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8885dc06190-2371/27/047:13 AM Page194mac76mac76:385 Chapter 6 Enzymes Transition state(+ either substrate or product is equally likely. The differ ence between the energy levels of the ground state and the transition state is the activation energy, AG+. The rate of a reaction reflects this activation energy: a higher activation energy corresponds to a slower reaction. Re- △G"° action rates can be increased by raising the tempera gRound ture, thereby increasing the number of molecules with sufficient energy to overcome the energy barrier. Alter state natively, the activation energy can be lowered by adding Reaction coordinate a catalyst(Fig. 6-3) Catalysts enhance reaction rates by lowering activation energies. FIGURE 6-2 Reaction coordinate diagram for a chemical reaction Enzymes are no exception to the rule that catalysts The free energy of the system is plotted against the progress of the re. do not affect reaction equilibria. The bidirectional ar- action S-P. A diagram of this kind is a description of the energy rows in Equation 6-1 make this point: any enzyme that hanges during the reaction, and the horizontal axis(reaction coor- catalyzes the reaction s-P also catalyzes the reaction dinate)reflects the progressive chemical changes (e., bond breakage P-S. The role of enzymes is to accelerate the inter or formation) as S is converted to P The activation energies, AG, for conversion of S and P. The enzyme is not used up in the the s-P and P-Sreactions are indicated AC" is the overall stan- process, and the equilibrium point is unaffected. How dard free-energy change in the direction S-P. ever, the reaction reaches equilibrium much faster when the appropriate enzyme is present, because the rate of the reaction is increased of each solute 1 M) and express the free-energy change This general principle can be illustrated by consid- for this reacting system as AG, the standard free- ering the conversion of sucrose and oxygen to carbon energy change Because biochemical systems commonly involve ht concentrations far below 1 M. biochemists define a biochemical standard free-energy change C12H22O1+1202→12CO2+11H2O AG, the standard free-energy change at pH 7.0, we This conversion, which takes place through a series of employ this definition throughout the book. A more separate reactions, has a very large and negative AG complete definition of AG is given in Chapter 13 and at equilibrium the amount of sucrose present is neg The equilibrium between S and P reflects the dif- ligible. Yet sucrose is a stable compound, because the ference in the free energies of their ground states. In activation energy barrier that must be overcome before the example shown in Figure 6-2, the free energy of the sucrose reacts with oxygen is quite high. Sucrose can ground state of P is lower than that of S, So AG for the be stored in a container with oxygen almost indefinitely reaction is negative and the equilibrium favors P. The without reacting. In cells, however, sucrose is readily position and direction of equilibrium are not affected by broken down to co and Ho in a series of reactions any catalyst catalyzed by enzymes. These enzymes not only accel A favorable equilibrium does not mean that the s-P conversion will occur at a detectable rate. The rate of a reaction is dependent on an entirely different parameter. There is an energy barrier between S and P Transition state(+) the energy required for alignment of reacting groups formation of transient unstable charges, bond re arrangements, and other transformations required for the reaction to proceed in either direction. This is il lustrated by the energy "hill"in Figures 6-2 and 6-3. To undergo reaction, the molecules must overcome this barrier and therefore must be raised to a higher energy level. At the top of the energy hill is a point at which decay to the s or P state is equally probable (it is down- Reaction coordinate hill either way). This is called the transition state. The FIgure 6-3 Reaction coordinat transition state is not a chemical species with any sig catalyzed and uncatalyzed reactions. In the reaction S-P, the ES nificant stability and should not be confused with a re- and EP intermediates occupy minima in the energy progress curve of action intermediate(such as Es or EP). It is simply a the enzyme-catalyzed reaction. The terms AGuncat and ACtat corre- fleeting molecular moment in which events such as bond spond to the activation energy for the uncatalyzed reaction and the breakage, bond formation, and charge development overall activation energy for the catalyzed reaction, respectively.The have proceeded to the precise point at which decay to activation energy is lower when the enzyme catalyzes the reactionof each solute 1 M) and express the free-energy