Institute for Molecular Bioscience, Division of Chemistry and Structural Biology, The University of Queensland, 306 Carmody Rd, St. Lucia, QLD 4067, Australia.
Biopolymers. 2010;94(5):647-58. doi: 10.1002/bip.21409.
Cyclotides are a large family of plant-derived proteins typified by their head-to-tail cyclic backbone and knotted arrangement of three disulfide bonds. Although they display a diverse range of biological activities, their native function is thought to be plant defense. Here we characterized the expression, three-dimensional structure, and hemolytic activity of the cyclotide kalata B5 from the African plant Oldenlandia affinis. Kalata B5 shows an interesting seasonal variation in its expression and can only be isolated during certain times of the year, when the plant is flowering. It displays a typical tightly folded cyclic Scystine knot structure. A range of pH and temperature titrations reveal that a conserved glutamic acid in loop 1 Sof the structure forms a key hydrogen bond network, similar to that reported previously for other cyclotides. However, specific line broadening in the NMR spectra of kalata B5 suggests that the hydrogen bonding network in this peptide is less rigid than in other cyclotides. Notably, the pK9a) of Glu6 of 4.5 is higher than the values for other cyclotides studied so far, which range from 3.0 to 4.0, providing a further indication of a weaker hydrogen bond network. Kalata B5 has only moderate hemolytic activity compared with other highly expressed cyclotides, and this reduced activity probably reflects its more flexible structure. As is the case with other cyclotides, kalata B5 has an exposed hydrophobic region on its surface, supporting suggestions that this hydrophobic patch is a key feature for membrane binding and biological activity of cyclotides.
环肽是一类广泛存在于植物中的蛋白质家族，其特征为头尾相连的环化骨架和三个二硫键的打结排列。尽管它们表现出多种生物活性，但它们的天然功能被认为是植物防御。在这里，我们对来自非洲植物 Oldenlandia affinis 的环肽 kalata B5 的表达、三维结构和溶血活性进行了表征。Kalata B5 的表达呈现出有趣的季节性变化，只能在一年中的某些时候分离出来，也就是植物开花的时候。它显示出典型的紧密折叠的环 Scystine 结结构。一系列 pH 和温度滴定表明，结构中环 1 的保守谷氨酸形成了一个关键的氢键网络，类似于以前报道的其他环肽。然而，kalata B5 的 NMR 光谱中的特定线宽加宽表明，该肽中的氢键网络不如其他环肽那么刚性。值得注意的是，Glu6 的 pK9a) 值为 4.5，高于迄今为止研究过的其他环肽的 pK9a) 值（3.0 至 4.0），这进一步表明氢键网络较弱。与其他高度表达的环肽相比，kalata B5 的溶血活性仅适中，这种活性降低可能反映了其更灵活的结构。与其他环肽一样，kalata B5 表面有一个暴露的疏水区，这支持了这样一种观点，即这个疏水区是环肽与膜结合和生物活性的关键特征。
