The quaternary structure of a molluscan (Helisoma trivolvis) extracellular hemoglobin

Abstract

The hemoglobin (erythrocruorin) of the planorbid mollusc Helisoma trivolvis has a molecular weight of 1.7-10(6) and a sedimentation coefficient (s0 20, w) of 33.8 S at pH 7.0. At pH 2.0, the pigment consists of 32 S and 13 S material. The hemoglobin exists as a 350 000 molecular weight submultiple in 6 M guanidine and can be further dissociated into a 175-200 000 dalton polypeptide in 6M guanidine, 0.1 M 2-mercaptoethanol or by sodium dodecyl sulfate gel electrophoresis of globin, performic acid oxidized globin or carboxymethylated globin. Electron microscope observations show a ten-membered ring structure measuring 200 A in diameter. It is proposed that Helisoma hemoglobin consists of a 1.7-10(6) dalton circular assembly of ten 175-200 000 dalton polypeptide chains. The amino acid composition of the pigment is reported. The hemoglobin contains one heme per 18-19 000 g protein. Limited proteolysis of the intact pigment shows 60 000, 40 000 and 17 000-18 500 dalton components when analyzed by sodium dodecylsulfate gel electrophoresis. It is likely that the 175-200 000 dalton polypeptide consists of a linear arrangement of 8-12 heme-containing domains, each domain having a molecular weight of 18-19 000.