Smith F R, Gingrich D, Hoffman B M, Ackers G K
Department of Biology, Johns Hopkins University, Baltimore, MD 21218.
Proc Natl Acad Sci U S A. 1987 Oct;84(20):7089-93. doi: 10.1073/pnas.84.20.7089.
In a previous study on cyanomethemoglobin the 10 tetrameric species (each with a unique combination of ligated and unligated subunits) were found to exhibit three distinct free energies of cooperative interaction. The distribution of these free energies among the partially ligated species is incompatible with a two-state mechanism of molecular switching and requires a minimum of three molecular structures with distinctly different free energies of heme-heme interaction. Ligand-linked transitions between the three cooperativity states were found to be "combinatorial"--i.e., dependent upon changes in both the number and specific configuration of bound ligands. Here we present results from two other chemical systems that mimic intermediate oxygenation states. In these systems the heme iron is replaced by manganese in certain of the subunits. We find the same distribution of cooperative free energies as reported for the cyanomethemoglobin system. These results demonstrate that the three-state combinatorial nature of cooperative switching is neither a special feature of the cyanomet reactions nor of the substitution of manganese for iron, but reflects a fundamental property of hemoglobin. These findings are compared with crystallographic structural results on partially ligated hemoglobins.
在先前一项关于氰化高铁血红蛋白的研究中，发现10种四聚体物种（每种都具有结合和未结合亚基的独特组合）表现出三种不同的协同相互作用自由能。这些自由能在部分结合物种中的分布与分子开关的双态机制不相符，并且需要至少三种具有明显不同血红素 - 血红素相互作用自由能的分子结构。发现三种协同状态之间的配体连接转变是“组合性的”——即，取决于结合配体的数量和特定构型的变化。在这里，我们展示了另外两个模拟中间氧化态的化学系统的结果。在这些系统中，某些亚基中的血红素铁被锰取代。我们发现协同自由能的分布与氰化高铁血红蛋白系统所报道的相同。这些结果表明，协同转换的三态组合性质既不是氰化反应的特殊特征，也不是用锰取代铁的特殊特征，而是反映了血红蛋白的基本特性。将这些发现与部分结合血红蛋白的晶体结构结果进行了比较。
