Dangott L J, Terwilliger R C
Biochim Biophys Acta. 1979 Aug 28;579(2):452-61. doi: 10.1016/0005-2795(79)90072-2.
The extracellular hemoglobin of the notostracan branchiopod Lepidurus bilobatus has an apparent molecular weight of 680,000 and may exist in a dissociation-association equilibrium dependent on pH and ligand state. The pigment contains one heme per 18,000 g protein. However, attempts to dissociate the hemoglobin by harsh denaturing conditions results in a 33-34,000 molecular weight polypeptide chain as well as traces of some 62-64,000 molecular weight material. Limited proteolysis of this hemoglobin with subtilisin produces 14,800 and 16,500 dalton heme-containing polypeptides (domains) which bind oxygen reversibly. These domains, isolated by column chromatography, have a heme content similar to the intact pigment. It is proposed that the intact 34,000 dalton subunit of Lepidurus hemoglobin consists of two linearly linked oxygen binding domains. Oxygen binding properties of the intact hemoglobin show a low oxygen affinity with a slight Bohr effect. In contrast, the isolated domains display a relatively high oxygen affinity and lack a Bohr effect between pH 7.0 and 8.0. It is apparent that the intact 34,000 dalton polypeptide is necessary for the expression of the heterotropic interactions of the native pigment.
背甲目鳃足动物双叶鲎的细胞外血红蛋白的表观分子量为680,000，可能存在依赖于pH和配体状态的解离-缔合平衡。该色素每18,000克蛋白质含一个血红素。然而，试图通过苛刻的变性条件使血红蛋白解离，会产生分子量为33 - 34,000的多肽链以及一些分子量约为62 - 64,000的物质的痕迹。用枯草杆菌蛋白酶对这种血红蛋白进行有限的蛋白水解会产生14,800和16,500道尔顿的含血红素多肽（结构域），它们可可逆地结合氧气。通过柱色谱分离得到的这些结构域的血红素含量与完整色素相似。有人提出，双叶鲎血红蛋白完整的34,000道尔顿亚基由两个线性连接的氧结合结构域组成。完整血红蛋白的氧结合特性显示出低氧亲和力和轻微的玻尔效应。相比之下，分离出的结构域在pH 7.0至8.0之间表现出相对较高的氧亲和力且没有玻尔效应。很明显，完整的34,000道尔顿多肽对于天然色素的异促相互作用的表达是必需的。
