Molecular architecture of annelid erythrocruorins. Extracellular hemoglobin of Arenicola marina (Polychaeta).

The respiratory protein, erythrocruorin, of the annelid Arenicola marina was investigated. Spectral properties show many analogies with those of vertebrate hemoglobine. 144 heme groups (ferroprotoporphyrin) were found in the whole molecule, which has a relative molecular mass of 3.56 X 10(6), as determined by sedimentation equilibrium, and an isoelectric point of 4.69. Protein dissociation patterns were analysed by electrophoresis after denaturation in the presence of dodecylsulfate, with and without 2-mercaptoethanol. A tentative model associating molecular mass of the native molecule, heme content, molecular mass of the polypeptide chains and functional properties is proposed. A twelfth subunit of A. marina erythrocruorin would contain twelve heme groups arranged in three functional units made up of four protomers, half of these being covalently bound to non-heme chains; two structural chains would be spatially arranged as bonds between the subunits.