change for this reacting system as G, the standard freeenergy change. Because biochemical systems commonly involve H concentrations far below 1 M, biochemists define a biochemical standard free-energy change, G, the standard free-energy change at pH 7.0; we employ this definition throughout the book. A more complete definition of G is given in Chapter 13. The equilibrium between S and P reflects the difference in the free energies of their ground states. In the example shown in Figure 6–2, the free energy of the ground state of P is lower than that of S, so G for the reaction is negative and the equilibrium favors P. The position and direction of equilibrium are not affected by any catalyst. A favorable equilibrium does not mean that the S n P conversion will occur at a detectable rate. The rate of a reaction is dependent on an entirely different parameter. There is an energy barrier between S and P: the energy required for alignment of reacting groups, formation of transient unstable charges, bond rearrangements, and other transformations required for the reaction to proceed in either direction. This is illustrated by the energy “hill” in Figures 6–2 and 6–3. To undergo reaction, the molecules must overcome this barrier and therefore must be raised to a higher energy level. At the top of the energy hill is a point at which decay to the S or P state is equally probable (it is downhill either way). This is called the transition state. The transition state is not a chemical species with any significant stability and should not be confused with a reaction intermediate (such as ES or EP). It is simply a fleeting molecular moment in which events such as bond breakage, bond formation, and charge development have proceeded to the precise point at which decay to either substrate or product is equally likely. The difference between the energy levels of the ground state and the transition state is the activation energy, G‡ . The rate of a reaction reflects this activation energy: a higher activation energy corresponds to a slower reaction. Reaction rates can be increased by raising the temperature, thereby increasing the number of molecules with sufficient energy to overcome the energy barrier. Alternatively, the activation energy can be lowered by adding a catalyst (Fig. 6–3). Catalysts enhance reaction rates by lowering activation energies. Enzymes are no exception to the rule that catalysts do not affect reaction equilibria. The bidirectional arrows in Equation 6–1 make this point: any enzyme that catalyzes the reaction S n P also catalyzes the reaction P n S. The role of enzymes is to accelerate the interconversion of S and P. The enzyme is not used up in the process, and the equilibrium point is unaffected. However, the reaction reaches equilibrium much faster when the appropriate enzyme is present, because the rate of the reaction is increased. This general principle can be illustrated by considering the conversion of sucrose and oxygen to carbon dioxide and water: C12H22O11 12O2 88n 12CO2 11H2O This conversion, which takes place through a series of separate reactions, has a very large and negative G, and at equilibrium the amount of sucrose present is negligible. Yet sucrose is a stable compound, because the activation energy barrier that must be overcome before sucrose reacts with oxygen is quite high. Sucrose can be stored in a container with oxygen almost indefinitely without reacting. In cells, however, sucrose is readily broken down to CO2 and H2O in a series of reactions catalyzed by enzymes. These enzymes not only accel- 194 Chapter 6 Enzymes Transition state (‡) Free energy, G Reaction coordinate S Ground state P Ground state G‡ S P G‡ P S G FIGURE 6–2 Reaction coordinate diagram for a chemical reaction. The free energy of the system is plotted against the progress of the reaction Sn P. A diagram of this kind is a description of the energy changes during the reaction, and the horizontal axis (reaction coordinate) reflects the progressive chemical changes (e.g., bond breakage or formation) as S is converted to P. The activation energies, G‡ , for the Sn P and Pn S reactions are indicated. G is the overall standard free-energy change in the direction Sn P. Transition state (‡) Reaction coordinate S P G‡ uncat G‡ cat ‡ ES EP Free energy, G FIGURE 6–3 Reaction coordinate diagram comparing enzymecatalyzed and uncatalyzed reactions. In the reaction Sn P, the ES and EP intermediates occupy minima in the energy progress curve of the enzyme-catalyzed reaction. The terms G‡ uncat and G‡ cat correspond to the activation energy for the uncatalyzed reaction and the overall activation energy for the catalyzed reaction, respectively. The activation energy is lower when the enzyme catalyzes the reaction. 8885d_c06_190-237 1/27/04 7:13 AM Page 194 mac76 mac76:385_reb: