A Kinetic Study of the pH‐Dependent Properties of the Ferric Undecapeptide of Cytochrome <i>c</i> (Microperoxidase)

Abstract

The ferric form of the haem undecapeptide, derived from horse cytochrome c by peptic digestion, undergoes at least three pH‐induced transitions with p K values of 3.4, 5.8 and 7.6. Temperature‐jump experiments suggest that the first of these is due to the binding of a deprotonated imidazole group to the ferric iron while the second and third arise from the binding of the two available amino groups present (the α‐NH 2 of valine and the ɛ‐NH 2 of lysine). Molecular models indicate that steric restraints on the peptide dictate that these amino groups may only coordinate to iron atoms via intermolecular bonds, thus leading to the polymerization of the peptide. Cyanide binding studies are in agreement with these conclusions and also yield a value of 3.6 × 10 6 M −1 s −1 for the intrinsic combination constant of CN − anion with the haem. A model is proposed which describes the pH‐dependent properties of the ferric undecapeptide.