The structure of Urechis caupo hemoglobin in the cyanomet state has been refined to R = 0.148 at 2.5 A resolution. Although the tertiary structure is similar to that of other vertebrate and invertebrate hemoglobins the quaternary structures of this tetramer is unique as suggested by the earlier determination of the 5.0 A resolution structure. The G and H helices of the hemoglobin are on the outside of the tetramer facing the solvent in contrast to human hemoglobin where the G and H helices form inter-subunit contacts. A substantial number of tightly bound water molecules help mediate interactions between subunits. The unusual arrangement of subunits is consistent with the general lack of co-operativity of oxygen uptake for Urechis caupo hemoglobin.
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