Shi Zhengshuang, Olson C Anders, Rose George D, Baldwin Robert L, Kallenbach Neville R
Department of Chemistry, New York University, New York, NY 10003, USA.
Proc Natl Acad Sci U S A. 2002 Jul 9;99(14):9190-5. doi: 10.1073/pnas.112193999. Epub 2002 Jun 28.
A sequence of seven alanine residues-too short to form an alpha-helix and whose side chains do not interact with each other-is a particularly simple model for testing the common description of denatured proteins as structureless random coils. The (3)J(HN alpha) coupling constants of individual alanine residues have been measured from 2 to 56 degrees C by using isotopically labeled samples. The results display a thermal transition between different backbone conformations, which is confirmed by CD spectra. The NMR results suggest that polyproline II is the dominant conformation at 2 degrees C and the content of beta strand is increased by approximately 10% at 55 degrees C relative to that at 2 degrees C. The polyproline II conformation is consistent with recent studies of short alanine peptides, including structure prediction by ab initio quantum mechanics and solution structures for both a blocked alanine dipeptide and an alanine tripeptide. CD and other optical spectroscopies have found structure in longer "random coil" peptides and have implicated polyproline II, which is a major backbone conformation in residues within loop regions of protein structures. Our result suggests that the backbone conformational entropy in alanine peptides is considerably smaller than estimated by the random coil model. New thermodynamic data confirm this suggestion: the entropy loss on alanine helix formation is only 2.2 entropy units per residue.
一段由七个丙氨酸残基组成的序列——短到无法形成α螺旋且其侧链之间不相互作用——是用于测试将变性蛋白质描述为无结构随机卷曲这一常见说法的特别简单的模型。通过使用同位素标记的样品，在2至56摄氏度范围内测量了各个丙氨酸残基的(3)J(HNα)耦合常数。结果显示了不同主链构象之间的热转变，这一点得到了圆二色光谱（CD光谱）的证实。核磁共振（NMR）结果表明，在2摄氏度时多聚脯氨酸II是主要构象，相对于2摄氏度时，在55摄氏度时β链的含量增加了约10%。多聚脯氨酸II构象与近期对短丙氨酸肽的研究一致，包括从头算量子力学的结构预测以及封闭丙氨酸二肽和丙氨酸三肽的溶液结构。圆二色光谱和其他光学光谱法在更长的“随机卷曲”肽中发现了结构，并涉及到多聚脯氨酸II，它是蛋白质结构环区域内残基的主要主链构象。我们的结果表明，丙氨酸肽中的主链构象熵比随机卷曲模型估计的要小得多。新的热力学数据证实了这一观点：丙氨酸螺旋形成时的熵损失仅为每个残基2.2熵单位。
