Kinetics and Regulation

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  • 7.1 Kinetics Is the Study of Reaction Rates

  • 7.2 The Michaelis–Menten Model Describes the Kinetics of Many Enzymes

  • 7.3 Allosteric Enzymes Are Catalysts and Information Sensors

  • 7.4 Enzymes Can Be Studied One Molecule at a Time

Much of life is motion, whether at the macroscopic level of our daily life or at the molecular level of a cell. Studying motion is what motivated Eadweard James Muybridge to use stop-motion photography to analyze the gallop of a horse in 1878. In biochemistry, kinetics (derived from the Greek kinesis, meaning “movement”) is used to capture the dynamics of enzyme activity.

The primary function of enzymes is to accelerate the rates, or velocities, of reactions so that they are compatible with the needs of the organism. Thus, to understand how enzymes function, we need a kinetic description of their activity. This description will help us to quantify such kinetic parameters as how fast an enzyme can operate, how fast it will operate at substrate concentrations found in a cell, and what substrate is most readily operated on by the enzyme.

Some enzymes, however, need to do more than enhance the velocity of reactions. Metabolism in the cell is a complex array of dozens of metabolic pathways composed of thousands of different reactions, each catalyzed by a different enzyme. If all of these reactions were to take place in an unregulated fashion, metabolic chaos would result. An important class of enzymes called allosteric enzymes prevents this chaos and allows for the efficient integration of metabolism. These remarkable enzymes are not only catalysts, but also information sensors. They sense signals in the environment that allow them to adjust the rates of their reactions to meet the metabolic needs of the cell and facilitate the efficient coordination of the various metabolic pathways.

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In this chapter, we will examine a kinetic model that describes the activity of a class of enzymes called Michaelis–Menten enzymes that do not display sophisticated regulatory properties. Then, we will consider another class of enzymes, the allosteric enzymes, that are highly regulated. We will see many examples of these information-sensing enzymes throughout our study of biochemistry.