Astbury Centre for Structural Molecular Biology, Faculty of Biological Sciences, University of Leeds, Leeds LS2 9JT, UK.
J Am Chem Soc. 2010 Jun 30;132(25):8682-9. doi: 10.1021/ja101362u.
In the present study we characterize the thermodynamics of binding of histamine to recombinant histamine-binding protein (rRaHBP2), a member of the lipocalin family isolated from the brown-ear tick Rhipicephalus appendiculatus. The binding pocket of this protein contains a number of charged residues, consistent with histamine binding, and is thus a typical example of a "hydrophilic" binder. In contrast, a second member of the lipocalin family, the recombinant major urinary protein (rMUP), binds small hydrophobic ligands, with a similar overall entropy of binding in comparison with rRaHBP2. Having extensively studied ligand binding thermodynamics for rMUP previously, the data we obtained in the present study for HBP enables a comparison of the driving forces for binding between these classically distinct binding processes in terms of entropic contributions from ligand, protein, and solvent. In the case of rRaHBP2, we find favorable entropic contributions to binding from desolvation of the ligand; however, the overall entropy of binding is unfavorable due to a dominant unfavorable contribution arising from the loss of ligand degrees of freedom, together with the sequestration of solvent water molecules into the binding pocket in the complex. This contrasts with binding in rMUP where desolvation of the protein binding pocket makes a minor contribution to the overall entropy of binding given that the pocket is substantially desolvated prior to binding.
在本研究中，我们描述了组胺与重组组胺结合蛋白（rRaHBP2）结合的热力学特性，rRaHBP2 是从棕色耳蜱 Rhipicephalus appendiculatus 中分离出的亲脂素家族的成员。该蛋白的结合口袋中含有许多带电荷的残基，与组胺结合一致，因此是典型的“亲水”结合物。相比之下，亲脂素家族的另一个成员，重组主要尿蛋白（rMUP），结合小的疏水性配体，与 rRaHBP2 相比，结合的总熵相似。由于我们之前已经广泛研究了 rMUP 的配体结合热力学，因此我们在本研究中获得的 HBP 数据可以根据配体、蛋白质和溶剂的熵贡献来比较这两种截然不同的结合过程的结合驱动力。对于 rRaHBP2，我们发现配体去溶剂化对结合有有利的熵贡献；然而，由于配体自由度的丧失以及溶剂水分子被囚禁在复合物的结合口袋中，导致结合的整体熵不利。这与 rMUP 中的结合形成对比，在 rMUP 中，由于结合口袋在结合之前已经基本上去溶剂化，因此蛋白质结合口袋的去溶剂化对结合的整体熵的贡献较小。
