溶液中におけるタンパク質の熱物性

摘要

To reveal the relationships between structure, property, and function of protein, we have determined the various thermodynamic properties (partial specific volume, compressibility, expansibility coefficient, intrinsic viscosity, activation free energy for viscous flow, preferential solvation, and T_m, △H, △S, △G, and △C_p for conformational change) of proteins in solution with various conditions as temperature, pressure, solvents, additives, and protein modifications. The cyclodextrin (CD) destabilized the folded state of lysozyme by stabilizing the unfolded state by including hydrophobic part of unfolded state into interior of CD. The destabilization effect of CD was weaker than guanidine hydrochloride and was dependent on the interior diameter and the substituent. The strong water-dimethyl sulfoxide (DMSO) interaction caused lysozyme unfolding with increasing partial specific volume and intrinsic viscosity. DMSO was bound to lysozyme at low DMSO concentration and was preferentially excluded from lysozyme at high DMSO concentration. The sorbitol stabilizes protein through enhancing the hydrophobic interaction in protein. The molten globule state induced by sorbitol had thermodynamic properties somewhat like native state by preferential solvent interactions and osmotic pressure of the medium. Thus, thermodynamic properties can sensitively reflect the conformational changes and interactions with solvent, and hence will be an effective approach to give quantitative and valuable index to protein.%電離性高分子であるタンパク質はそれぞれが固有のアミノ酸配列を持ち，規則正しく折れたたまった固有の立体構造を形成している。Ⅹ線結晶構造解析で決定されるタンパク質の構造はある瞬間の安定構造（スナップショット）であり，溶液という溶媒分子の海に浮かぶタンパク質の構造は熱振動の影響を受けて大きく揺らいでいる。酵素活性の至適温度や反応速度に反映されるように，タンパク質が十分に機能するためには安定性やダイナミクス（揺らぎ）などの物理化学的性質（物性）を無視することはできず，近年ではアミロイドに代表されるようなタンパク質由来の疾患に注目が集まるなど，タンパク質の物性が一層重要視されている。