Structural basis for cooperative oxygen binding and bracelet-assisted assembly of Lumbricus terrestris hemoglobin.

Wei-Ting Chen,Chih-Yu Chao,Horng-Huei Liou,Yu-Chuen Chen

doi:10.1038/srep09494

Wei-Ting Chen, Chih-Yu Chao + Show 2 more

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https://doi.org/10.1038/srep09494

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Abstract

The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood. Among various kinds of Hbs, the earthworm Hbs are the champions in carrying oxygen due to not only their large size but also the unusually high cooperativity of ligand binding. However, the cooperative oxygen binding mechanisms are still mostly unknown. Here we report the cryo-electron microscopy structure of Lumbricus terrestris Hb in its native, oxygenated state at 9.1 Å resolution, showing remarkable differences from the carbon monoxide-binding X-ray structure. Our structural analysis first indicates that the cooperative ligand binding of L. terrestris Hb requires tertiary and quaternary transitions in the heme pocket and a global subunit movement facilitated by intra-ring and inter-ring contacts. Moreover, the additional sinusoidal bracelet provides the confirmation for the long-standing debate about the additional electron densities absent in the X-ray crystal structure.

Highlights

The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood
To investigate the conformational change induced upon O2 binding, we carried out single particle cryo-EM analysis of L. terrestris Hb in its oxygenated state (Fig. 1a and 1b)
The focus of our research was to investigate the structural transitions induced by different ligand binding to the giant hexagonal-bilayer Hb of L. terrestris

Summary

Introduction

The iron-containing hemoglobins (Hbs) are essential proteins to serve as oxygen transporters in the blood. We report the cryo-electron microscopy structure of Lumbricus terrestris Hb in its native, oxygenated state at 9.1 Aresolution, showing remarkable differences from the carbon monoxide-binding X-ray structure. Packing a lot of functional units into one particle can prevent too high viscosity of the blood while maximizing O2 transport Due to their large size, extracellular nature, and resistance to oxidation, recent studies have shown that the Hb from the common earthworm Lumbricus terrestris could be a promising O2 carrier to be used in transfusion medicine[2]. The 3.5 Aresolution crystal structure of L. terrestris Hb reported by Royer and co-workers[7] is a remarkable milestone This structure provides the first atomic model for an entire megadalton respiratory protein, and reveals detailed hierarchical arrangement of 180 polypeptide chains. The smaller central cavity observed in some cryo-EM structures[6,14,15] and the sinusoidal pillars observed in the 14.9 Acryo-EM structure[4] imply the plausible existence of additional central densities which are still absent in the X-ray structure

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