Oxygen equilibrium ofCucumaria miniata hemoglobin and the absence of the Bohr effect

Phylogeny of Echinoderm Hemoglobins

BACKGROUND

Recent genomic information has revealed that neuroglobin and cytoglobin are the two principal lineages of vertebrate hemoglobins, with the latter encompassing the familiar myoglobin and α-globin/β-globin tetramer hemoglobin, and several minor groups. In contrast, very little is known about hemoglobins in echinoderms, a phylum of exclusively marine organisms closely related to vertebrates, beyond the presence of coelomic hemoglobins in sea cucumbers and brittle stars. We identified about 50 hemoglobins in sea urchin, starfish and sea cucumber genomes and transcriptomes, and used Bayesian inference to carry out a molecular phylogenetic analysis of their relationship to vertebrate sequences, specifically, to assess the hypothesis that the neuroglobin and cytoglobin lineages are also present in echinoderms.

RESULTS

The genome of the sea urchin Strongylocentrotus purpuratus encodes several hemoglobins, including a unique chimeric 14-domain globin, 2 androglobin isoforms and a unique single androglobin domain protein. Other strongylocentrotid genomes appear to have similar repertoires of globin genes. We carried out molecular phylogenetic analyses of 52 hemoglobins identified in sea urchin, brittle star and sea cucumber genomes and transcriptomes, using different multiple sequence alignment methods coupled with Bayesian and maximum likelihood approaches. The results demonstrate that there are two major globin lineages in echinoderms, which are related to the vertebrate neuroglobin and cytoglobin lineages. Furthermore, the brittle star and sea cucumber coelomic hemoglobins appear to have evolved independently from the cytoglobin lineage, similar to the evolution of erythroid oxygen binding globins in cyclostomes and vertebrates.

CONCLUSION

The presence of echinoderm globins related to the vertebrate neuroglobin and cytoglobin lineages suggests that the split between neuroglobins and cytoglobins occurred in the deuterostome ancestor shared by echinoderms and vertebrates.

Functional and biochemical properties of the hemoglobins of the burrowing brittle star Hemipholis elongata say (Echinodermata, Ophiuroidea)

The burrowing brittle star Hemipholis elongata (Say) possesses hemoglobin-containing coelomocytes (RBCs) in its water vascular system. The RBCs, which circulate between the arms and body, are thought to play a role in oxygen transport. The hemoglobin of adult animals has a moderate affinity for oxygen (P(50) = 11.4 mm Hg at pH 8, 20 degrees C, measured in cellulo) and exhibits cooperativity (Hill coefficient > 1.7). The hemoglobin of juveniles has a higher affinity (P(50) = 2.3 mmHg at pH 8.0, 20 degrees C) and also exhibits cooperativity. The oxygen-binding properties of the hemoglobin are relatively insensitive to pH, temperature, and hydrogen sulfide. Adult hemoglobin is a heterogeneous mixture composed of three major fractions. The combined results of electrospray mass spectrometry and oxygen-binding experiments performed on purified fractions indicate that the native hemoglobin is in the form of homopolymers. A partial amino acid sequence (about 40 amino acids) of adult hemoglobin reveals little homology with holothurian hemoglobins.

Oxygen equilibrium and subunit aggregation of a holothurian hemoglobin

The hemoglobin of the sea cucumber Cucumaria miniata Brandt has a mol. wt of about 36000 in the oxy- form with a s20,w equal to 2.9 and a subunit molecular weight of 18000 by sodium dodecylsulfate gel electrophoresis. This pigment aggregates when deoxygenated to an oligomer with a s20,w equal to 4.7, an aggregation which is reversible upon subsequent oxygenation. The hemoglobin shows a sigmoid binding equilibrium with "n" equal to 1.8 and a decrease in oxygen affinity with an increase in pigment concentration. This hemoglobin is compared with other hemoglobins showing oxygenation-linked subunit aggregation.

Sea cucumber sibling species: polypeptide chain types and oxygen equilibrium of hemoglobin

The hemoglobin of the "thin" sibling species of Thyonella gemmata (phylum: Echinodermata; class: Holothuria) has three electrophoretically distinct polypeptide chains. In "stout" sibling species of T. gemmata there are only two chain types. These results account for the greater number of multiple hemoglobins in "thins" than in "stouts," as well as for differences in the amounts of some of the multiple hemoglobins when comparisons are mnade of hemolyzates of erythrocytes from the water vascular systemn and from the main body cavity of the "thin" but not the "stout" sibling species.