Smith F R, Ackers G K
Proc Natl Acad Sci U S A. 1985 Aug;82(16):5347-51. doi: 10.1073/pnas.82.16.5347.
Tetrameric human hemoglobin can assume ten molecular forms that differ in the number and configuration of ligands bound at the four heme sites. For each of these species we have determined the cooperative free energy--i.e., the deviation in free energy of ligation from that which would obtain for the same sites binding as independent alpha and beta subunits. These cooperative free energies were resolved from measurements on the dissociation into dimers of tetramers in which each subunit is either unligated (Fe2+ deoxy) or "ligated" by conversion into the cyanomet form (Fe3+ CN). The results indicate that each hemoglobin tetramer acts as a three-level molecular switch. During the course of ligation, the total cooperative free energy (6 kcal/mol over all four binding steps) is expended in two transitions that are synchronized with particular ligation steps. Whether a cooperative energy transition occurs or not depends upon how the ligation step changes both the number and configuration of ligated subunits. The hemoglobin tetramer is thus a "combinatorial switch." The finding of three distinct free energy levels for the ten ligation states suggests the existence of three major structural forms of the hemoglobin tetramer.
四聚体人血红蛋白可呈现十种分子形式，这些形式在四个血红素位点结合的配体数量和构型上有所不同。对于这些物种中的每一种，我们都确定了协同自由能——即，与独立的α和β亚基结合的相同位点相比，配体结合自由能的偏差。这些协同自由能是通过对四聚体解离为二聚体的测量来解析的，其中每个亚基要么未结合配体（亚铁脱氧），要么通过转化为氰化高铁形式（高铁CN）而“结合”配体。结果表明，每个血红蛋白四聚体都作为一个三级分子开关。在配体结合过程中，总的协同自由能（在所有四个结合步骤中为6千卡/摩尔）在与特定配体结合步骤同步的两个转变中消耗。协同能量转变是否发生取决于配体结合步骤如何改变结合配体的亚基数量和构型。因此，血红蛋白四聚体是一个“组合开关”。十种配体结合状态存在三个不同的自由能水平，这表明血红蛋白四聚体存在三种主要的结构形式。
