Redox equilibria of liganded forms of methemoglobin

The authors examined the redox equilibria of azidomethemoglobin (low spin) and fluoromethemoglobin (high spin). A modified Hill equation was derived which includes the tetramer dimer equilibrium of the oxidized form, and also generalized the 2 state model to incorporate ligand binding to the ferriheme. The pH dependence of the redox Hill's constant for fluoromethemoglobin is the same as that for methemoglobin, demonstrating that this dependence and the marked cooperativity achieved (n =2.2) are not coupled to changes of the ferriheme spin state. The redox Hill's constant for azidomethemoglobin, however, is as large as the oxygenation Hill's constant (n 2.7) and is also roughly pH independent.