Effect of Removal of a Salt-Bridge on the Oxygen Binding Properties and the Electronic Structure of Heme in Cobalt-Iron Hybrid Hemoglobin

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Abstract
In order to clarify the role of salt-bridges in hemoglobin, the oxygen equilibrium curves and electron paramagnetic resonance (EPR) spectra of cobalt-iron hybrid hemoglobins were determined. The EPR spectra of deoxy α(Co)2β(Fe)2 could be interpreted as a mixture of two distinct paramagnetic species: one showed a maximum of the first derivative spectrum at g=2.39 and the other at g=2.33. The oxygen equilibrium curves of the hybrid indicated that the former is assignable to the T structure and the latter to the R structure. The cooperativity of oxygen binding of α(Co)2β(Fe) was abolished by removal of Arg-141 or. The EPR spectrum of stripped deoxy des-Arg α(Co)2β(Fe)2 exhibited a maximum at g=2.33, which is characteristic of the R structure, regardless of the pH. Addition of inositol hexaphosphate (1HP) to des-Arg α(Co)2β(Fe)2 restored the cooperativity of oxygen binding, which implies that the deoxygenated form of des-Arg α(Co)2β(Fe)2 is converted to the T structure upon addition of IHP. However, the EPR signal at g=2.39 was not restored upon conversion to the T structure by addition of IHP. It is therefore concluded that the EPR spectrum of the deoxy α(Co) subunit depends both on the quaternary structure and on the localized strain at the heme.