The quarternary structure of an unusual high-molecular-weight intracellular haemoglobin from the bivalve mollusc Barbatia reeveana

Abstract

The arcid clam Barbatia reeveana contains an intracellular haemoglobin with an unusual structure. First, compared with other intracellular haemoglobins, it is extremely large, with a mol.wt. of 430000 and an s20,w of 13.6S. A minor component (mol.wt.=220000; s20,w=9.7S) is also present as a probable dissociation product of the major component. Secondly, this haemoglobin has an unusual subunit structure. It contains 1mol of haem per 16000g of protein, in common with most other haemoglobins. However, the smallest polypeptide that could be obtained after treatment with sodium dodecyl sulphate or 6m-guanidine with reducing agent has a mol.wt. of 32000–37000. Digestion of the haemoglobin with the proteinase subtilisin produces both 57000- and 30000-mol.wt. aggregates that contain 1mol of haem per 16000g of protein and that can be dissociated into 16500-mol.wt. polypeptides by treatment with sodium dodecyl sulphate. The intact polymer shows slight co-operativity (h=1.7), lacks a Bohr effect between pH7 and 8, and has a low oxygen affinity [P50=4.8kPa (36mmHg) at 20°C] relative to other haemoglobins. The 30000-mol.wt. aggregate obtained by digestion of the polymer binds oxygen reversibly with an affinity greater than that of the polymer, but with some co-operativity (h=1.7). These results are consistent with the hypothesis that the subunits of this unusually large intracellular haemoglobin are 32000-mol.wt. polypeptides that in turn are composed of two covalently linked haem-containing oxygen-binding domains. This is the first report of an intracellular haemoglobin with such a structure.