The primary structure of hemoglobin D from the Aldabra giant tortoise, Geochelone gigantea

Molecular cloning and expression of α-globin and β-globin genes from crocodile (Crocodylus siamensis)

The first report of complete nucleotide sequences for α- and β-globin chains from the Siamese hemoglobin (Crocodylus siamensis) is given in this study. The cDNAs encoding α- and β-globins were cloned by RT-PCR using the degenerate primers and by the rapid amplification of cDNA ends method. The full-length α-globin cDNA contains an open reading frame of 423 nucleotides encoding 141 amino acid residues, whereas the β-globin cDNA contains an open reading frame of 438 nucleotides encoding 146 amino acid residues. The authenticity of both α- and β-globin cDNA clones were also confirmed by the heterologous expression in Escherichia coli (E. coli). This is the first time that the recombinant C. siamensis globins were produced in prokaryotic system. Additionally, the heme group was inserted into the recombinant proteins and purified heme-bound proteins were performed by affinity chromatography using Co(2+)-charged Talon resins. The heme-bound proteins appeared to have a maximum absorbance at 415 nm, indicated that the recombinant proteins bound to oxygen and formed active oxyhemoglobin (HbO2). The results indicated that recombinant C. siamensis globins were successfully expressed in prokaryotic system and possessed an activity as ligand binding protein.

Side-necked turtle (Pleurodira, Chelonia, reptilia) hemoglobin: cDNA-derived primary structures and X-ray crystal structures of Hb A

Red blood cells of yellow-spotted river turtles (Podocnemis unifilis, Pleurodira, Chelonia, REPTILIA) have two hemoglobin (Hb) components, Hb A and Hb D. We purified the hemoglobin component homologous to amniote (reptiles, birds, and mammals) adult Hb A which comprises two identical α(A) -globin polypeptides and two identical β-globin polypeptides. To establish the crystal structure of Podocnemis Hb A, we first determined the globin primary structures using cDNA nucleotide sequencing with the assistance of protein sequencing. The purified Podocnemis Hb A produced a different form of crystal for each of the two different buffer systems used: form A, tetragonal crystals (space group, P4₁2₁2), produced under neutral pH (pH 7-8) conditions; and form B, hexagonal crystals (space group, P6₁22), produced under high alkaline pH (pH 11-13) conditions. Single crystals of the two forms were examined by Raman microscopy with an excitation of 532 nm, indicating their structural differences. The crystal structures of the two forms were constructed by X-ray crystallographic diffraction at a resolution of 2.20 Å for form A and 2.35 Å for form B. The differences of the tertiary and quaternary structures of the two forms were marginal; however, one clear difference was found in helix structure. When comparing Podocnemis Hb A with Hb A from specimens in other taxa, such as Anser indicus (birds) and Homo sapiens (mammals) by SHELXPRO, the root mean square deviation (RMSD) between the corresponding Cα atoms of the two globins does not exceed 2.0 Å. These low values indicate the crystal structures resemble each other. Our data on X-ray crystal structures and Raman spectra not only reveal the first findings on the two crystal forms of Podocnemis unifilis Hb A but also provide the first refined models for reptilian adult Hb A.

Organisation of the Hb 1 genes of the Antarctic skate Bathyraja eatonii: new insights into the evolution of globin genes

An extensive investigation of the organisation of globin genes has greatly contributed to the understanding of universal mechanisms of gene evolution and expression. Cartilaginous fish are the first organisms that have evolved the tetrameric form of hemoglobin (Hb). So far, there has been absolute lack of data about globin genes in chondrichthyans. Bathyraja is the dominant rajid south of 60 degrees S. In the framework of the investigations on globin genes of Antarctic red-blooded and Hb-less fish we obtained the cloning of the alpha- and beta-globin cDNAs of the main Hb (Hb 1) of the skate Bathyraja eatonii. Then, a genomic fragment of 6.2 kb was isolated where the Hb 1 alpha and beta genes are linked in a tail-to-head (3' to 5') orientation. The beta-globin gene promoter region and the chromosomal organisation of the Hb 1 genes of B. eatonii have been compared to their homologues in other vertebrates. The finding of a tail-to-head linkage of the Hb 1 alpha- and beta-globin genes in B. eatonii is the first characterisation of the organisation of globin genes in chondrichthyes; such finding offers a novel contribution to the understanding of the evolution of this class of genes. Moreover, the characterisation of chondrichthyan genes is very important for gaining insight into the ancestral state of vertebrate genomes.

The alphaD-globin gene originated via duplication of an embryonic alpha-like globin gene in the ancestor of tetrapod vertebrates

Gene duplication is thought to play an important role in the co-option of existing protein functions to new physiological pathways. The globin superfamily of genes provides an excellent example of the kind of physiological versatility that can be attained through the functional and regulatory divergence of duplicated genes that encode different subunit polypeptides of the tetrameric hemoglobin protein. In contrast to prevailing views about the evolutionary history of the alpha-globin gene family, here we present phylogenetic evidence that the alpha(A)- and alpha(D)-globin genes are not the product of a single, tandem duplication of an ancestral globin gene with adult function in the common ancestor of extant birds, reptiles, and mammals. Instead, our analysis reveals that the alpha(D)-globin gene of amniote vertebrates arose via duplication of an embryonic alpha-like globin gene that predated the radiation of tetrapods. The important evolutionary implication is that the distinct biochemical properties of alpha(D)-hemoglobin (HbD) are not exclusively derived characters that can be attributed to a post-duplication process of neofunctionalization. Rather, many of the distinct biochemical properties of HbD are retained ancestral characters that reflect the fact that the alpha(D)-globin gene arose via duplication of a gene that had a larval/embryonic function. These insights into the evolutionary origin of HbD illustrate how adaptive modifications of physiological pathways may result from the retention and opportunistic co-option of ancestral protein functions.

