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標題: | 帶電荷氨基酸側鏈長短對蛋白質二級結構中”離子對作用力”的影響 Effect of Charged Amino Acid Side Chain Length on Ion Pairing Interactions in Protein Secondary Structures |
作者: | Hsiou-Ting Kuo 郭秀婷 |
指導教授: | 陳平 |
關鍵字: | 胺基酸,胜?鏈,鏈長短,離子對作用力,α-螺旋,β-摺板, amino acids,peptides,side chain length,ion pairing interaction,α-helices,β-sheets, |
出版年 : | 2014 |
學位: | 博士 |
摘要: | 自然界帶電荷胺基酸是由帶電荷之親水性官能基經由不同數目的亞甲基連接到主鏈而組成,其靜電力之特性在蛋白質中形成離子對作用力。為了探討這些具有不同側鏈長短之含胍基和羧基殘基之間所形成的離子作用力的影響,我們經由圓二色光譜來研究由Zbb-Agx 和Agx-Zbb (i, i+3), (i, i+4)以及(i, i+5)模式所組成的α-螺旋胜肽鏈。由ZbbAgxN胜肽鏈得到的結果得知,含有Aad和Glu胺基酸之胜肽鏈皆具有類似的螺旋程度,且大部分都不隨著pH值而改變。然而對含有Asp胺基酸的胜肽鏈,其螺旋程度則因pH值不同而改變。另外,含有Arg胺基酸之胜肽鏈其螺旋程度都高於其對應具Agp, Agb以及Agh胺基酸的胜肽鏈。而AgxZbbN胜肽鏈得到的結果,AgbAad3和AgxAsp4胜肽鏈之螺旋程度皆隨著pH值從7到2而遞減,顯示失去有利離子對作用力對螺旋程度的影響要比移走帶電荷殘基與螺旋偶極之間的斥力來得大。愈短的側鏈也愈固定不動,因此Agb-Asp (i, i+4)所形成的靜電作用力顯示有較穩定的能量。而AgbAad3胜肽鏈奇特地顯示其靜電離子對作用力大於帶電荷殘基與螺旋偶極之間的斥力。所有的AgbZbb4和AghZbb4胜肽鏈皆顯示有明顯地離子對作用力,這意味著特別構型與特定長度的胍基對促進離子對作用力是必需的。重要地,在所有的AgxZbbN胜肽鏈中,ArgAad4胜肽鏈顯示其具有很小的離子對作用力,但卻有最好的螺旋程度。
帶有相反電荷的胺基酸常見於具有互相平行但方向相反的β-摺板結構中,且直接跨越於相鄰二鏈之間,這意味著離子對作用力對穩定跨鏈的重要性。因此我們探討不同側鏈長度之帶電荷胺基酸在兩鏈中相對且非氫鍵位置上,形成離子對作用力對β-髮夾型胜肽鏈穩定度的影響。摺疊百分比是由化學位移偏差而得。而利用雙向改變循環分析方法來測定此相對離子對作用力的能量。對Lys衍生物來說,只有具較短且穩固地側鏈如Asp/Glu-Dap之間的離子對作用力,以及較長且柔韌側鏈如Aad-Orn/Lys之間的生成的疏水性作用力顯示具穩定能量。而對Arg的衍生物來看,則只有具較長側鏈如Aad-Arg/Agh之間的作用力有清楚顯示穩定能量,這可能是來自於側鏈之間的疏水性作用力。顯然正電荷會散佈在Arg衍生物的胍基上,並減弱其與對面跨鏈上羧基之間的靜電作用力,導致較短側鏈殘基之間的作用力被減弱。 The charged hydrophilic functionalities of encoded charged amino acids are linked to the backbone via different number of hydrophobic methylenes, despite the apparent electrostatic nature of protein ion pairing interactions. To investigate the effect of side chain length of guanidinium- and carboxylate-containing residues on ion pairing interactions, α-helical peptides containing Zbb-Agx and Agx-Zbb (i, i+3), (i, i+4) and (i, i+5) sequence patterns were studied by circular dichroism spectroscopy. For the ZbbAgxN peptides, the helicity of the Aad- and Glu-containing peptides was similar and mostly pH independent, whereas the helicity of Asp-containing peptides was mostly pH dependent. Also, the Arg-containing peptides consistently exhibited higher helicity compared to the corresponding Agp-, Agb-, and Agh-containing peptides. For the AgxZbbN pepitdes, the helicity of peptides AgbAad3 and AgxAsp4 were attenuated upon changing the pH from 7 to 2, suggesting that the loss of favorable ion pairing interactions outweighed the removal of the repulsive interactions between the charged residues and the helix dipole. The shorter and more rigid side chain in the Agb-Asp (i, i+4) interactions exhibited stabilizing energetics due to electrostatics. Peptides AgbAad3 was unusual in which the electrostatic ion pairing interaction outweighed the unfavorable charge-helix dipole interaction. All the AgbZbb4 and AghZbb4 peptides exhibited significantly ion pairing interactions, suggesting the need for particular chain lengths and geometry of the gaunidinium group. Importantly, peptide ArgAad4 exhibited minimal ion pairing interaction but showed the most helical content among the AgxZbbN peptides. Oppositely charged amino acids are frequently observed directly across one another in antiparallel sheet structures, suggesting the importance of cross-strand ion pairing interactions. The effect of charged amino acid side chain length on cross-strand ion pairing interactions at lateral non-hydrogen bonded positions was investigated in a β-hairpin motif. The fraction folded population was derived from the chemical shift deviation data. Double mutant cycle analysis was used to determine the interaction energy for the potential lateral ion pairs. For the Lys analogs, only the Asp/Glu-Dap interactions with shorter side chains and the Aad-Orn/Lys interactions with longer side chains exhibited stabilizing energetics, mostly relying on electrostatics and hydrophobics, respectively. For Arg analogues, only the Aad-Arg/Agh interactions with long side chains clearly exhibited stabilizing energetics, most likely relying on hydrophobics. Apparently, the diffused positive charge on the guanidinium group of Arg analogues resulted in weakened cross-strand electrostatic interactions with the carboxylate side chain, leading to minimal interaction between residues with short side chains. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/57346 |
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