Crystal structure of extracellular giant hemoglobin of pogonophoran Oligobrachia mashikoi

Graduate School of Science, Kyoto University* Graduate School of Natural Science and Technology, Kanazawa University, Japan** Research Reactor Institute, Kyoto University, Japan*** Institute of Nature and Environmental Technology, Kanazawa University, Japan**** Natural Science and Technology, Kanazawa University, Japan***** , Graduate School of Science, Kyoto University, Japan / RIKEN SPring-8 Center at Harima Institution, Japan******
â—‹Nobutaka Numoto* Taro Nakagawa** Akiko Kita*** Yuichi Sasayama**** Yoshihiro Fukumori***** Kunio Miki******

Pogonophorans obtain their nutrition mainly from their symbiotic bacteria because they have no mouth or gut. These animals live in sulfide rich seabed and the symbiont is thought to be chemoautotrophic sulfur-oxidizing bacteria. Pogonophorans have 400 kDa extracellular giant hemoglobin (Hb) which can transports oxygen and sulfide simultaneously. The symbiotic bacteria are supplied with sulfide by the giant Hb and afford host organic materials. We have recently determined the crystal structure of a oxygenated giant Hb from pogonophoran Oligobrachia mashikoi at 2.85 Å resolution1). The structure is hollow-spherical, with outer and inner diameters of about 120 Å and 50 Å, respectively, and composed of a total of 24 globins as a dimer of dodecamer. Crystal structure of the reconstructed Hb dodecamer core complex of annelids giant Hb and vestimentiferan coelomic Hb have also been reported as CO and Oxy form, respectively. These giant Hbs have almost the same dodecameric assemblage and it would be a common unit to pogonophorans, vestimentiferans, and annelids giant Hbs. The structure of pogonophoran Hb suggests sulfide-binding mechanism that free Cys residues bind sulfide by forming a persulfide group and it could be stabilized by aromatic-electrostatic interactions by the surrounding Phe residues.
We also determined the crystal structure of partially oxygenated giant Hb of O. mashikoi at higher resolution. The structure of the partially Oxy-Hb is compared with those of other reported Oxy/CO- giant Hbs, especially with that of the Oxy form of O. mashikoi Hb.
1) Numoto, N. et al., Proc. Natl. Acad. Sci. USA, 102, 14521-14526 (2005).