8885dc061962/2/042:50 PM Page196mac76mac76:385reb Chapter 6 Enzymes this may be interpreted(qualitatively) to mean that 3% amino acid side chains, metal ions, and coenzymes). Cat of the available S will be converted to P in I s. A reac- alytic functional groups on an enzyme may form a tran tion with a rate constant of 2.000s will be over in a sient covalent bond with a substrate and activate it for small fraction of a second. If a reaction rate depends reaction, or a group may be transiently transferred from on the concentration of two different compounds, or the substrate to the enzyme. In many cases, these re- if the reaction is between two molecules of the same actions occur only in the enzyme active site. covalent compound, the reaction is second order and k is a interactions between enzymes and substrates lower the second-order rate constant, with units of M s. The activation energy (and thereby accelerate the reaction) rate equation then becomes by providing an alternative, lower-energy reaction path. V=kSS The specific types of rearrangements that occur are de- scribed in section 6.4 From transition-state theory we can derive an expres The second part of the explanation lies in the non- sion that relates the magnitude of a rate constant to the covalent interactions between enzyme and substrate Much of the energy required to lower activation ener gies is derived from weak, noncovalent interactions be- kT -AG+/RT tween substrate and enzyme. What really sets enzymes apart from most other catalysts is the formation of a where k is the Boltzmann constant and h is Planck's specific ES complex. The interaction between substrate constant. The important point here is that the relation- and enzyme in this complex is mediated by the same ship between the rate constant k and the activation en- forces that stabilize protein structure, including hydro- ergy AGt is inverse and exponential In simplified terms gen bonds and hydrophobic and ionic interactions this is the basis for the statement that a lower activa-(Chapter 4). Formation of each weak interaction in the tion energy means a faster reaction rate ES complex is accompanied by release of a small amount Now we turn from what enzymes do te ey of free energy that provides a degree of stability to the interaction. The energy derived from enzyme-substrate interaction is called binding energy, AGB. Its signifi A Few Principles Explain the Catalytic Power cance extends beyond a simple stabilization of the and Specificity of Enzyme enzyme-substrate interaction. Binding energy is a major source of free energy used by enzymes to lower Enzymes are extraordinary catalysts. The rate en- the activation energies of reactions hancements they bring about are in the range of 5 to 17 Two fundamental and interrelated principles pro orders of magnitude(Table 6-5). Enzymes are also very vide a general explanation for how enzymes use nonce- specific, readily discriminating between substrates with valent binding energy quite similar structures. How can these enormous and highly selective rate enhancements be explained? What 1. Much of the catalytic power of enzymes ultimately derived from the free energy released the activation energies for specific reactions? in forming many weak bonds and interactions The answer to these questions has two distinct but between an enzyme and its substrate. This binding interwoven parts. The first lies in the rearrangements energy contributes to specificity as well as to of covalent bonds during an enzyme-catalyzed reaction. catalysis Chemical reactions of many types take place between substrates and enzymes functional groups (specific Weak interactions are optimized in the reaction transition state; enzyme active sites are complementary not to the substrates per se but to TABLE 6-5 Some Rate Enhancements the transition states through which substrates pass Produced by Enzymes as they are converted to products during an enzymatic reaction. Cyclophilin Carbonic anhydrase These themes are critical to an understanding of en- Triose phosphate isomerase zymes, and they now become our primary focus Carboxypeptidase A 10 Phosphoglucomutase 1012 Weak Interactions between Enzyme and Substrate Succinyl-CoA transferase Are Optimized in the Transition State ease How does an enzyme use binding energy to lower the Orotidine monophosphate decarboxylase activation energy for a reaction? Formation of the es complex is not the explanation in itself, although somethis may be interpreted (qualitatively) to mean that 3% of the available S will be converted to P in 1 s. A reac￾tion with a rate constant of 2,000 s
1 will be over in a small fraction of a second. If a reaction rate depends on the concentration of two different compounds, or if the reaction is between two molecules of the same compound, the reaction is second order and k is a second-order rate constant, with units of M
1 s
1 . The rate equation then becomes V k[S1][S2] (6–5) From transition-state theory we can derive an expres￾sion that relates the magnitude of a rate constant to the activation energy: k k h T e
G‡/RT (6–6) where k is the Boltzmann constant and h is Planck’s constant. The important point here is that the relation￾ship between the rate constant k and the activation en￾ergy G‡ is inverse and exponential. In simplified terms, this is the basis for the statement that a lower activa￾tion energy means a faster reaction rate. Now we turn from what enzymes do to how they do it. A Few Principles Explain the Catalytic Power and Specificity of Enzymes Enzymes are extraordinary catalysts. The rate en￾hancements they bring about are in the range of 5 to 17 orders of magnitude (Table 6–5). Enzymes are also very specific, readily discriminating between substrates with quite similar structures. How can these enormous and highly selective rate enhancements be explained? What is the source of the energy for the dramatic lowering of the activation energies for specific reactions? The answer to these questions has two distinct but interwoven parts. The first lies in the rearrangements of covalent bonds during an enzyme-catalyzed reaction. Chemical reactions of many types take place between substrates and enzymes’ functional groups (specific amino acid side chains, metal ions, and coenzymes). Cat￾alytic functional groups on an enzyme may form a tran￾sient covalent bond with a substrate and activate it for reaction, or a group may be transiently transferred from the substrate to the enzyme. In many cases, these re￾actions occur only in the enzyme active site. Covalent interactions between enzymes and substrates lower the activation energy (and thereby accelerate the reaction) by providing an alternative, lower-energy reaction path. The specific types of rearrangements that occur are de￾scribed in Section 6.4. The second part of the explanation lies in the non￾covalent interactions between enzyme and substrate. Much of the energy required to lower activation ener￾gies is derived from weak, noncovalent interactions be￾tween substrate and enzyme. What really sets enzymes apart from most other catalysts is the formation of a specific ES complex. The interaction between substrate and enzyme in this complex is mediated by the same forces that stabilize protein structure, including hydro￾gen bonds and hydrophobic and ionic interactions (Chapter 4). Formation of each weak interaction in the ES complex is accompanied by release of a small amount of free energy that provides a degree of stability to the interaction. The energy derived from enzyme-substrate interaction is called binding energy,
GB. Its signifi￾cance extends beyond a simple stabilization of the enzyme-substrate interaction. Binding energy is a major source of free energy used by enzymes to lower the activation energies of reactions. Two fundamental and interrelated principles pro￾vide a general explanation for how enzymes use nonco￾valent binding energy: 1. Much of the catalytic power of enzymes is ultimately derived from the free energy released in forming many weak bonds and interactions between an enzyme and its substrate. This binding energy contributes to specificity as well as to catalysis. 2. Weak interactions are optimized in the reaction transition state; enzyme active sites are complementary not to the substrates per se but to the transition states through which substrates pass as they are converted to products during an enzymatic reaction. These themes are critical to an understanding of en￾zymes, and they now become our primary focus. Weak Interactions between Enzyme and Substrate Are Optimized in the Transition State How does an enzyme use binding energy to lower the activation energy for a reaction? Formation of the ES complex is not the explanation in itself, although some 196 Chapter 6 Enzymes Cyclophilin 105 Carbonic anhydrase 107 Triose phosphate isomerase 109 Carboxypeptidase A 1011 Phosphoglucomutase 1012 Succinyl-CoA transferase 1013 Urease 1014 Orotidine monophosphate decarboxylase 1017 TABLE 6–5 Some Rate Enhancements Produced by Enzymes 8885d_c06_196 2/2/04 2:50 PM Page 196 mac76 mac76:385_reb: