Red blood with blue-blood ancestry: Intriguing structure of a snail hemoglobin

Abstract

The phylogenetic enigma of snail hemoglobin, its isolated occurrence in a single gastropod family, the Planorbidae, and the lack of sequence data, stimulated the present study. We present here the complete cDNA and predicted amino acid sequence of two hemoglobin polypeptides from the planorbid Biomphalaria glabrata (intermediate host snail for the human parasite Schistosoma mansoni). Both isoforms contain 13 different, cysteine-free globin domains, plus a small N-terminal nonglobin ''plug'' domain with three cysteines for subunit dimerization (total M r Ϸ 238 kDa). We also identified the native hemoglobin molecule and present here a preliminary 3D reconstruction from electron microscopical images (3 nm resolution); it suggests a 3 ؋ 2-mer quaternary structure (M r Ϸ 1.43 MDa). Moreover, we identified a previously undescribed rosette-like hemolymph protein that has been mistaken for hemoglobin. We also detected expression of an incomplete hemocyanin as trace component. The combined data show that B. glabrata hemoglobin evolved from pulmonate myoglobin, possibly to replace a less-efficient hemocyanin, and reveals a surprisingly simple evolutionary mechanism to create a high molecular mass respiratory protein from 78 similar globin domains.

Hemoglobin of the bloody clam Tegillarca granosa (Tg-HbI) is involved in the immune response against bacterial infection

“…Hb is rare in mollusks, with most harboring copper-containing hemocyanin instead [2]. However, like humans and other vertebrates, the bivalves, belonging to the family Arcidae, contain abundant Hbs in circulating erythrocytes [3,4].…”

Recombinant functional multidomain hemoglobin from the gastropod Biomphalaria glabrata

“…1d); it was identified by immunoblotting (not shown). An apparent molecular mass of 180 kDa on SDS gels is also exhibited by the native BgHb subunits, although from the amino acid sequence, a molecular mass of 240 kDa is calculated; this discrepancy has been interpreted as an example of irregular migration on SDS gels (6). As deduced from its reddish color, also the supernatant contained hemoglobin (see Fig.…”

“…In some invertebrate groups, they occur as extracellular, multisubunit complexes, which have been extensively studied in some annelids, notably Lumbricus terrestris (1,2). Another type of giant hemoglobin occurs in planorbid snails (3)(4)(5)(6), which is amazing because all other gastropod groups express hemocyanin as a respiratory protein (for details of the structure of gastropod hemocyanin, see 7). Hemoglobin of the planorbid Biomphalaria glabrata shows a moderate-to-high oxygen affinity and apparently evolved from ancestral planorbid myoglobin, probably to replace hemocyanin, which might be less efficient in hypoxic habitats (5,6).…”

“…Another type of giant hemoglobin occurs in planorbid snails (3)(4)(5)(6), which is amazing because all other gastropod groups express hemocyanin as a respiratory protein (for details of the structure of gastropod hemocyanin, see 7). Hemoglobin of the planorbid Biomphalaria glabrata shows a moderate-to-high oxygen affinity and apparently evolved from ancestral planorbid myoglobin, probably to replace hemocyanin, which might be less efficient in hypoxic habitats (5,6). The members of the genus Biomphalaria are typically found in tropical ponds often exposed to drastic variations in physicochemical factors also affecting oxygen-binding conditions (5).…”

“…Both complete subunits, BgHb1 and BgHb2, are composed of 13 paralogus heme domains (termed BgHba to BgHb-m), and a nonheme plug domain, termed BgHb-p (6). As cysteines are only detected in the N-terminal plug domain, the latter is responsible for subunit dimerization, which is required for establishing the quaternary structure (6). To collect more data, recombinant expression of BgHb components was attempted, and the products were analyzed structurally and functionally.…”