Quiocho F A
Howard Hughes Medical Institute, Houston, Texas, USA.
Kidney Int. 1996 Apr;49(4):943-6. doi: 10.1038/ki.1996.132.
We have determined, by the method of x-ray crystallography, the 1.7 A resolution three-dimensional structures of the ligand-bound form of the phosphate receptor as well as the sulfate receptor. These protein structures provide an unprecedented atomic-level understanding of the mechanism governing the exquisite specificity of each receptor. Although they lack amino acid sequence homology, both receptors have very similar three-dimensional structure. The structure consists of two globular domains separated by a deep cleft which contains the ligand-binding site. The bound phosphate and sulfate are totally devoid of water of hydration. The bound phosphate is tightly held in place by 12 hydrogen bonds, 11 with donor and 1 with acceptor groups. The acceptor group (an Asp carboxylate side chain) plays three key roles. It confers specificity by directly recognizing one proton of either the monobasic or dibasic phosphate. It also assists in the recognition of another proton of the monobasic phosphate. Finally, because of charge repulsion, it disallows binding of fully ionized sulfate. The sulfate bound to the sulfate receptor makes seven hydrogen bonds with uncharged polar groups exclusively. The absence of an acceptor group in the binding site of the sulfate receptor is not conducive to phosphate binding.
我们通过X射线晶体学方法确定了磷酸受体和硫酸受体的配体结合形式的分辨率为1.7埃的三维结构。这些蛋白质结构为支配每个受体的精细特异性的机制提供了前所未有的原子水平理解。尽管它们缺乏氨基酸序列同源性，但两种受体具有非常相似的三维结构。该结构由两个球状结构域组成，中间由一个含有配体结合位点的深裂缝隔开。结合的磷酸根和硫酸根完全没有水化水。结合的磷酸根通过12个氢键紧密固定在位，其中11个与供体基团形成氢键，1个与受体基团形成氢键。受体基团（一个天冬氨酸羧酸盐侧链）发挥三个关键作用。它通过直接识别一元或二元磷酸根的一个质子来赋予特异性。它还协助识别一元磷酸根的另一个质子。最后，由于电荷排斥，它不允许完全电离的硫酸根结合。与硫酸受体结合的硫酸根仅与不带电荷的极性基团形成七个氢键。硫酸受体结合位点中不存在受体基团不利于磷酸根结合。
