The Structure of Erythrocruorins and Chlorocruorins, the Invertebrate Extracellular Hemoglobins

Abstract

The intermittent occurrence of intracellular and extracellular hemoglobins among the invertebrates may be due either to the intermittent presence of functional globin genes or to defects in the mechanism regulating their expression. In the absence of information concerning the structure and expression of invertebrate globin genes, I would like to focus on the identification of common structural themes among these molecules. All the extracellular invertebrate hemoglobins whose subunit structures are known can be classified into four separate groups: (A) single-domain, single-subunit molecules, i. e. monomeric hemoglobins, (B) two-domain, multi-subunit hemoglobins, consisting of aggregates of dimeric polypeptide chains, each containing two heme-binding domains, (C) multi-domain, multi-subunit hemoglobins, consisting of two or more polypeptide chains, each comprising many heme-binding domains and (D) single domain, multi-subunit hemoglobins, consisting of aggregates of monomeric and dimeric subunits and disulfide-bonded trimers or tetramers. A diagrammatic representation of the subunits found in the four classes is shown in figure 1.