Analysis of the ionization constants and heats of ionization of reduced and oxidized horse heart cytochrome c

✍ Scribed by M. A. Marini; C. J. Martin; R. L. Berger; L. Forlani

• Publisher: Wiley (John Wiley & Sons)
• Year: 1981
• Tongue: English
• Weight: 480 KB
• Volume: 20
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.1981.360201017

✦ Synopsis

Abstract

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).