Movement of Fe with respect to the heme plane in the R-T transition of carp hemoglobin. An extended x-ray absorption fine structure study.

Carp Hb undergoes a well known change in kinetics over the pH range 6-9. X-ray absorption spectroscopy, in conjunction with refined data analysis procedures, shows no difference in iron-ligand distances when carp HbCO is switched from R (high affinity) to T (low affinity) states. These distances are 2.015 +/- 0.015 A for the average iron-pyrrole nitrogen distance, 2.14 +/- 0.04 A for the iron-nitrogen (of histidine) distance, and 1.89 +/- 0.05 A for the Fe-C (of CO) distance. Examination of the region from 30 to 100 eV above the threshold, called the ligand field indicator region, reveals spectral differences, which when compared to model compounds suggest that the iron and the heme are less coplanar in the T-like forms. These results are consistent with the iron being 0.1 A more out of the mean heme plane in both carp HbCO and carp Hb T states, relative to the R forms, and that the change in iron position on ligation to either T or R state is four times larger than that occurring with the quaternary switch.