title: Structure of an extracellular giant hemoglobin of the gutless beard worm Oligobrachia mashikoi.
abstract: Mouthless and gutless marine animals, pogonophorans and vestimentiferans, obtain their nutrition solely from their symbiotic chemoautotrophic sulfur-oxidizing bacteria. These animals have sulfide-binding 400-kDa and/or 3,500-kDa Hb, which transports oxygen and sulfide simultaneously. The symbiotic bacteria are supplied with sulfide by Hb of the host animal and use it to provide carbon compounds. Here, we report the crystal structure of a 400-kDa Hb from pogonophoran Oligobrachia mashikoi at 2.85-A resolution. The structure is hollow-spherical, composed of a total of 24 globins as a dimer of dodecamer. This dodecameric assemblage would be a fundamental structural unit of both 400-kDa and 3,500-kDa Hbs. The structure of the mercury derivative used for phasing provides insights into the sulfide-binding mechanism. The mercury compounds bound to all free Cys residues that have been expected as sulfide-binding sites. Some of the free Cys residues are surrounded by Phe aromatic rings, and mercury atoms come into contact with these residues in the derivative structure. It is strongly suggested that sulfur atoms bound to these sites could be stabilized by aromatic-electrostatic interactions by the surrounding Phe residues.
authors.affiliation: Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan.
authors.forename: Nobutaka
authors.initials: N
authors.lastname: Numoto
authors.forename: Taro
authors.initials: T
authors.lastname: Nakagawa
authors.forename: Akiko
authors.initials: A
authors.lastname: Kita
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Sasayama
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Fukumori
authors.forename: Kunio
authors.initials: K
authors.lastname: Miki
mesh_terms: D000818:Animals; D000845:Annelida / Q000737:chemistry* / Q000235:genetics; D001483:Base Sequence; D018360:Crystallography, X-Ray; D006454:Hemoglobins / Q000737:chemistry* / Q000235:genetics / Q000648:ultrastructure; D008628:Mercury / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017422:Sequence Analysis, DNA; D013440:Sulfides / Q000737:chemistry
chemical_list: D006454:Hemoglobins; D013440:Sulfides; D008628:Mercury