A naturally occurring protective system in urea-rich cells: Mechanism of osmolyte protection of proteins against urea denaturation
A naturally occurring protective system in urea-rich cells: Mechanism of osmolyte protection of proteins against urea denaturation
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DOI：
10.1021/bi970247h
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发表时间：
1997-07-29
期刊：
影响因子：
2.9
通讯作者：
Bolen, DW
中科院分区：
文献类型：
--
作者：
Wang, AJ;Bolen, DW
氧化三甲胺（TMAO）是一种溶质，浓缩在富含尿素的板鳃类和腔棘鱼细胞中，以抵消尿素对细胞内蛋白质结构和功能的破坏作用。根据转移自由能的测量，TMAO与氨基酸侧链的有利相互作用促进蛋白质变性。这种效应被非常不利的TMAO-肽骨架相互作用抵消，TMAO-肽骨架相互作用不仅对抗变性，而且还提供针对尿素变性的稳定性。通过将侧链和主链的转移自由能与核糖核酸酶T1的天然和未折叠状态下的表面积暴露相结合，天然和未折叠蛋白质从水到1 M TMAO的转移自由能估计为1.7和5.9 kcal/mol，这些估计值与Lin和Timasheff [（1994）Biochemistry 33，12695-12701]。天然和未折叠蛋白质从水到TMAO的不利转移自由能为渗透剂优先水合蛋白质提供了分子水平的基本原理。发现尿素对变性的促进作用被TMAO抵消，其方式大致是两种溶质的组合效应的加和。尿素与主链的有利相互作用提供了该变性剂对蛋白质去折叠的主导驱动力，而TMAO与主链的不利相互作用是反对尿素变性的主导力。在含有显著有机溶质浓度的溶液中，肽主链的作用超过侧链的作用的优势可以解释许多观察到的由各种稳定和不稳定有机溶质诱导的蛋白质变性和稳定性的影响。
Trimethylamine N-oxide (TMAO) is a solute concentrated in the urea-rich cells of elasmobranchs and coelacanth to offset the damaging effects of urea on intracellular protein structure and function. On the basis of transfer free energy measurements, favorable interaction of TMAO with amino acid side chains promote protein denaturation. This effect is more than offset by highly unfavorable TMAO-peptide backbone interactions that not only oppose denaturation but also provide stabilization against denaturation by urea. By combining transfer free energies of side chains and backbone with surface area exposure in the native and unfolded states of ribonuclease T1, the transfer free energies of native and unfolded protein from water to 1 M TMAO are estimated as 1.7 and 5.9 kcal/mol, respectively These estimates agree favorably with the respective values of 1.2 and 5.4 kcal/mol determined experimentally by Lin and Timasheff [(1994) Biochemistry 33, 12695-12701]. The unfavorable transfer free energies of native and unfolded protein from water to TMAO provides a molecular level rationale for preferential hydration of proteins by osmolytes. Promotion of denaturation by urea is found to be offset by TMAO in a manner that is roughly additive of the combined effects of both solutes. The favorable interaction of urea with the backbone provides the dominant driving force for protein unfolding by this denaturant, and the unfavorable interaction of TMAO with backbone is the dominant force opposing urea denaturation. In solutions that contain significant organic solute concentration, the ascendance of the role of the peptide backbone over that of side chains can explain many observed effects in protein denaturation and stability induced by a variety of stabilizing and destabilizing organic solutes.