Bioinformatic characterization of Lucina pectinata hemoglobins (778.9)

Abstract

Lucina pectinata is a bivalve clam that is found in sulfide rich muds environments and lives in symbiosis with chemoautotrophic bacteria. Three different hemoglobins have been described in L. pectinata and in addition to their physiological role in the clam, are believed to be responsible for transportation hydrogen sulfide (H2S) and oxygen (O2) to a chemoautotrophic bacterial symbiont. Hemoglobin I (HbI) is a sulfide‐reactive hemeprotein while Hemoglobin II (HbII) and Hemoglobin III (HbIII) are oxygen‐reactive hemeproteins. Multiple sequence alignment analysis carried with L. pectinata hemoglobins and other mollusk globin sequences showed several conserved residues within this subfamily. Motif analysis using MEME also revealed a motif shared only between HbII and HbIII sequences. A phylogenetic tree was constructed the 19 molluscan globin sequences obtain after doing the CD‐Hit clustering. The tree separated them into three main clusters, a cluster for the class of Pteriomorphia, another for Heteroconchia and other for Gastropods. Hemoglobins of L. pectinata, which are found in the heteroconchia cluster, are group with the globins of those of the Calyptogena sp, where HbI is more related to this species while HbII and HbIII seen to be more distant. Based on the phylogenetic analysis and the reconciliation with the species tree, we suggest that a first gene duplication at an early evolutionary stage resulted in the sulphide binding and oxygen binding genes. A more recent gene duplication of the oxygen binding gene occurred to give rise to two oxygen binding proteins, HbII and HbIII. This hypothesis is supported with the fact that HbI shows low sequence homology to both HbII and HbIII, while there are more similarities between HbII and HbIII. In addition, HbI appears to have been changing over time, apparently not subject to the constraint of binding oxygen and acquired a unique function for a specialized environment.