The spectral (e.p.r. and absorbance) properties of the NO and deoxy derivatives of ferrous horseradish peroxidase (HRP; EC 188.8.131.52) and baker's-yeast cytochrome c peroxidase (CCP; EC 184.108.40.206) were investigated between pH 7 and pH 2; over the same pH range the kinetics for CO binding were also determined. At neutral pH the e.p.r. and absorption spectra of the NO and deoxy derivatives of HRP and CCP are typical of systems in which the haem iron is in the hexaco-ordinated state and the pentaco-ordinated state respectively. By lowering pH, the e.p.r. and absorption spectra of HRP and CCP undergo reversible transitions, with pK(a) values of 4.1 for the NO derivatives and ≤ 3 for the deoxy derivatives of the ferrour forms. By analogy with O2-carrying proteins and haem model compounds, the pH-dependent spectral changes of HRP and CCP were interpreted as indicative of the protonation of the N((ε)) atom of the proximal histidine residue and of the cleavage of the Fe-N((ε)) bond. However, the slow second-order rate constant (0.003 μM-1·s-1) for CO binding to deoxy ferrous HRP and CCP does not increase substantially even at pH 2.6, suggesting that changes in the Fe-haem plane geometry, presumably associated with the cleavage of the Fe-N((ε)) bond, do not affect appreciably the observed ligand association rate constant.
|Number of pages||6|
|Publication status||Published - 1989|
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