The iron-oxygen bond in human oxyhaemoglobin

Knowledge of the exact nature of the iron-oxygen bond in oxyhaemoglobin (oxyHb) is essential for the understanding of cooperative oxygen binding to haemoglobin (Hb)¹. However, X-ray studies of oxyHb have previously been hindered by the tendency of oxyHb crystals to autoxidize. Here we report the stereochemistry of the haem-oxygen complex in human oxyHb, as determined by single-crystal X-ray analysis. Bent end-on geometry of the iron-oxygen bond in haem proteins, as predicted by Pauling², was first observed in the 'picket fence' complex³ and has since been observed in oxymyoglobin (oxyMb)^4,5; in oxyerythrocruorin, however, the Fe - O - O bond is almost linear⁶. In oxyHb the Fe - O - O bond angle is 156°, intermediate between the 'picket fence' compound and erythrocruorin. The position of N^ε of His E7 in the α-subunit suggests that it forms a hydrogen bond with the bound oxygen, as in oxyMb⁷. In the β-subunit, however, N^ε of His E7 is located further from the oxygen, suggesting that the hydrogen bond is weaker.