Changes in quaternary structure on binding PALA PALA与ATCase:结合导致酶蛋白四级结构发生显著变化 The two catalytic trimers move 12 A farther apart and rotate approximately 10 degrees;the regulatory dimers rotate approximately 15 degrees. 两个催化三聚体亚基之间疏远了12A,旋转了10°。相应地，调节亚基也 旋转了15以适应这些变化。 6A 10 PALA PAL PALA 6A T state R state Figure 10.9 The T-to-R state transition in ATCase. ATCase:有两种四级结构模式。 1.紧缩型结构(T),是没有与底物或底物类似物结合时处于优势的结构； 2.松弛型结构(R),是与底物或底物类似物结合时占优势的结构
Figure 10.9 The T-to-R state transition in ATCase. ATCase有两种四级结构模式。 1. 紧缩型结构(T)，是没有与底物或底物类似物结合时处于优势的结构； 2. 松弛型结构(R)，是与底物或底物类似物结合时占优势的结构。 The two catalytic trimers move 12 Å farther apart and rotate approximately 10 degrees; the regulatory dimers rotate approximately 15 degrees. 两个催化三聚体亚基之间疏远了12 Å ，旋转了10o。相应地，调节亚基也 旋转了15o以适应这些变化。 Changes in quaternary structure on binding PALA PALA与ATCase结合导致酶蛋白四级结构发生显著变化
The enzyme's sigmoidal kinetics In light of the structural observations? T(全无状态)oibU四R (全有状态) In the absence of substrate,almost all the enzyme molecules are in the T state.没有底物分子存在时，几乎所有的酶蛋白都是T态。 T state:a low affinity for substrate and hence shows a low catalytic activity. The occasional binding of a substrate molecule to one active site in an enzyme increases the likelihood that the entire enzyme shifts to the R state with its higher binding affinity.偶然与底物分子结合， 导致整个酶分子转变成亲和力高的R态。 增加底物亲和力产生两种效应： 一是使每个酶分子至少结合一个底物分子的几率大大增加； 其次是增加每个酶分子结合底物的平均数值
In light of the structural observations? In the absence of substrate, almost all the enzyme molecules are in the T state.没有底物分子存在时，几乎所有的酶蛋白都是T态。 T state：a low affinity for substrate and hence shows a low catalytic activity. The occasional binding of a substrate molecule to one active site in an enzyme increases the likelihood that the entire enzyme shifts to the R state with its higher binding affinity.偶然与底物分子结合， 导致整个酶分子转变成亲和力高的R态。 T（全无状态） R （全有状态） Equilibrium The enzyme's sigmoidal kinetics 增加底物亲和力产生两种效应： 一是 使每个酶分子至少结合一个底物分子的几率大大增加； 其次 是增加每个酶分子结合底物的平均数值
Cooperatively协同性 The presence of additional substrate will increase the fraction of enzyme molecules in the more active R state because the position of the equilibrium depends on the number of active sites that are occupied by substrate底物增加了R态酶的比例， 平衡取决于活性位点填充底物的数量 10 20 30 40 [Aspartate],mM Cooperatively(协同性)：the Substrate subunits cooperate with one another. 协同性：亚基协同变为某一状态。 酶蛋白分子显示“全有”或“全无” 两种状态。 inactive form Active form stabillzed of enzyme by a substrate molecule
The presence of additional substrate will increase the fraction of enzyme molecules in the more active R state because the position of the equilibrium depends on the number of active sites that are occupied by substrate 底物增加了R态酶的比例， 平衡取决于活性位点填充底物的数量 Cooperatively 协同性 Cooperatively (协同性) ：the subunits cooperate with one another. 协同性：亚基协同变为某一状态。 酶蛋白分子显示“全有”或“全无” 两种状态
The concerted mechanism(协同机制)：the change in the enzyme is "all or none",the entire enzyme is converted from T into R,affecting all of the catalytic sites equally..酶蛋白分子 显示“全有”或“全无”两种状态。酶蛋白从T转化成R影响所有 的活性位点。对酶分子所有催化位点的影响是等同的 The sequential model(序变模型)：the binding of ligand to one site on the complex can affect neighboring sites without causing all subunits to undergo the T-to-R transition.假定底物 分子与酶蛋白结合仅增加相邻位点活性，而不会导致所有位点都发 生T向R的转化
The concerted mechanism (协同机制) ： the change in the enzyme is “all or none”, the entire enzyme is converted from T into R, affecting all of the catalytic sites equally.酶蛋白分子 显示“全有”或“全无”两种状态。酶蛋白从T转化成R影响所有 的活性位点。对酶分子所有催化位点的影响是等同的 The sequential model (序变模型) ：the binding of ligand to one site on the complex can affect neighboring sites without causing all subunits to undergo the T-to-R transition.假定底物 分子与酶蛋白结合仅增加相邻位点活性，而不会导致所有位点都发 生T向R的转化
Basis for the sigmoidal curve The sigmoidal curve for R-state curve ATCase can be pictured as a composite of two Michaelis- ↑ Menten curves,An increase in substrate concentration T-state curve favors a transition from the T- state curve to the R-state curve. ATCase的S型曲线可以看作是两 [Aspartate]→ 种米氏酶（一种是T态酶，另一种 是R态酶)酶促反应的叠加。底物 One with a high value of Km:T state 浓度的增加有助于酶促反应从T One with a low value of Km:R state. 态曲线转化成R态曲线
One with a high value of Km： T state One with a low value of Km：R state. The sigmoidal curve for ATCase can be pictured as a composite of two MichaelisMenten curves, An increase in substrate concentration favors a transition from the Tstate curve to the R-state curve. ATCase的S型曲线可以看作是两 种米氏酶(一种是T态酶，另一种 是R态酶)酶促反应的叠加。底物 浓度的增加有助于酶促反应从T 态曲线转化成R态曲线 Basis for the sigmoidal curve