Ladurner A G, Itzhaki L S, Fersht A R
MRC Cambridge Centre for Protein Engineering, MRC Centre, UK.
Fold Des. 1997;2(6):363-8. doi: 10.1016/S1359-0278(97)00050-3.
Chymotrypsin inhibitor 2 (CI2) is a member of the class of fast-folding small proteins, which is very suitable for testing theories of folding. CI2 folds around a diffuse extended nucleus consisting of the single alpha helix and a set of hydrophobic residues. In particular, Ala16 has been predicted and independently found to interact with Leu49 and Ile57, hydrophobic residues that are highly conserved among homologues. We have characterised in detail the interactions between these residues in the folding nucleus of the protein by using double-mutant cycles.
Surprisingly, we find that there is some destabilising strain in the transition state for folding of the wild-type protein between Ala16 and Ile57. Further, we find that the strain is larger in the native state of the protein. This is shown directly in the unfolding kinetics, which clearly show a release of strain. The net result of this is that the presence of both residues speeds up folding. Ala16 and Leu49 interact favourably in the transition state, but have no net interaction energy in the native state.
Part of the folding nucleus of the protein fits together more snugly in the transition state than it does in the native state. Interactions between some of the closely packed residues in the folding nucleus of CI2 may perhaps be optimised for the rate of folding and not for stability.
胰凝乳蛋白酶抑制剂2（CI2）是快速折叠小蛋白家族的成员，非常适合用于检验折叠理论。CI2围绕由单个α螺旋和一组疏水残基组成的弥散伸展核进行折叠。特别地，已预测并独立发现Ala16与Leu49和Ile57相互作用，这两个疏水残基在同源物中高度保守。我们通过使用双突变循环详细表征了该蛋白折叠核中这些残基之间的相互作用。
令人惊讶的是，我们发现在野生型蛋白从Ala16到Ile57的折叠过渡态中存在一些不稳定应变。此外，我们发现该应变在蛋白的天然状态下更大。这在解折叠动力学中直接显示出来，其清楚地表明了应变的释放。其最终结果是这两个残基的存在加速了折叠。Ala16和Leu49在过渡态中相互作用良好，但在天然状态下没有净相互作用能。
该蛋白折叠核的一部分在过渡态中比在天然状态下结合得更紧密。CI2折叠核中一些紧密堆积残基之间的相互作用可能是为了折叠速率而非稳定性而优化的。
