Bioinformatic Characterization and Molecular Evolution of the Hemoglobins

(1) Introduction: is a clam found in sulfide-rich mud environments that has three hemoglobins believed to be responsible for the transport of hydrogen sulfide (HbI) and oxygen (HbII and HbIII) to chemoautotrophic endosymbionts. The physiological roles and evolution of these globins in sulfide-rich environments are not well understood. (2) Methods: We performed bioinformatic and phylogenetic analyses with 32 homologous mollusk globin sequences. Phylogenetics suggests a first gene duplication resulting in sulfide binding and oxygen binding genes. A more recent gene duplication gave rise to the two oxygen-binding hemoglobins. Multidimensional scaling analysis of the sequence space shows evolutionary drift of HbII and HbIII, while HbI was closer to the hemoglobins. Further corroboration is seen by conservation in the coding region of hemoglobins from compared to those from . (3) Conclusions: Presence of glutamine in position E7 in organisms living in sulfide-rich environments can be considered an adaptation to prevent loss of protein function. In HbI a substitution of phenylalanine in position B10 is accountable for its unique reactivity towards HS. It appears that HbI has been changing over time, apparently not subject to functional constraints of binding oxygen, and acquired a unique function for a specialized environment.