Structure of a crystal form of human methemoglobin indicative of fiber formation

Steven B Larson,Chieugiang Nguyen,Robert Cudney,John S Day,Alexander Mcpherson

doi:10.1107/s0907444910040370

Steven B Larson, Chieugiang Nguyen + Show 3 more

Open Access

https://doi.org/10.1107/s0907444910040370

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Abstract

Human methemoglobin was crystallized in a unique unit cell and its structure was solved by molecular replacement. The hexagonal unit cell has unit-cell parameters a = b = 54.6, c = 677.4 Å, with symmetry consistent with space group P6₁22. The unit cell has the second highest aspect ratio of all unit cells contained in the PDB. The 12 molecules in the unit cell describe a right-handed helical filament having no polarity, which is different from the filament composed of HbS fibers, which is the only other well characterized fiber of human hemoglobin. The filaments reported here can be related to canonical sickle-cell hemoglobin filaments and to an alternative sickle-cell filament deduced from fiber diffraction by slight modifications of intermolecular contacts.

Highlights

In the course of our investigations on the propensity of certain small molecules to promote macromolecular crystallization (McPherson & Cudney, 2006; Larson et al, 2007), we grew crystals of human hemoglobin in a number of trials
The crystallographic unit cell has the second highest aspect ratio of any contained in the PDB and the structure implies that the hexagonal crystals are composed of filaments of methemoglobin molecules lying parallel to the c axis
The symmetry and dimensions suggested that the protein had crystallized in the form of continuous helical filaments having 6122 symmetry, a periodicity of 677 Aand 12 hemoglobin molecules per turn of the helix

Summary

Introduction

In the course of our investigations on the propensity of certain small molecules to promote macromolecular crystallization (McPherson & Cudney, 2006; Larson et al, 2007), we grew crystals of human hemoglobin in a number of trials. Methemoglobin does not bind oxygen, but neither is it reported to assume the quaternary arrangement of deoxyhemoglobin This is significant because it is deoxyhemoglobin that forms fibers of sickle-cell hemoglobin as a consequence of the mutation of glutamic acid at position 6 of the chain in normal human hemoglobin to valine in the sickle-cell protein. A second report suggesting the existence of alternate hemoglobin filament forms comes from a study of sickle-cell fibers by Mu & Magdoff-Fairchild (1992). They observed a second filament form that was clearly different from the. Filaments could transform in time to a filament of different structure

Experimental procedures

Results

The dyad interface

Comparison with ‘classical’ sickle-cell filaments