Structural studies of a branchiopod crustacean (Lepidurus bilobatus) extracellular hemoglobin. Evidence for oxygen-binding domains.

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.