Occurrence of two architectural types of hexagonal bilayer hemoglobin in annelids: Comparison of 3D reconstruction volumes of Arenicola marina and Lumbricus terrestris hemoglobins

A 3D reconstruction at 25 Angstrom resolution of native hemoglobin of the p olychaete worm Arenicola marina was carried out from frozen-hydrated specim ens examined in the electron microscope. The reconstruction volume of this large extracellular multimeric respiratory pigment appears as a hexagonal b ilayer structure with eclipsed vertices in its upper and lower hexagonal la yers. Conversely, in hemoglobins of oligochaetes, achaetes, and vestimentif erans and in chlorocruorins of the Sabellidae (polychaete) family, the vert ices of the upper layer are 16 degrees clockwise rotated with respect to th ose of the lower layer. The fact that two other polychaete hemoglobins (Alv inella pompejana and Tylorrhynchus heterochaetus) have the same architectur e as Arenicola led us to define two types of hexagonal bilayer hemoglobins/ chlorocruorins: (i) type-I present in oligochaete, achaete, and vestimentif eran hemoglobins and in Sabellidae chlorocruorins; and (ii) type-II present in polychaete hemoglobins, a comparative study of the hemoglobins of Lumbr icus terrestris (type-I) and Arenicola marina (type-II) showed that only tw o small differences located in the c4 and c5 linking units are responsible of the important architectural difference present in oligomers. A likely sc heme proposed to explain the phylogenic distribution of the two types sugge sts that Clitellata, Sabellida (polychaete), and vestimentiferan hemoglobin s and chlorocruorins derive from a type-I ancestral molecule, while Terebel lida (Alvinella), Phyllodocida (Tylorrhynchus), and Scolecida (Arenicola) a nd possibly other polychaetes derive from an ancestor molecule with type-II hemoglobin. The architectures of the hollow globular substructures are hig hly similar in Arenicola and Lumbricus hemoglobins, with 12 globin chains a nd three linking units (c3a, c3b, and c4). The central piece of Arenicola h emoglobin is an ellipsoid while that of Lumbricus is a toroid. No phylogeni c correlation could be found between the structure of the central pieces an d the architecture type. (C) 2001 Academic Press.