Trabi M, Schirra H J, Craik D J
Institute for Molecular Bioscience (Centre for Drug Design and Development), University of Queensland, Brisbane, QLD 4072, Australia.
Biochemistry. 2001 Apr 10;40(14):4211-21. doi: 10.1021/bi002028t.
Most mammalian defensins are cationic peptides of 29-42 amino acids long, stabilized by three disulfide bonds. However, recently Tang et al. (1999, Science 286, 498-502) reported the isolation of a new defensin type found in the leukocytes of rhesus macaques. In contrast to all the other defensins found so far, rhesus theta defensin-1 (RTD-1) is composed of just 18 amino acids with the backbone cyclized through peptide bonds. Antibacterial activities of both the native cyclic peptide and a linear form were examined, showing that the cyclic form was 3-fold more active than the open chain analogue [Tang et al. (1999) Science 286, 498-502]. To elucidate the three-dimensional structure of RTD-1 and its open chain analogue, both peptides were synthesized using solid-phase peptide synthesis and tert-butyloxycarbonyl chemistry. The structures of both peptides in aqueous solution were determined from two-dimensional (1)H NMR data recorded at 500 and 750 MHz. Structural constraints consisting of interproton distances and dihedral angles were used as input for simulated-annealing calculations and water refinement with the program CNS. RTD-1 and its open chain analogue oRTD-1 adopt very similar structures in water. Both comprise an extended beta-hairpin structure with turns at one or both ends. The turns are well defined within themselves and seem to be flexible with respect to the extended regions of the molecules. Although the two strands of the beta-sheet are connected by three disulfide bonds, this region displays a degree of flexibility. The structural similarity of RTD-1 and its open chain analogue oRTD-1, as well as their comparable degree of flexibility, support the theory that the additional charges at the termini of the open chain analogue rather than overall differences in structure or flexibility are the cause for oRTD-1's lower antimicrobial activity. In contrast to numerous other antimicrobial peptides, RTD-1 does not display any amphiphilic character, even though surface models of RTD-1 exhibit a certain clustering of positive charges. Some amide protons of RTD-1 that should be solvent-exposed in monomeric beta-sheet structures show low-temperature coefficients, suggesting the possible presence of weak intermolecular hydrogen bonds.
大多数哺乳动物防御素是由29 - 42个氨基酸组成的阳离子肽，通过三个二硫键稳定。然而，最近Tang等人（1999年，《科学》286卷，498 - 502页）报道了在恒河猴白细胞中发现的一种新类型防御素。与迄今发现的所有其他防御素不同，恒河猴θ防御素-1（RTD-1）仅由18个氨基酸组成，其主链通过肽键环化。对天然环肽和线性形式的抗菌活性进行了检测，结果表明环化形式的活性比开链类似物高3倍[Tang等人（1999年）《科学》286卷，498 - 502页]。为了阐明RTD-1及其开链类似物的三维结构，使用固相肽合成和叔丁氧羰基化学方法合成了这两种肽。根据在500和750 MHz记录的二维（1）H NMR数据确定了两种肽在水溶液中的结构。由质子间距离和二面角组成的结构约束被用作输入，用于使用CNS程序进行模拟退火计算和水精制。RTD-1及其开链类似物oRTD-1在水中采用非常相似的结构。两者都包含一个延伸的β-发夹结构，在一端或两端有转角。转角自身定义明确，相对于分子的延伸区域似乎是灵活的。尽管β-折叠的两条链通过三个二硫键连接，但该区域表现出一定程度的灵活性。RTD-1及其开链类似物oRTD-1的结构相似性，以及它们相当的灵活性程度，支持了这样一种理论，即开链类似物末端的额外电荷而非结构或灵活性的整体差异是oRTD-1抗菌活性较低的原因。与许多其他抗菌肽不同，RTD-1不表现出任何两亲性特征，尽管RTD-1的表面模型显示出一定的正电荷聚集。RTD-1中一些在单体β-折叠结构中应暴露于溶剂的酰胺质子显示出低温系数，表明可能存在弱的分子间氢键。
