Thermodynamics of assembly of Escherichia coli aspartate transcarbamoylase.
Thermodynamics of assembly of Escherichia coli aspartate transcarbamoylase.
复制标题
大肠杆菌天冬氨酸转氨甲酰酶组装的热力学。
DOI：
10.1073/pnas.80.22.6824
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发表时间：
1983
影响因子：
11.1
通讯作者：
Allewell,NM
中科院分区：
文献类型：
--
作者：
McCarthy,MP;Allewell,NM
反应微量热法和电位测定法已被用于确定大肠杆菌天冬氨酸氨基甲酸转移酶（天冬氨酸氨基甲酰转移酶，氨基甲酰磷酸：L-天冬氨酸氨基甲酰转移酶，EC 2.1.3.2）从其催化和调节亚基组装的热力学以及组装和质子结合之间的联系。在pH范围7-9.5和温度范围15-30摄氏度内，组装的特征在于负焓和热容变化以及正熵变化。焓变和熵变对pH值的依赖性很复杂;然而，负的热容变化导致这两个量随着温度的升高而变得更负。组装与质子的结合有关;观察到的效应可以拟合到涉及六个或更多个可电离基团的模型，其pK值为7.3-7.4，8.5-8.8和9.2-9.5，它们相互协作。不能排除其他群体的贡献，事实上，这是预期的。热力学效应的总体模式意味着一组复杂的亚基间相互作用。质子化反应和增加的氢键可能是负焓变的主要来源;然而，负热容量变化主要是由于与疏水和静电键形成相关的溶剂结构的变化，低频振动模式的变化起次要作用。同样，相对较小的熵变内观察到的温度范围内检查可能反映了积极的贡献之间的平衡，从增加的疏水性和静电键合和消极的贡献，从增加氢键和阻尼的低频振动模式。
Reaction microcalorimetry and potentiometry have been used to define the thermodynamics of assembly of Escherichia coli aspartate transcarbamoylase (aspartate carbamoyltransferase, carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) from its catalytic and regulatory subunits and the linkage between assembly and proton binding. Over the pH range 7-9.5 and the temperature range 15-30 degrees C, assembly is characterized by negative enthalpy and heat capacity changes and positive entropy changes. The dependence of the enthalpy and entropy changes on pH is complex; however, the negative heat capacity change results in both quantities becoming more negative with increasing temperature. Assembly is linked to the binding of protons; the effects observed can be fit to models involving six or more ionizable groups with pK values of 7.3-7.4, 8.5-8.8, and 9.2-9.5, which ionize cooperatively. Contributions from additional groups cannot be ruled out and are in fact expected. The overall pattern of thermodynamic effects implies a complex set of intersubunit interactions. Protonation reactions and increased hydrogen bonding are likely to be the major sources of the negative enthalpy change; however, the negative heat capacity change results primarily from changes in solvent structure associated with hydrophobic and electrostatic bond formation with changes in low-frequency vibrational modes making a secondary contribution. Similarly, the relatively small entropy change observed within the temperature range examined probably reflects the balance between positive contributions from increased hydrophobic and electrostatic bonding and negative contributions from increased hydrogen bonding and damping of low-frequency vibrational modes.