Crystal Structures of Deoxy- and Carbonmonoxyhemoglobin F1 from the Hagfish Eptatretus burgeri

Abstract

Hagfish are extremely primitive jawless fish of disputed ancestry. Although generally classed with lampreys as cyclostomes ("round mouths"), it is clear that they diverged from them several hundred million years ago. The crystal structures of the deoxy and CO forms of hemoglobin from a hagfish (Eptatretus burgeri) have been solved at 1.6 and 2.1 Å, respectively. The deoxy crystal contains one dimer and two monomers in a unit cell, with the dimer being similar to that found in lamprey deoxy-Hb, but with a larger interface and different relative orientation of the partner chains. Ile(E11) and Gln(E7) obstruct ligand binding in the deoxy form and make room for ligands in the CO form, but no interaction path between the two hemes could be identified. The BGH core structure, which forms the ␣ 1 ␤ 1 interface of all vertebrate ␣ 2 ␤ 2 tetrameric Hbs, is conserved in hagfish and lamprey Hbs. It was shown previously that human and cartilaginous fish Hbs have independently evolved stereochemical mechanisms other than the movement of the proximal histidine to regulate ligand binding at the hemes. Our results therefore suggest that the formation of the ␣ 2 ␤ 2 tetramer using the BGH core and the mechanism of quaternary structure change evolved between the branching points of hagfish and lampreys from other vertebrates.In human hemoglobin, the interactions between a ligand bound to a heme and the globin structure can be divided into proximal effects (interactions with the proximal histidine imidazole group and the heme upon ligand binding) and distal effects (interactions with the distal amino acids). The proximal effect moves the F helix and FG corner upon ligand binding and connects the heme to the structure change at the ␣ 1 ␤ 2 interface associated with the quaternary structure change. The distal effect is more important in the ␤ subunit than in the ␣ subunit (1). When the x-ray structures of two cartilaginous fish Hbs were solved (2, 3), the quaternary structure and its change upon ligand binding were preserved, as expected. The proximal effect linking the quaternary structure change to the hemes was also preserved, but the distal effect, especially the role of Val(E11), was altered. The Bohr proton sites and the 2,3-diphosphoglycerate-binding site were not conserved, indicating that stereochemical mechanisms other than the proximal effect have evolved independently in the different species.Hbs from hagfish and lampreys are not of the ␣ 2 ␤ 2 tetramer type. The evolution of the tetramer, including the duplication and differentiation of the globin gene into the ␣ and ␤ genes, the formation of the subunit interfaces, and the mechanism of the quaternary structure change, took place within a comparatively short period between the branching points of hagfish and lampreys from cartilaginous fish.

Water regulates oxygen binding in hagfish (Myxine glutinosa)hemoglobin

“…This response is opposite to that of human hemoglobin, where water stabilizes the oxygenated, high-affinity state of the hemoglobin, but similar to that found in the dimeric hemoglobin from the gastropod mollusc Scapharca inaequivalvis, where 6-8 water molecules bind at the dimer interface in the deoxy conformation (Royer et al, 1996). Interestingly, the recently solved crystal structure of dimeric (deoxy) hagfish hemoglobin (Mito et al, 2002) indicates a subunit arrangement similar (but not identical) to that of lamprey and dimeric Scapharca hemoglobins, with the two heme groups in close contact with each other (Heaslet and Royer, 1999;Royer, 1994).…”

Crystal structures of two hemoglobin components from the midge larva Propsilocerus akamusi (Orthocladiinae, Diptera)

“…For comparison, the parameters from the structures of human Oxy-Hb (PDB code 2DN1), CO-Hb (2DN3), and Deoxy-Hb (2DN2) (Park et al, 2006) are also included. , 1994;Mito et al, 2002). Our comparative studies of the crystal structure of P. akamusi Hb components revealed the characteristics of the tertiary structures of each component, and especially the interesting heme region of component V. It seems that the heme region of aquomet component V is in the intermediate state between a typical ligated (aquomet) and unligated or hemichrome form.…”

“…One is between Cδ of the proximal His and the nitrogen of pyrrole 3 of porphyrin, and the other is between Cε of the proximal His and the nitrogen of pyrrole 1. This tilting links the shift of the F helix toward the FG corner upon ligand binding (Mito et al, 2002). For comparison, the parameters from the structures of human Oxy-Hb (PDB code 2DN1), CO-Hb (2DN3), and Deoxy-Hb (2DN2) (Park et al, 2006) are also included.…”

Gene cooption and convergent evolution of oxygen transport hemoglobins in jawed and jawless vertebrates

“…The formation of heteromultimers composed of unlike subunits appears superficially similar to the α 2 β 2 heterotetramers of most gnathostomes, but the oxygenation-linked transition in quaternary structure is completely different. The intersubunit contacts of heterotetrameric gnathostome Hbs primarily involve the C, G, and H helices of the globin chain subunits (51), whereas the contact surfaces of the deoxygenated, homodimeric cyclostome Hbs involve the E helix and the AB corner, such that the heme groups are in almost direct contact (52)(53)(54)(55). The heme-heme interactions of cyclostome Hbs are intriguingly similar to those of the homodimer of Cygb (23,38,39,56,57), an observation that makes sense in light of our inferred phylogenetic relationship between these two globin proteins (Fig.…”