Biemann H P, Koshland D E
Department of Molecular and Cell Biology, University of California at Berkeley 94720.
Biochemistry. 1994 Jan 25;33(3):629-34. doi: 10.1021/bi00169a002.
The aspartate receptors of Escherichia coli and Salmonella typhimurium which mediate chemotactic responsiveness to aspartate have 79% amino acid sequence identity but exhibited apparently quite different aspartate binding plots. The Scatchard plot of the Salmonella receptor was concave upward whereas the E. coli receptor gave a straight line. Because the two binding sites in the Salmonella receptor lacking aspartate have a 2-fold crystallographic symmetry axis and do not overlap, the observation of more than one class of binding sites must be due to a ligand-induced conformational change giving negative cooperativity. The closely related E. coli receptor was found to bind with only one class of sites but with a stoichiometry of one aspartate per dimer. The E. coli receptor thus binds with half-of-sites reactivity, an extreme form of negative cooperativity in which the second ligand is not observed to bind at all. Comparison of the X-ray crystal structure of the ligand binding domain with and without bound aspartate revealed ligand-induced conformational changes that explain the two examples of negative cooperativity.
介导对天冬氨酸趋化反应的大肠杆菌和鼠伤寒沙门氏菌的天冬氨酸受体，其氨基酸序列同一性为79%，但呈现出明显不同的天冬氨酸结合曲线。沙门氏菌受体的斯卡查德图向上凹，而大肠杆菌受体给出的是一条直线。由于沙门氏菌受体中缺少天冬氨酸的两个结合位点具有二重晶体对称轴且不重叠，观察到不止一类结合位点必定是由于配体诱导的构象变化导致负协同效应。发现密切相关的大肠杆菌受体仅与一类位点结合，但化学计量比为每个二聚体一个天冬氨酸。因此，大肠杆菌受体以半位点反应性结合，这是负协同效应的一种极端形式，其中根本未观察到第二个配体结合。对结合和未结合天冬氨酸的配体结合结构域的X射线晶体结构进行比较，揭示了配体诱导的构象变化，这解释了负协同效应的两个例子。
