(2)Double-Displacement(Ping-Pong)Reactions 乒乓反应机制 In double-displacement,or ping-pong,reactions,one or more products are released before all substrates bind the enzyme. 在所有底物结合到酶前，就有一个或多个产物时放出来。 The defining feature of double-displacement reactions is the existence of a substituted enzyme intermediate,in which the enzyme is temporarily modified. 存在酶修饰的中间体 S1 P1 S2 P2 Enzyme Enzyme E (E-NH3) (-NH)= (E-NH3) E (aspartate) (oxaloacetate) (ketoglutarate) (glutamate)
(2)Double-Displacement (Ping-Pong) Reactions 乒乓反应机制 In double-displacement, or ping-pong, reactions, one or more products are released before all substrates bind the enzyme. 在所有底物结合到酶前，就有一个或多个产物时放出来。 The defining feature of double-displacement reactions is the existence of a substituted enzyme intermediate, in which the enzyme is temporarily modified. 存在酶修饰的中间体 S1 S2 P1 P2
Shuttle amino groups between amino acids and a-keto acids The aspartate aminotransferase门冬氨酸转氨酶 catalyzes the transfer of an amino group from aspartate to a-ketoglutarate. 00C C00- CH2 C00 CH2 H20 H29 H2 C00 —C00 H3N C00 +H3N C00 Aspartate a-Ketoglutarate Oxaloacetate Glutamate (草酰乙酸) not free enzyme Aspartate Oxaloacetate o-Ketoglutarate Glutamate Enzyme Enzyme E (E-NH3) =(-NH)三 (E-NH3) E (aspartate) (oxaloacetate) (c-ketoglutarate) (glutamate)
The aspartate aminotransferase 门冬氨酸转氨酶 catalyzes the transfer of an amino group from aspartate to α-ketoglutarate. Shuttle amino groups between amino acids and α-keto acids （草酰乙酸） not free enzyme
Allosteric Enzymes别构酶 Michaelis-Menten model cannot account for the kinetic properties of many enzymes.(局限性) An important group of enzymes that do not obey Michaelis-Menten kinetics are the allosteric enzymes (mu/tipie subunits and multiple active sites). 别构酶不符合米氏动力学；通常酶存在多个亚基和多个活性部 位 Figure 8.14 Kinetics for an allosteric enzyme.Allosteric enzymes display a sigmoidal dependence of reaction velocity on substrate concentration. S形曲线 Substrate concentration,[S]- 别构酶中，底物结合到一个酶活性部位后，会改变其他酶 结合部位的性质
Figure 8.14 Kinetics for an allosteric enzyme. Allosteric enzymes display a sigmoidal dependence of reaction velocity on substrate concentration. S形曲线 Allosteric Enzymes 别构酶 Michaelis- Menten model cannot account for the kinetic properties of many enzymes. (局限性） An important group of enzymes that do not obey Michaelis-Menten kinetics are the allosteric enzymes (multipie subunits and multiple active sites). 别构酶不符合米氏动力学；通常酶存在多个亚基和多个活性部 位 别构酶中，底物结合到一个酶活性部位后，会改变其他酶 结合部位的性质
Possible outcome of this interaction between subunits Cooperative effecti协同效应： the binding of substrate to one active site of the enzyme facilitates substrate binding to the other active sites. Substrate concentration,[S]- 存在别构分子： Allosteric molecule:regulatory molecules are reversibly bound to specific sites other than the catalytic sites. 别构分子：可逆结合到酶特定部位（非活性部位），起到 调节酶催化功能的作用。 酶活性可以迅速改变，因此在代谢途径中起到关键调节作 用（第10章介绍）
Cooperative effect协同效应： the binding of substrate to one active site of the enzyme facilitates substrate binding to the other active sites. Possible outcome of this interaction between subunits 存在别构分子： Allosteric molecule: regulatory molecules are reversibly bound to specific sites other than the catalytic sites. 别构分子：可逆结合到酶特定部位（非活性部位），起到 调节酶催化功能的作用。 酶活性可以迅速改变，因此在代谢途径中起到关键调节作 用（第10章介绍）
OUTLINES 1.Enzymes Are Powerful and Highly Specific Catalysts一概论 2.Free Energy Is a Useful Thermodynamic Function for Understanding Enzymes一热力学 3.Enzymes Accelerate Reactions by Facilitating the Formation of the Transition State一化学 4.The Michaelis-Menten Model Accounts for the Kinetic Properties of Many Enzymes一动力学 5.Enzymes Can Be Inhibited by Specific Molecules一抑制作用
1. Enzymes Are Powerful and Highly Specific Catalysts － 概论 2. Free Energy Is a Useful Thermodynamic Function for Understanding Enzymes －热力学 3. Enzymes Accelerate Reactions by Facilitating the Formation of the Transition State －化学 4. The Michaelis-Menten Model Accounts for the Kinetic Properties of Many Enzymes －动力学 5. Enzymes Can Be Inhibited by Specific Molecules －抑制作用 OUTLINES