. Isomerases – Enzymes catalyzing interconversion of optical, geometrical or positional isomers. Ligases – (ligare = to bind ) These enzyme catalyze the formation of bonds with ATP cleavages in molecules containing C-O, C-S, C-N & C-C linkages.

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Enzyme inhibitors Inhibition – means the prevention or reduction of function Enzyme inhibitors are molecules that interact in some way with the enzyme to prevent it from working in the normal manner. Since blocking an enzyme's activity can kill a pathogen or correct a metabolic imbalance, many drugs are enzyme inhibitors. They are also used as herbicides and pesticides. Not all molecules that bind to enzymes are inhibitors; enzyme activators bind to enzymes and increase their enzymatic activity.

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Enzyme Inhibitors in Medicine Inhibitors have great potential as drug by selectively blocking certain metabolic pathways, decreasing the concentration of enzymatic product, or increasing the concentration of enzymatic substrate. An important area of drug design is the development of agents against microorganisms & parasites in humans. e.g. Alanine racemase inhibitor (L-Ala-P) Clavulanic acid & Sulbactam

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A highly specific inhibitor may be able to exploit existing subtle structural difference between isoenzyme. e.g. Dihydrofolate reductase inhibitor (Trimethoprim) Acyclovir

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Anticancer agents which are designed to kill tumor cells selectively without harming the normal cells in the organism. Unfortunately the identification of appropriate target isoenzymes in tumor cells appears to quite challenging. A number of diseases have been correlated with either dysfunction of an enzyme or an imbalance of metabolites, & can often be treated by the inhibition of particular enzyme. e.g. Captopril ,Enalapril

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Enzyme inhibitors in Basic Research They serve as useful tools for the elucidation of structure & function of enzymes. e.g. irreversible inhibitors have assisted in determination of half-site reactivity of enzymes or measurement of enzyme turnover. They have been applied to quantify & localized enzymes in organs in vivo. Covalently binding enzyme inhibitors have been used to identify active site amino acid residues.

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Reversible enzyme inhibitors have been used to facilitate enzyme purification by using the inhibitor as a ligand for affinity chromatography. Enzyme inhibitor can also be used to mimic genetic disease in animal models. The another application of enzyme inhibitor is the screening of microorganisms for inhibitor-resistant mutants.

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Study Of The Rates Of Enzyme-catalyzed Reactions Velocity of enzymatic reactions generally follows hyperbolic kinetics

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Km in the Michaelis-Menten mechanism • For enzymes that utilize the true Michaelis-Menten mechanism, kcat is rate-limiting. In this case, … kcat<<k2 Km ≈ k2 / k1 = [E][S] / [E·S] = E·S dissociation constant Km is an apparent dissociation constant of E·S

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What is an “apparent” value? • Basically a value that is acknowledged to be altered from the “true” value by some factor(s) • Examples: Km ≈ (but not =) dissociation constant of E·S The presence of an inhibitor can affect the values you determine for Km and/or Vmax of a reaction, but they don’t affect the true values inherent to the enzyme (obtained in the absence of inhibitor) Apparent Km = K’m Apparent Vmax = V’max

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Enzyme Inhibitors Major classes of enzyme inhibitor Reversible E + I E.I Usually from non-covalent interaction with enzyme. Irreversible E + I E.I Usually from covalent bond with enzyme.

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Reversible Inhibitors • Can bind either to free enzyme or enzyme-substrate complex (usually non-covalently)

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Classification of Reversible Inhibitors • Competitive – bind only to free enzyme • Uncompetitive – bind only to enzyme-substrate Complex • Noncompetitive – bind equally well to free enzyme and enzyme-substrate complex • Mixed – binds to both free enzyme and enzymesubstrate complex, but with different affinities

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Why does a competitive inhibitor have these effects ? • Km = [S] when V = (1/2) Vmax • In the presence of a competitive inhibitor, more substrate is required to reach the uninhibited Velocity • But, given high enough [S], the same Vmax can still be reached

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Why does a competitive inhibitor have these effects? • “When stress is applied to a system at chemical equilibrium, the equilibrium will shift in the direction that tends to relieve or counteract that stress.” • A competitive inhibitor reduces [E] Equilibrium shifts toward [E] [E] [S] = K’m increases [ES]

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Why does a noncompetitive inhibitor have these effects? • E and E·S are bound equally by the noncompetitive Inhibitor • Amount of uninhibited enzyme is decreased, so V’max is decreased • E vs. E·S equilibrium is not altered, so K’m is Unchanged • Amount of substrate required to reach ½ Vmax is unchanged, so K’m is unchanged

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Why does an uncompetitive inhibitor have these effects? • Only E·S complex is bound by the uncompetitive Inhibitor • Decrease in E·S means less active enzyme, so apparent maximal velocity (V’max) decreases as well • E vs. E·S equilibrium shifts toward E·S [E] [S] = K’m decreases [ES]

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References – Medicinal Chemistry, by Berger , 4th edition A Wiley Interscience Publication, New York. Foye’s Principles of Medicinal Chemistry 5th edition, B.I. Publications. Drug Design, by Dr. V.M.Kulkarni, Dr. K.G.Bothara 3rd edition ,Nirali Publication. Biopharmaceutics & Pharmacokinetics, by D.M.Brahmankar, Sunil B. Jaiswal, 1st edition, Vallabh Prakashan. Search on www.google.com

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Thank you !