Chapter 8

After studying this chapter, you should be able to:

• Describe the scope and overall purposes of the study of enzyme kinetics.
• 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 temperature and the concentration of hydrogen ions, enzyme, and substrate affect the rate of an enzyme-catalyzed reaction.
• Indicate why laboratory measurement of the rate of an enzyme-catalyzed reaction typically employs initial rate conditions.
• Describe the application of linear forms of the Michaelis–Menten equation to the determination of Km and Vmax.
• Give one reason why a linear form of the Hill equation is used to evaluate the substrate-binding kinetics exhibited by some multimeric enzymes.
• Contrast the effects of an increasing concentration of substrate on the kinetics of simple competitive and noncompetitive inhibition.
• Describe the ways in which substrates add to, and products depart from, an enzyme that follows a ping–pong mechanism and do the same for an enzyme that follows a rapid-equilibrium mechanism.
• Illustrate the utility of enzyme kinetics in ascertaining the mode of action of drugs.

Enzyme kinetics is the field of biochemistry concerned with the quantitative measurement of the rates of enzyme-catalyzed reactions and the systematic study of factors that affect these rates. Kinetic analysis can reveal the number and order of the individual steps by which enzymes transform substrates into products. Together with site-directed mutagenesis and other techniques that probe the protein structure, kinetic analyses can reveal details of the catalytic mechanism of a given enzyme.

A complete, balanced set of enzyme activities is of fundamental importance for maintaining homeostasis. An understanding of enzyme kinetics thus is important to understanding how physiologic stresses such as anoxia, metabolic acidosis or alkalosis, toxins, and pharmacologic agents affect that balance. 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 and comparative pharmacodynamics, as well as in elucidating the mode of action of drugs.

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 correct proportions or stoichiometry. For example, balanced equation (1) describes the reaction of one molecule each of substrates A and B to form one molecule each of products P and Q:

The double arrows indicate reversibility, an intrinsic property of all chemical reactions. Thus, for reaction (1), if ...

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