Borin G, Ruzza P, Rossi M, Calderan A, Marchiori F, Peggion E
Biopolymers. 1989 Jan;28(1):353-69. doi: 10.1002/bip.360280134.
The conformational and ion binding properties of the sequences 93-104, 96-104, and 93-98 of domain III of bovine brain calmodulin (CaM) have been studied by CD and Tb3+-mediated fluorescence. In aqueous solution the interaction of all fragments with Ca2+ and Mg2+ ions is very weak and without any effect on the peptide conformation, which remains always random. In trifluoroethanol the interaction is very strong and the different fragments exhibit very distinct binding properties. In particular, the dodecapeptide fragment 93-104, and its N-terminal hexapeptide 98-104, bind calcium and magnesium with a very high binding constant (Kb greater than 10(5) M-1), undergoing a substantial conformational change. The structural rearrangement is particularly evident in the hexapeptide fragment, which tend to form a beta-bend. The C-terminal nonapeptide fragment 96-104 interacts with calcium and magnesium more weakly, and the binding process causes a decrease of ordered structure. These results suggest that, even in the entire dodecapeptide sequence corresponding to the loop of domain III of CaM, the calcium binding site is shifted toward the N-terminal hexapeptide segment. This interpretation is consistent with the results of crystallographic studies of CaM, which show that the calcium ions are located toward the amino terminal portion of the loop.
通过圆二色光谱（CD）和铽离子（Tb3+）介导的荧光法，研究了牛脑钙调蛋白（CaM）结构域III的93 - 104、96 - 104和93 - 98序列的构象及离子结合特性。在水溶液中，所有片段与钙离子和镁离子的相互作用非常弱，对肽的构象没有任何影响，肽的构象始终保持随机状态。在三氟乙醇中，相互作用非常强烈，不同片段表现出非常明显的结合特性。特别是十二肽片段93 - 104及其N端六肽98 - 104，以非常高的结合常数（Kb大于10(5) M-1）结合钙和镁，同时发生显著的构象变化。这种结构重排在六肽片段中尤为明显，该片段倾向于形成一个β-转角。C端九肽片段96 - 104与钙和镁的相互作用较弱，结合过程导致有序结构减少。这些结果表明，即使在与CaM结构域III环相对应的整个十二肽序列中，钙结合位点也向N端六肽段偏移。这一解释与CaM的晶体学研究结果一致，该研究表明钙离子位于环的氨基末端部分。
