Callihan D E, Logan T M
Department of Chemistry and Program in Structural Biology, Florida State University, Tallahassee, FL, 32306-4380, USA.
J Mol Biol. 1999 Feb 5;285(5):2161-75. doi: 10.1006/jmbi.1998.2440.
The helix-forming tendency of seven peptide fragments corresponding with the entire sequence of the FK506 binding protein (FKBP) has been investigated in aqueous buffer and in 2,2,2-trifluoroethanol (TFE) using CD and NMR spectroscopy. All fragments exhibited random coil conformations in aqueous buffer, whereas the amount of helix induced in the peptide fragments by TFE varied. The fragment with the highest degree of helicity in TFE corresponded with the single (alpha-helix in native FKBP. Fragments corresponding with beta-strands 2 and 3 also exhibited strong propensity towards helix formation. In contrast, the fragment corresponding with beta-strand 1 did not form helix in TFE. The inherent helix-forming tendencies are interpreted in light of the native structure to suggest possible folding nucleation sites. Conformational sampling in each peptide fragment was also compared with that observed in urea-denatured FKBP. With the exception of the fragment corresponding with beta-strand 2, the formation of helical structures in the peptide fragments in TFE was correlated with the observation of turn and/or helix conformers in urea-unfolded FKBP. Surprisingly, peptide fragments in aqueous solution were less structured than the corresponding regions in urea-denatured FKBP. The conformational differences between the peptide fragments and unfolded FKBP were not due to the urea buffer or to differences in their rotational correlation times. We conclude that local amino acid interactions are not generally sufficient to account for the formation of non-random conformations in unfolded FKBP. Formation of non-random structures in unfolded FKBP may require stabilization of incipient turn or helical conformations through transient contact with non-local non-polar residues.
利用圆二色光谱（CD）和核磁共振光谱（NMR），研究了与FK506结合蛋白（FKBP）全序列对应的七个肽片段在水性缓冲液和2,2,2-三氟乙醇（TFE）中的螺旋形成倾向。所有片段在水性缓冲液中均呈现无规卷曲构象，而TFE诱导肽片段中形成的螺旋量有所不同。在TFE中螺旋度最高的片段对应于天然FKBP中的单个α螺旋。与β链2和3对应的片段也表现出强烈的螺旋形成倾向。相比之下，与β链1对应的片段在TFE中不形成螺旋。根据天然结构解释了固有的螺旋形成倾向，以提示可能的折叠成核位点。还将每个肽片段中的构象采样与在尿素变性的FKBP中观察到的情况进行了比较。除了与β链2对应的片段外，TFE中肽片段中螺旋结构的形成与尿素展开的FKBP中转角和/或螺旋构象的观察结果相关。令人惊讶的是，水溶液中的肽片段比尿素变性的FKBP中的相应区域结构更少。肽片段与展开的FKBP之间的构象差异不是由于尿素缓冲液或它们的旋转相关时间的差异。我们得出结论，局部氨基酸相互作用通常不足以解释未折叠的FKBP中形成非随机构象的原因。未折叠的FKBP中形成非随机结构可能需要通过与非局部非极性残基的短暂接触来稳定初始转角或螺旋构象。