The mammalian αD-globin gene lineage and a new model for the molecular evolution of α-globin gene clusters at the stem of the mammalian radiation

We have explored the evolution of the alpha-globin gene family by comparative sequence and phylogenetic analyses of mammalian alpha-globin genes. Our analyses reveal the existence of a new alpha-globin gene lineage in mammals that is related to the alpha(D)-globin genes of birds, squamates and turtles. The gene is located in the middle of the alpha-globin gene cluster of a marsupial, Sminthopsis macroura and of humans. It exists in a wide variety of additional mammals, including pigs, cows, cats, and dogs, but is a pseudogene in American marsupials. Evolutionary analyses suggest that the gene has generally evolved under purifying selection, indicative of a functional gene. The presence of mRNA products in humans, pigs, and cows also suggest that the gene is expressed and likely to be functional. The analyses support the hypothesis that the alpha(D)-globin gene lineage has an ancient evolutionary origin that predates the divergence of amniotes. The structural similarity of alpha-globin gene clusters of marsupials and humans suggest that an eight gene cluster (5'-zeta2-zeta1-alpha(D)-alpha3-alpha2-alpha1-theta-omega-3'), including seven alpha-like genes and one beta-like globin gene (omega-globin) existed in the common ancestor of all marsupial and eutherian mammals. This basic structure has remained relatively stable in marsupials and in the lineage leading to humans, although omega-globin has been lost from the alpha-globin gene cluster of humans.

Axolotl hemoglobin: cDNA-derived amino acid sequences of two α globins and a β globin from an adult Ambystoma mexicanum

Erythrocytes of the adult axolotl, Ambystoma mexicanum, have multiple hemoglobins. We separated and purified two kinds of hemoglobin, termed major hemoglobin (Hb M) and minor hemoglobin (Hb m), from a five-year-old male by hydrophobic interaction column chromatography on Alkyl Superose. The hemoglobins have two distinct alpha type globin polypeptides (alphaM and alpham) and a common beta globin polypeptide, all of which were purified in FPLC on a reversed-phase column after S-pyridylethylation. The complete amino acid sequences of the three globin chains were determined separately using nucleotide sequencing with the assistance of protein sequencing. The mature globin molecules were composed of 141 amino acid residues for alphaM globin, 143 for alpham globin and 146 for beta globin. Comparing primary structures of the five kinds of axolotl globins, including two previously established alpha type globins from the same species, with other known globins of amphibians and representatives of other vertebrates, we constructed phylogenetic trees for amphibian hemoglobins and tetrapod hemoglobins. The molecular trees indicated that alphaM, alpham, beta and the previously known alpha major globin were adult types of globins and the other known alpha globin was a larval type. The existence of two to four more globins in the axolotl erythrocyte is predicted.

Mapping of alpha- and beta-globin genes on Antarctic fish chromosomes by fluorescence in-situ hybridization

The pathways and mechanisms of genomic change that have led to the peculiar haemoglobinless phenotype of the white-blooded Antarctic icefishes (16 species in the family Channichthyidae) constitute an important model for understanding the rapid diversification of the Antarctic notothenioid fish flock. To provide complementary structural information on genomic change at globin-gene loci in Antarctic fish species, cytogenetic studies and in-situ chromosomal mapping have been undertaken. Using a DNA probe containing one alpha- and one beta-globin gene from the embryonic/juvenile globin gene cluster of the red-blooded species Notothenia coriiceps, we mapped the cluster on the chromosomes of Antarctic teleosts by fluorescence in-situ hybridization. As anticipated on the basis of its molecular organization, the cluster was located on a single chromosome pair in all of the red-blooded fish species probed (N. coriiceps, N. angustata, Trematomus hansoni, T. pennellii). In contrast, the alpha/beta-globin probe did not recognize complementary sequences on the chromosomes of the white-blooded species Chionodraco hamatus and Channichthys rhinoceratus. These results represent the first example of chromosomal mapping of embryonic/juvenile globin genes in teleostean fishes. Beyond its relevance to the evolutionary history of Antarctic notothenioids, this work contributes to our understanding of the evolution of the chromosomal loci of globin genes in fishes and other vertebrates.

Molecular characterization of hemoglobin alpha-D chains from Geochelone carbonaria and Geochelone denticulata land turtles

In order to help elucidate the evolution of alpha-globins, the complete cDNA and amino acid sequences of Geochelone carbonaria and Geochelone denticulata land turtles alpha-D chains have been described. In G. carbonaria, the cDNA is 539 bp with ATG start codon located at position 46, TGA stop codon at position 469 and AATAAA polyadenylation signal at position 520. In G. denticulata, the cDNA is 536 bp with ATG start codon located at position 46, TGA stop codon at position 469 and AATAAA polyadenylation signal at position 517. Both cDNAs codify 141 amino acid residues, differing from each other in only four amino acid residues. When comparing with human Hb alpha-chain, alterations in important regions can be noted: alpha110 Ala-Gly, alpha114 Pro-Gly, alpha117 Phe-Tyr and alpha122 His-Gln. There is a high homology between the amino acids of these turtles when compared with chicken alpha-D chains, progressively decreasing when compared with human, crocodile, snake, frog and fish alpha-chains. Phylogenetic analysis of alpha-D chains shows that those of turtles are closer to those of birds than to snakes and lizards.