Otzen D E, Fersht A R
MRC Unit for Protein Function and Design, University Chemical Laboratory, Cambridge, UK.
Biochemistry. 1995 May 2;34(17):5718-24. doi: 10.1021/bi00017a003.
beta-Sheet propensities of different amino acids depend on the context of both secondary and tertiary structure. In an attempt to establish general empirical relationships that determine this context dependence, we have determined the free energy of unfolding of a series of mutants at six positions in the beta-sheet of chymotrypsin inhibitor 2 (CI2). We have generated the series Val-->Ala-->Gly and Val<==>Thr at five positions, as well as the side-chain deletion Ile-->Val at residue 49 and Ala-->Gly at residue 77. In the series Val-->Ala-->Gly, the ranking order in terms of stability is Val > Ala > Gly at all positions. However, the change in free energy on deletion of methylene groups varies greatly. When Val and Thr are interchanged, the wild-type residue is always the more stable, but by a different amount at each position. We have attempted to rationalize the data by relating it to changes in solvent-accessible surface area, packing density, and statistically derived pseudo-energy functions that depend on phi, psi angles. There is no significant correlation of the energies with any of the variables except with the pseudo-energy function, but the deviations from these values are large. We conclude that thermodynamic scales for beta-sheet propensity are currently of insufficient precision for general design purposes, although they may be useful in special cases.
不同氨基酸的β-折叠倾向取决于二级和三级结构的背景。为了建立确定这种背景依赖性的一般经验关系，我们测定了一系列在胰凝乳蛋白酶抑制剂2（CI2）的β-折叠六个位置处突变体的解折叠自由能。我们在五个位置生成了Val→Ala→Gly和Val⇔Thr系列，以及在49位残基处的侧链缺失Ile→Val和在77位残基处的Ala→Gly。在Val→Ala→Gly系列中，就稳定性而言，在所有位置的排名顺序都是Val＞Ala＞Gly。然而，亚甲基缺失时自由能的变化差异很大。当Val和Thr互换时，野生型残基总是更稳定，但在每个位置的稳定程度不同。我们试图通过将数据与溶剂可及表面积、堆积密度以及依赖于φ、ψ角的统计推导伪能量函数的变化联系起来，对数据进行合理化解释。除了伪能量函数外，能量与任何变量都没有显著相关性，但与这些值的偏差很大。我们得出结论，尽管β-折叠倾向的热力学标度在特殊情况下可能有用，但目前其精度不足以用于一般设计目的。
