Boudker O, Todd M J, Freire E
Department of Biology, Johns Hopkins University Baltimore, MD 21218, USA.
J Mol Biol. 1997 Oct 10;272(5):770-9. doi: 10.1006/jmbi.1997.1263.
The structural stability of the co-chaperonin GroES has been studied by high sensitivity differential scanning calorimetry and circular dichroism under different solvent conditions. The thermal folding/unfolding of GroES is a spontaneous reversible process involving a highly cooperative transition between folded heptamers and unfolded monomers. During the denaturation process folded monomers are energetically unfavourable and consequently never become populated to an appreciable degree. Analysis of the high resolution structure indicates that isolated folded monomers of GroES bury a significantly smaller fraction of their total surface than typical globular proteins of similar molecular mass. For this reason the intramolecular interactions within each GroES monomer appear not to be sufficient for thermodynamic stabilization. The stabilization of the heptameric structure is due primarily to intersubunit interactions rather than intrasubunit interactions. These interactions favor oligomerization both enthalpically and entropically. Despite the high density of charged residues, the stability of GroES shows no measurable dependence on salt concentration at pH 7. On the other hand, millimolar concentrations of magnesium stabilize GroES, presumably by specific binding. The stabilization elicited by Mg2+ is consistent with a dissociation constant of the order of 0.5 mM and approximately three binding sites per heptamer. These results emphasize the role of quaternary structure in the stabilization of small oligomeric proteins.
已通过高灵敏度差示扫描量热法和圆二色性在不同溶剂条件下研究了伴侣蛋白GroES的结构稳定性。GroES的热折叠/去折叠是一个自发的可逆过程，涉及折叠的七聚体和未折叠的单体之间的高度协同转变。在变性过程中，折叠的单体在能量上是不利的，因此其数量从未达到可观的程度。对高分辨率结构的分析表明，与类似分子量的典型球状蛋白相比，孤立的折叠GroES单体掩埋的总表面积比例要小得多。因此，每个GroES单体内部的分子内相互作用似乎不足以实现热力学稳定。七聚体结构的稳定主要归因于亚基间相互作用而非亚基内相互作用。这些相互作用在焓和熵方面都有利于寡聚化。尽管带电残基密度很高，但在pH 7时，GroES的稳定性对盐浓度没有可测量的依赖性。另一方面，毫摩尔浓度的镁可稳定GroES，推测是通过特异性结合。Mg2+引起的稳定作用与约0.5 mM的解离常数和约每个七聚体三个结合位点一致。这些结果强调了四级结构在小寡聚蛋白稳定中的作用。
