School of Biotechnology, Yeungnam University, Gyeongsan 712-749, South Korea.
Centre for Interdisciplinary Research in Basic Sciences, Jamia Millia Islamia, New Delhi 110025, India.
Int J Biol Macromol. 2019 Jul 1;132:666-676. doi: 10.1016/j.ijbiomac.2019.03.236. Epub 2019 Apr 1.
In many tissues and organisms, large amount of urea gets accumulated to maintain osmotic balance. To evade the threatening impact of urea, living organisms accumulate methylamines, a class of osmolytes, in proportion of 2:1 (urea:methylamine). To understand underlying cause(s) for protein-specific counteraction behavior, thermodynamic stability (ΔG) of three disulfide free proteins (myoglobin, bovine cytochrome c and barstar) in the mixture of urea and methylamine has been estimated from guanidinium chloride-(GdmCl) driven denaturation curves. Using the experimentally measured values of ΔG obtained in the presence of individual methylamines and urea, we predicted the molar ratio of urea and a methylamine required for perfect compensation for each of the proteins. Interestingly, for all proteins studied, a similar ratio has been observed for perfect compensation. The predicted ratio for perfect compensation in terms of thermodynamic parameters was about 2:1 M ratio of urea to methylamine. Furthermore, a partial counteraction was observed in the myoglobin and barstar. However, for bovine cytochrome c, perfect compensation was observed in both GdmCl- and heat-driven denaturations. Our observations clearly suggest that the counteraction phenomenon depends on the extent of the unfolding of the denatured states of proteins.
在许多组织和生物中,大量的尿素会积累以维持渗透平衡。为了避免尿素的威胁,生物会以 2:1(尿素:甲基胺)的比例积累甲基胺,这是一类渗透调节剂。为了了解蛋白质特异性拮抗行为的潜在原因,我们从胍盐酸盐(GdmCl)驱动的变性曲线估计了三种无二硫键蛋白(肌红蛋白、牛细胞色素 c 和 barnase)在尿素和甲基胺混合物中的热力学稳定性(ΔG)。利用在单个甲基胺和尿素存在下测量的 ΔG 的实验值,我们预测了每种蛋白质完全补偿所需的尿素和甲基胺的摩尔比。有趣的是,对于所有研究的蛋白质,都观察到了完全补偿的相似比例。在热力学参数方面,完全补偿的预测比值约为 2:1,即尿素与甲基胺的摩尔比。此外,在肌红蛋白和 barnase 中观察到部分拮抗作用。然而,对于牛细胞色素 c,在 GdmCl 和热驱动变性中都观察到了完全补偿。我们的观察结果清楚地表明,拮抗现象取决于蛋白质变性状态的展开程度。