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Ligand binding is the principal determinant of stability for the p21(H-ras) protein

机译：配体结合是p21（H-ras）蛋白稳定性的主要决定因素

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p21(H-ras) is a 21 kDa, alpha/beta sheet protein that, as a member of the GTPase superfamily, acts as a molecular switch in signal transduction pathways. The essential role of GDP and Mg2+ in maintaining the inactive conformation of p21(H-ras) prompted a study of the influence of these ligands on its structure and stability. The urea-induced equilibrium unfolding transitions for the ternary (p21 . GDP . Mg2+), binary (p21 GDP) and apo (p21) forms of p21(H-ras) at pH 7.5 and 25 degrees C were monitored by absorbance and circular dichroism spectroscopies. The cooperative disruptions of the secondary and tertiary structures for all three forms are well-described by a two-state model. Global analysis of the equilibrium unfolding data yields a free energy of folding in the absence of urea and under standard state conditions of 14.1 +/- 0.2 kcal mol(-1), 7.5 +/- 0.4 kcal mol(-1) and 1.8 +/- 0.2 kcal mol(-1) for ternary, binary and apo forms, respectively. Near- and far-UV circular dichroism spectra of these three forms of p21(H-ras) show that removal of the Mg2+ from the ternary complex loosens the aromatic side chain packing but leaves the secondary structure largely unchanged. The removal of both GDP and Mg2+ completely releases the side chain packing but leaves a substantial fraction of the secondary structure intact. These results demonstrate that ligands play a significant role in the stability and structure of the p21 GDP Mg2+ complex. The amino acid sequence itself only contains sufficient information to direct the formation of a large portion of the secondary structure in a molten globule-like state. Ligand binding is required to drive the formation of specific tertiary structure. [References: 49]
机译：p21（H-ras）是一种21 kDa的alpha / beta折叠蛋白，作为GTPase超家族的成员，在信号转导途径中充当分子开关。 GDP和Mg2 +在维持p21（H-ras）的非活性构象中的重要作用促使人们研究了这些配体对其结构和稳定性的影响。通过吸光度和圆二色性监测了在pH 7.5和25摄氏度下三元（p21。GDP。Mg2 +），二元（p21 GDP）和apo（p21）形式的p21（H-ras）脲诱导的平衡展开转变。光谱学。这两种形式的二级和三级结构的协同破坏都可以通过二态模型很好地描述。平衡展开数据的全局分析会在没有尿素且标准状态下分别为14.1 +/- 0.2 kcal mol（-1），7.5 +/- 0.4 kcal mol（-1）和1.8 +的情况下产生折叠自由能分别为三元，二元和载脂蛋白形式的0.2 kcal mol（-1）。这三种形式的p21（H-ras）的近紫外和远紫外圆二色性光谱表明，从三元络合物中除去Mg2 +会使芳族侧链堆积松散，但二级结构基本上保持不变。 GDP和Mg2 +的去除都完全释放了侧链堆积，但大部分二级结构保持完整。这些结果表明，配体在p21 GDP Mg2 +复合物的稳定性和结构中起着重要作用。氨基酸序列本身仅包含足以指导大部分二级结构以熔融小球状状态形成的信息。需要配体结合以驱动特定三级结构的形成。 [参考：49]

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