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INTRODUCTION

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OBJECTIVES

After studying this chapter, you should be able to:

  • Describe the scope and objectives of enzyme kinetic analysis.

  • Indicate whether ΔG, the overall change in free energy for a reaction, is dependent on reaction mechanism.

  • Indicate whether ΔG is a function of the rates of reactions.

  • Explain the relationship between Keq, concentrations of substrates and products at equilibrium, and the ratio of the rate constants k1/k−1.

  • Outline how the concentration of hydrogen ions, of enzyme, and of substrate affect the rate of an enzyme-catalyzed reaction.

  • Utilize collision theory to explain how temperature affects the rate of a chemical reaction.

  • Define initial rate conditions and explain the advantage obtained from measuring the velocity of an enzyme-catalyzed reaction under these conditions.

  • Describe the application of linear forms of the Michaelis-Menten equation to estimate Km and Vmax.

  • Give one reason why a linear form of the Hill equation is used to evaluate how substrate-binding influences the kinetic behavior of certain multimeric enzymes.

  • Contrast the effects of an increasing concentration of substrate on the kinetics of simple competitive and noncompetitive inhibition.

  • Describe how substrates add to, and products depart from, an enzyme that follows a ping–pong mechanism.

  • Describe how substrates add to, and products depart from, an enzyme that follows a rapid-equilibrium mechanism.

  • Provide examples of the utility of enzyme kinetics in ascertaining the mode of action of drugs.

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BIOMEDICAL IMPORTANCE

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A complete and balanced set of enzyme activities is required for maintaining homeostasis. Enzyme kinetics, the quantitative measurement of the rates of enzyme-catalyzed reactions and the systematic study of factors that affect these rates, constitutes a central tool for the analysis, diagnosis, and treatment of the enzymic imbalances that underlie numerous human diseases. For example, kinetic analysis can reveal the number and order of the individual steps by which enzymes transform substrates into products, and in conjunction with site-directed mutagenesis, kinetic analyses can reveal details of the catalytic mechanism of a given enzyme. In the blood, the appearance or a surge in the levels of particular enzymes serve as clinical indicators for pathologies such as myocardial infarctions, prostate cancer, and damage to the liver. The involvement of enzymes in virtually all physiologic processes makes them the targets of choice for drugs that cure or ameliorate human disease. Applied enzyme kinetics represents the principal tool by which scientists identify and characterize therapeutic agents that selectively inhibit the rates of specific enzyme-catalyzed processes. Enzyme kinetics thus plays a central and critical role in drug discovery, in comparative pharmacodynamics, and in elucidating the mode of action of drugs.

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CHEMICAL REACTIONS ARE DESCRIBED USING BALANCED EQUATIONS

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A balanced chemical equation lists the initial chemical species (substrates) present and the new chemical species (products) formed for a particular chemical reaction, all in their respective proportions or stoichiometry. For example, balanced equation ...

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