Electron transfer between α and β haem groups in haemoglobin. An electron spin resonance study

Abstract

We have previously shown that when haemoglobin hybrids containing Fe^III and (FeO₂) units are exposed to ⁶⁰Co γ-rays, ejected electrons add in a highly specific manner to both units, addition to the (FeO₂) units being preferred. This suggested that intramolecular electron transfer from the unstable (FeO₂)^– centres to Fe^III to give the stable Fe^II form is immeasurably slow in the 77–150 K range. [We cannot use higher temperatures because the (FeO₂)^– units change, probably as a result of protonation to give Fe—O₂H units.] This result was unexpected in view of the relatively fast reactions occurring over similar distances in proteins, including haemoglobin hybrids. One possible error in our argument lies in the cooperative effect of oxygen binding which could negate the results, since the transfer considered is clearly impossible within (FeO₂)₃(FeO^–₂) molecules. Thus, even if the transfer within hybrid units were rapid, (FeO₂)^– centres would remain unchanged in (FeO₂)₃(FeO^–₂) molecules, and it could have been these (FeO₂)^– units that were studied, those formed in hybrids having undergone electron transfer so rapidly that they were lost prior to e.s.r. studies.

Working at low concentrations of (FeO₂), we have established that reaction to give (FeO₂)^– remains favourable and that the units remain stable. The results rule out the possibility of fast electron transfer . As a further check, we have studied the stability of (FeO₂)^– units in the mutant haemoglobin M Iwate. The form used, (a₂Mmet β₂oxy) has all its α chains fixed as Fe^III, whereas the β-chains take up oxygen normally to give oxyhaem units. Again, electron capture on irradiation was favoured at the (FeO₂) units, and, at low doses, these must be in molecules containing at least one Fe^III units. The (FeO₂)^– centres were stable in the 77–150 K region, having e.s.r. properties closely similar to those in the β-chains of normal oxyhaemoglobin.

We conclude that electrons are not transferred from (FeO₂)^– to Fe^III at low temperatures when the (FeO₂)^– unit is in the β-chains and Fe^III is in the α-chains.