Electron addition to the (FeO2) unit of oxyhaemoglobin Glycera

Exposure of glassy solutions containing the monomeric fraction of the oxyhaemoglobin of the polychaete annelid Glycera dibranchiata to 60Co gamma-rays at 77 K resulted in electron addition to the (FeO2) moiety. The form of the g tensor components obtained from the ESR spectrum indicates that the spin-density on oxygen is much greater than that observed for similar paramagnetic centres formed in haemoglobin A or myoglobin. A major difference between these monomer haem units and normal haem units is that the distal histidine (E7 58) is replaced by leucine. We therefore postulate that the oxygen in the (FeO2)- units formed in haemoglobin A and myoglobin is hydrogen-bonded to the NH group of the distal histidine, whilst that of the (FeO2)- units in haemoglobin Glycera are not hydrogen-bonded. However, on annealing to approx. 160 K the spectrum changed irreversibly into one resembling those for (FeO2)- units in haemoglobin A and myoglobin. We postulate that this is caused by hydrogen-bonding to a water molecule in the haem pocket. Exposure of the polymeric fractions of haemoglobin Glycera to gamma-rays gave an (FeO2)- unit with an ESR spectrum remarkably similar to that obtained from oxymyoglobin. The X-ray structure of this protein is unknown but we suggest that our results could indicate the presence of a distal histidine in this material.