Stereochemistry of iron in deoxyhaemoglobin.

Abstract

X-ray crystallography has shown that in deoxyhaemoglobin the iron atoms are displaced by 0.56±0.03 A from the mean porphyrin plane, whereas in liganded haemoglobins they lie either in or close to that plane1,2. Perutz3 proposed this shift as one principal factor responsible for haem–haem interaction. Eisenberger et al. used extended X-ray absorption fine structure (EXAFS) to measure the Fe–N distance in deoxyhaemoglobin and concluded that the irons lie only 0.2+0.1−0.2 A from the plane of the porphyrin nitrogens and that the mechanism of Perutz is therefore invalid4. We have now compared the EXAFS of deoxyhaemoglobin with that of the ferrous ‘picket fence’ 2-methylimidazole complex in which the displacement of the iron from the plane of the porphyrin nitrogens is known to be 0.399±0.004 and 0.426±0.004 A from the mean porphyrin plane (ref. 5). The two EXAFS spectra are very similar, consistent with similar displacements of the irons. We find the same Fe–N distance of 2.06±0.01 A in deoxyhaemoglobin as Eisenberger et al., but show that the displacement of the iron cannot be calculated from that distance.