Ogawa K, Nishimura S, Doi M, Kyogoku Y, Hayashi M, Kobayashi Y
Institute for Protein Research, Osaka University, Japan.
Eur J Biochem. 1994 Jun 1;222(2):659-66. doi: 10.1111/j.1432-1033.1994.tb18910.x.
The conformational characteristics of elcatonin, an analogue of eel calcitonin having a disulfide bond Cys1-Cys7 replaced by an ethylene linkage between residues 1 and 7, have been analyzed in aqueous trifluoroethanol solutions. Circular dichroic spectra of elcatonin and eel calcitonin itself reveal the presence of alpha-helices at trifluoroethanol concentrations above 15%. The spectral changes caused by the trifluoroethanol content of the solutions are interesting. An isosbetic point is detected for eel calcitonin indicating that a conformational transition occurs between two states, namely alpha-helical and random coil states. On the other hand, the CD curves of elcatonin at less than 15% trifluoroethanol deviate from the isosbetic point while those at higher concentration are similar to those of eCT. This can probably be attributed to the third element of the ordered structure of elcatonin which is formed in 15% trifluoroethanol. The solution conformation of elcatonin in a mixture of 60% water and 40% trifluoroethanol has been determined by the combined use of 1H-NMR spectroscopy and distance geometry calculations. The conformation is characterized by an amphiphilic alpha-helix between Thr6 and Thr21, which extends into the constrained cyclic portion of the molecule to Thr6. The third structural element of elcatonin found in the CD analysis is detected by some turn structures in the region between residue 1 and Ser5 in the calculated structure.
依降钙素是鳗鱼降钙素的类似物，其1位和7位残基之间的二硫键Cys1-Cys7被乙烯连接所取代，本文已对其在三氟乙醇水溶液中的构象特征进行了分析。依降钙素和鳗鱼降钙素本身的圆二色光谱表明，在三氟乙醇浓度高于15%时存在α-螺旋。溶液中三氟乙醇含量引起的光谱变化很有趣。鳗鱼降钙素检测到一个等吸收点，表明在α-螺旋和无规卷曲两种状态之间发生了构象转变。另一方面，三氟乙醇含量低于15%时依降钙素的CD曲线偏离等吸收点，而在较高浓度时其CD曲线与鳗鱼降钙素的相似。这可能归因于依降钙素有序结构的第三个元素，它在15%的三氟乙醇中形成。通过结合使用1H-NMR光谱和距离几何计算，确定了依降钙素在60%水和40%三氟乙醇混合物中的溶液构象。其构象的特征是在Thr6和Thr21之间有一个两亲性α-螺旋，延伸到分子的受限环状部分直至Thr6。在CD分析中发现的依降钙素的第三个结构元素在计算结构中残基1和Ser5之间的区域通过一些转角结构检测到。
