308_p3 - CHAPTER By the end of this section you should be able to B a s i c 1 Describe the relations between the enzyme catalysis of a reaction the

308_p3 - CHAPTER By the end of this section you should be...

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C H A P T E R / 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 andHhe 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. 6.1 Enzymes Are Powerful and Highly Specific Catalysts 6.2 Many Enzymes Require Cofactors for Activity 6.3 Gibbs Free Energy Is a Useful Thermodynamic Function for Understanding Enzymes 6.4 Enzymes Facilitate the Formation of the Transition State B a s i c C o n c e p t s o f E n z y m e A c t i o n The activity of an enzyme is responsible for the glow of the luminescent jellyfish. The enzyme aequorin catalyzes the oxidation of a compound by oxygen in the presence of calcium to release C0 2 and light. [chain45154/Cetty Images.] HI iiiiiiiiiiiini iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiMiii iiiiHiiiiiiiiiiiiiiiiiiiii iiiiiiiiiiuii iiiiiiiiiiiiiiiiiiiiiiniiiii nun i iiiiiiii i iiiiiiii 0 C + H 2 0 0 HO" ^OH 96 T he energy and information processing that takes place inside a cell consists of thousands of individual chemical reactions. For these reactions to take place in a physiologically useful fashion, they must occur at a rate that meets the cell's needs, and they must display specificity; that is, a particular reactant should always yield a particular product. Side reactions leading to the formation of useless or hazardous by-products must be minimized. In this chapter, we consider the key prop- erties of enzymes, with a special look at the energetics of enzyme-catalyzed reactions. IItlf«fl9StSfll[l(llllllIflttltltlll11fllltlIllllIllfltllIIIIIiIlttilfIlllllIIIIIIIIItlIIIIlllllllilllllllliltllIII1lllri(lllillIljtlltlllllIllllIlIillilllllIilI»llllll 6.1 Enzymes Are Powerful and Highly Specific Catalysts Enzymes accelerate the rate of reactions by factors of as much as a million or more (Table 6.1). Indeed, most reactions in biological systems do not take place at perceptible rates in the absence of enzymes. Even a reaction as simple as adding water to carbon dioxide is catalyzed by an enzyme—namely, carbonic anhydrase. This reaction facilitates the transport of carbon dioxide from the tissues where it is produced to the lungs where it is exhaled. Carbonic anhydrase is one of the fastest known enzymes. Each enzyme molecule can hydrate 10 6 molecules of C0 2 per second. This catalyzed reaction is 10 7 times as fast as the uncatalyzed one. The transfer of C0 2
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