Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States.
J Am Chem Soc. 2011 Sep 7;133(35):14017-26. doi: 10.1021/ja2045293. Epub 2011 Aug 15.
The hydrophobic effect, the free-energetically favorable association of nonpolar solutes in water, makes a dominant contribution to binding of many systems of ligands and proteins. The objective of this study was to examine the hydrophobic effect in biomolecular recognition using two chemically different but structurally similar hydrophobic groups, aliphatic hydrocarbons and aliphatic fluorocarbons, and to determine whether the hydrophobicity of the two groups could be distinguished by thermodynamic and biostructural analysis. This paper uses isothermal titration calorimetry (ITC) to examine the thermodynamics of binding of benzenesulfonamides substituted in the para position with alkyl and fluoroalkyl chains (H(2)NSO(2)C(6)H(4)-CONHCH(2)(CX(2))(n)CX(3), n = 0-4, X = H, F) to human carbonic anhydrase II (HCA II). Both alkyl and fluoroalkyl substituents contribute favorably to the enthalpy and the entropy of binding; these contributions increase as the length of chain of the hydrophobic substituent increases. Crystallography of the protein-ligand complexes indicates that the benzenesulfonamide groups of all ligands examined bind with similar geometry, that the tail groups associate with the hydrophobic wall of HCA II (which is made up of the side chains of residues Phe131, Val135, Pro202, and Leu204), and that the structure of the protein is indistinguishable for all but one of the complexes (the longest member of the fluoroalkyl series). Analysis of the thermodynamics of binding as a function of structure is compatible with the hypothesis that hydrophobic binding of both alkyl and fluoroalkyl chains to hydrophobic surface of carbonic anhydrase is due primarily to the release of nonoptimally hydrogen-bonded water molecules that hydrate the binding cavity (including the hydrophobic wall) of HCA II and to the release of water molecules that surround the hydrophobic chain of the ligands. This study defines the balance of enthalpic and entropic contributions to the hydrophobic effect in this representative system of protein and ligand: hydrophobic interactions, here, seem to comprise approximately equal contributions from enthalpy (plausibly from strengthening networks of hydrogen bonds among molecules of water) and entropy (from release of water from configurationally restricted positions).
疏水效应是指非极性溶质在水中自由能有利的缔合，对许多配体和蛋白质系统的结合起主要贡献。本研究的目的是使用两种化学性质不同但结构相似的疏水分子，脂肪烃和脂肪氟碳，来研究生物分子识别中的疏水效应，并确定这两种基团的疏水性是否可以通过热力学和生物结构分析来区分。本文使用等温滴定量热法（ITC）来研究对取代苯磺酰胺与烷基和氟烷基链（H 2 NS0 2 C 6 H 4 -CONHCH 2 （CX 2 ）（n）CX 3 ，n = 0-4，X = H，F）结合到人碳酸酐酶 II（HCA II）的热力学。烷基和氟烷基取代基都有利于结合的焓和熵；随着疏水取代基链长的增加，这些贡献增加。蛋白质-配体复合物的晶体学表明，所有研究的苯磺酰胺配体的苯磺酰胺基团都以相似的几何形状结合，尾部基团与 HCA II 的疏水性壁（由残基 Phe131、Val135、Pro202 和 Leu204 的侧链组成）结合，并且除了一个复合物（氟烷基系列中最长的成员）外，所有复合物的蛋白质结构都无法区分。结合热力学作为结构函数的分析与假设一致，即烷基和氟烷基链对碳酸酐酶疏水性表面的疏水结合主要归因于释放非最佳氢键合水分子，这些水分子水合 HCA II 的结合腔（包括疏水性壁）和包围配体疏水链的水分子。本研究定义了该代表性蛋白质和配体系统中疏水效应的焓和熵贡献的平衡：在这里，疏水相互作用似乎包含来自焓（可能来自增强水分子之间氢键网络）和熵（来自从构象受限位置释放水）的大致相等的贡献。
