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SECTION 3

Basic Concepts and Kinetics of Enzymes

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CHAPTER 6 Basic Concepts of Enzyme Action

CHAPTER 7 Kinetics and Regulation

CHAPTER 8 Mechanisms and Inhibitors

CHAPTER 9 Hemoglobin, an Allosteric Protein

In Section 2, we considered the chemical workhorses of life—proteins. In this section, we examine an important class of proteins called enzymes. Protein enzymes are the most prominent catalysts of biological systems. Catalysts are chemicals that enhance the rate of reactions without being permanently altered themselves. The role of enzymes, then, is to make biochemically required reactions take place at a rate compatible with life.

Proteins as a class of macromolecules are well suited to be catalysts because of their capacity to form complex three-dimensional structures that can recognize one or a few molecules with high specificity. Collectively, the range of molecules on which enzymes can act is virtually unlimited. Because of their specificity, enzymes bring substrates (reactants) together at a particular site on the enzyme, called the active site, where they are oriented to facilitate the making and breaking of chemical bonds. The most striking characteristics of enzymes are their catalytic power and specificity.

Some enzymes are information sensors as well as catalysts. In addition to active sites, allosteric enzymes have distinct regulatory sites that bind to environmental signals. This binding modifies the activity of the active site.

We begin this section with a look at the basic properties of enzymes, with special emphasis on the energetics of enzyme-catalyzed reactions. We then move on to a kinetic analysis of enzymes. Next, we see how enzyme activity is modified by environmental conditions and examine the mechanism of action of chymotrypsin, a protein-digesting enzyme. We end the section with an examination of hemoglobin. This oxygen-transporting protein is a source of insight into the properties of allosteric proteins.

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✓ By the end of this section, you should be able to:

✓ 1 Describe the relations between the enzyme catalysis of a reaction, the thermodynamics of the reaction, and the formation of the transition state.
✓ 2 Explain the relation between the transition state and the active site of an enzyme, and list the characteristics of active sites.
✓ 3 Explain what reaction velocity is.
✓ 4 Explain how reaction velocity is determined and how reaction velocities are used to characterize enzyme activity.
✓ 5 Identify the key properties of allosteric proteins, and describe the structural basis for these properties.
✓ 6 List environmental factors that affect enzyme activity, and describe how these factors exert their effects on enzymes.
✓ 7 Explain how allosteric properties contribute to hemoglobin function.
✓ 8 Identify the key regulators of hemoglobin function.