General Chemistry, Vrije University Brussel, Brussels, Belgium.
Antioxid Redox Signal. 2013 Jan 1;18(1):94-127. doi: 10.1089/ars.2012.4521. Epub 2012 Sep 20.
Many cellular functions involve cysteine chemistry via thiol-disulfide exchange pathways. The nucleophilic cysteines of the enzymes involved are activated as thiolate. A thiolate is much more reactive than a neutral thiol. Therefore, determining and understanding the pK(a)s of functional cysteines are important aspects of biochemistry and molecular biology with direct implications for redox signaling. Here, we describe the experimental and theoretical methods to determine cysteine pK(a) values, and we examine the factors that control these pK(a)s. Drawing largely on experience gained with the thioredoxin superfamily, we examine the roles of solvation, charge-charge, helix macrodipole, and hydrogen bonding interactions as pK(a)-modulating factors. The contributions of these factors in influencing cysteine pK(a)s and the associated chemistry, including the relevance for the reaction kinetics and thermodynamics, are discussed. This analysis highlights the critical role of direct hydrogen bonding to the cysteine sulfur as a key factor modulating the equilibrium between thiol S-H and thiolate S(-). This role is easily understood intuitively and provides a framework for biochemical functional insights.
许多细胞功能都涉及巯基-二硫键交换途径中的半胱氨酸化学。参与其中的酶的亲核半胱氨酸被激活为硫醇盐。硫醇盐的反应性比中性硫醇强得多。因此，确定和理解功能半胱氨酸的 pK(a) 值是生物化学和分子生物学的重要方面，对氧化还原信号有直接影响。在这里，我们描述了确定半胱氨酸 pK(a) 值的实验和理论方法，并研究了控制这些 pK(a) 值的因素。主要借鉴了硫氧还蛋白超家族获得的经验，我们研究了溶剂化、电荷-电荷、螺旋宏观偶极子和氢键相互作用作为 pK(a) 调节因素的作用。讨论了这些因素对半胱氨酸 pK(a) 和相关化学的影响，包括对反应动力学和热力学的相关性。这种分析强调了直接氢键与半胱氨酸硫之间的关键作用，这是调节巯基 S-H 和硫醇盐 S(-)之间平衡的关键因素。这种作用很容易从直观上理解，并为生化功能见解提供了一个框架。
