Whittington Shelly J, Creamer Trevor P
Center for Structural Biology, Department of Molecular and Cellular Biochemistry, University of Kentucky, 800 Rose Street, Lexington, Kentucky 40536-0298, USA.
Biochemistry. 2003 Dec 16;42(49):14690-5. doi: 10.1021/bi035565x.
Interactions between side chains, and in particular salt bridges, have been shown to be important in the stabilization of secondary structure. Here we investigate the contribution of a salt bridge formed between a lysine and a glutamate to the polyproline II (P(II)) helical content of proline-rich peptides. Since this structure has precisely three residues per turn, charged residues spaced three residues apart are on the same side of the helix and are best situated to interact. By contrast, computer simulations show that charged residues spaced four residues apart are both too far apart to interact strongly and are oriented such that interactions are unlikely. We have measured the P(II) content of peptides containing a lysine and glutamate pair spaced three or four residues apart using circular dichroism spectroscopy. Somewhat surprisingly we find that the P(II) content is insensitive to both the spacing and the pH. These findings indicate that i --> i + 3 salt bridges do not stabilize the P(II) helical conformation. The implications of these observations for both P(II) helix formation and denatured protein conformations are discussed.
侧链之间的相互作用，尤其是盐桥，已被证明对二级结构的稳定很重要。在此，我们研究了赖氨酸和谷氨酸之间形成的盐桥对富含脯氨酸肽段的聚脯氨酸II（P(II)）螺旋含量的贡献。由于这种结构每圈恰好有三个残基，相隔三个残基的带电残基位于螺旋的同一侧，且最适合相互作用。相比之下，计算机模拟表明，相隔四个残基的带电残基距离太远，无法强烈相互作用，而且其取向使得相互作用不太可能发生。我们使用圆二色光谱法测量了含有相隔三个或四个残基的赖氨酸和谷氨酸对的肽段的P(II)含量。有点令人惊讶的是，我们发现P(II)含量对间距和pH均不敏感。这些发现表明，i --> i + 3盐桥并不能稳定P(II)螺旋构象。本文讨论了这些观察结果对P(II)螺旋形成和变性蛋白质构象的影响。
