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Erythrocruorin from the crustacean Caenestheria inopinata. Quaternary structure and arrangement of subunits.

The subunit structure of erythrocruorin from the crustacean Caenestheria inopinata was studied. The native protein was found to have a sedimentation coefficient of 12.0 S and a molecular weight, as determined by sedimentation equilibrium, of 302,000. Iron and heme determinations gave 0.346 and 3.98% corresponding to minimal molecular weights of 16,100 and 15,500, respectively. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis gave one band with mobility corresponding to a molecular weight of 30,000. The molecular weight of the polypeptide chain was determined to be 30,500 by sedimentation equilibrium in 6 M guanidine hydrochloride and 0.1 M 2-mercaptoethanol. Dissociation of the 12S molecule was observed at acidic and alkaline pH. A dissociation species of 2.7 S was isolated and its molecular weight determined to be 28,000 by sedimentation equilibrium. On a molecular weight basis, the native molecule is composed of ten 2.7S subunits, each of which consists of a single polypeptide chain carrying two hemes. We propose a model for the molecule composed of ten spheres, each representing a 2.7S subunit, arranged in two layers stacked in an eclipsed orientation, in five spheres of each layer occupying the vertices of a regular pentagon. Support for this arrangement is provided by a comparison of projections of the model with molecular profiles seen in the electron microscope.