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

Megumi Mito,Khoon Tee Chong,Gentaro Miyazaki,Shin-Ichi Adachi,Sam-Yong Park,Jeremy R.H Tame,Hideki Morimoto

doi:10.1074/jbc.m111492200

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 A, 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 alpha1beta1 interface of all vertebrate alpha2beta2 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 alpha2beta2 tetramer using the BGH core and the mechanism of quaternary structure change evolved between the branching points of hagfish and lampreys from other vertebrates.

Highlights

The interactions between a ligand bound to a heme and the globin structure can be divided into proximal effects and distal effects
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 EF corner and G and H helices are shorter than their counterparts in human Hb by 2, 3, and 7 residues, respectively; the F helix is longer by 6 residues, starting earlier in the sequence

Summary

Data collection and structure refinement statistics

Rmerge ϭ ¥h¥iI(h,i) ϪI(h)͉͘/¥h¥iI(h,i)͉, where I(h,i) is the intensity value of the ith measurement of h andI(h)͘ is the corresponding mean value of I(h,i) for all i measurements; the summation is over the reflections with I/␴(I) Ͼ 1.0. Rfree is the R factor calculated for 5% of reflections that were randomly selected and were excluded from the refinement. Rfactor ϭ ¥ʈFo ϪFcʈ/¥͉Fo, where Fo is the observed structure factor and Fc is that calculated from the model

Crystal data Space group Unit cell parameters

EXPERIMENTAL PROCEDURES

RESULTS

Glycera Hb

CO Deoxy CO Deoxy

DISCUSSION