d位置胺基酸的側鏈結構對於雙螺旋穩定度的影響

Abstract

雙螺旋由2到5個α螺旋互相纏繞而成，是個生物體內常見的結構單元。雙螺旋存在於轉錄因子、肌肉收縮系統、細胞骨架等系統之中，而這些生理功能來自於雙螺旋明確且穩定的結構。此研究探討了位在雙螺旋交界面的d位置殘基的側鏈結構對於雙螺旋穩定度的影響。研究所使用的雙螺旋系統為GCN4，將其第二個d位置殘基 (Leu12) 替換成帶有各式結構與極性的胺基酸。IaLd雙螺旋則用來測定胺基酸本身對於雙螺旋結構的偏好程度。實驗利用固相胜肽合成法來合成出各個雙螺旋，藉由圓二色光譜儀來監控由鹽酸胍促成的雙螺旋變性過程，並計算出雙螺旋變性的自由能。除此之外，也利用薄層色層分析法量測了胺基酸的疏水性。至於胺基酸的側鏈結構則以各種結構參數去描述，這些參數包含Es, MR, [L, B1, B5]和側鏈體積。數據分析顯示，當雙螺旋的d位置殘基為烷基時，側鏈結構和胺基酸疏水性兩者相比之下，側鏈結構對於雙螺旋穩定度的影響較大。

Coiled coil is a superhelical twist formed by two to five wrapping α-helices. It is a common structural motif that can be found in transcription factors, cytoskeletal systems, contractile systems and etc. These biochemical roles rely on well-defined and stable structures of coiled coils. Accordingly, the effect of side chain structure of d-position residue, which is buried in the coiled coil interface, on coiled coil stability was investigated. GCN4 coiled coil was employed, of which the 2nd d-residue (Leu12) was substituted with various amino acids. IaLd coiled coils were used to obtain the coiled coil propensities of these amino acids. Guanidinium denaturation of the coiled coils was monitored by circular dichroism spectroscopy. Free energy of unfolding was derived from the guanidinium denaturation data. The hydrophobicities of the amino acids were measured by thin layer chromatography. Structural parameters Es, MR, [L, B1, B5], and side chain volume were employed in the analysis. Results show that the shape and size of the residue side chain contribute more than hydrophobicity to the coiled coil stability in coiled coils with the d-position residue bearing aliphatic side chains.