Structure of human methaemoglobin: The variation of a theme

There has been considerable interest in the variability of the structure of the liganded haemoglobin after characterization of the R2 state in addition to the original relaxed R state. The structures of three crystallographically independent relaxed haemoglobin molecules have been determined through the X-ray structure analysis of the crystals of human methaemoglobin. The three molecules have quaternary structures intermediate between those of the R and R2 structures. The same is true about the disposition of residues in the switch region. Thus it would appear that haemoglobin can access different relaxed states with varying degrees of similarity among them