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Analysis of the ionization constants and heats of ionization of reduced and oxidized horse heart cytochrome c
Author(s) -
Marini M. A.,
Martin C. J.,
Berger R. L.,
Forlani L.
Publication year - 1981
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1981.360201017
Subject(s) - chemistry , potentiometric titration , ionization , cytochrome c , cytochrome , analytical chemistry (journal) , titration , histidine , titration curve , crystallography , ion , chromatography , organic chemistry , enzyme , biochemistry , mitochondrion , electrode
Simultaneous curve fitting for the ionization parameters of oxidized and reduced horse heart cytochrome c in 0.15 M KCl and 20°C yields values for the ionization constants (as p K ′) and the heats of ionization (Δ H i ) which can reconstruct either the potentiometric or thermal titration curves. Reduced cytochrome c requires 8 sets of groups, whereas oxidized cytochrome c requires 10 sets of groups. The additional groups in the oxidized preparation appear to involve the ferriheme (p K ′, 9.25; Δ H i , 13.7 kcal/mol) and a tyrosine (p K ′ ≃ 10.24) that is not present in the reduced form. The potentiometric and thermal difference curves (reduced – oxidized) involve the appearance of 17 kcal/mol centered at pH 9.7 and 5.8 kcal/mol centered at pH 4.9. The carboxyl groups in both species appear to be normal for the hydrogen‐bonded form. Only one histidine has normal ionization properties (p K ′, 6.7; Δ H i , 7.5 kcal/mol), as do 17 of the lysine residues (p K ′, 10.8; Δ H i , 11.5 kcal/mol).