6533b851fe1ef96bd12aa0e0

RESEARCH PRODUCT

8 Å cryo-EM structure of the giant hemoglobin from the planorbid snail Biomphalaria glabrata

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Until 2006, snail red hemoglobin remained a phylogenetic enigma because it occurs quite isolated in a single gastropod family, the Planorbidae, whereas all other gastropods use blue hemocyanin as a respiratory protein (for recent cryo-EM of hemocyanin, see [1,2]). Moreover, sequence data on this snail hemoglobin were completely lacking. In 2006, our group published the complete cDNA and predicted amino acid sequence of two Biomphalaria glabrata hemoglobin polypeptides, termed BgHb1 and BgHb2 [3]. (Biomphalaria is intermediate host of the human parasite Schistosoma mansoni that causes Bilharziosis.) Resembling pearl-chains, both polypeptide subunits encompass 13 different, cysteine-free globin domains (∼17 kDa each; total subunit Mr ∼238 kDa). In addition, there is a small N-terminal non-globin “plug” domain (∼10 kDa) which contains three cysteines for subunit dimerization. Phylogenetic interpretation of the sequence data showed that Biomphalaria hemoglobin evolved from myoglobin which is present in the radula muscle of most gastropods. Possibly, this hemoglobin evolved to replace a less efficient hemocyanin which was detected in Biomphalaria hemolymph as a trace component [3].