Hydrogen ion titration of horse heart ferricytochrome c

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Abstract
Continuous hydrogen ion titration curves of deionized solutions of horse heart ferricytochrome c were obtained at 25.degree. C at a constant ionic strength of 0.10 from pH 3.0 to 11.0. Titration of the oxidized protein in KCl required 28.4 eq over that pH range, and a small hysteresis between the forward and reverse limbs was displayed. The Linderstrom-Lang approximation, which takes into account electrostatic interactions between charged groups on the protein surface, was used in a computer simulation program to analyze the forward and reverse limbs of the titration curve spearately. The results indicated 1 .alpha.-, 12 .beta.- and .gamma.- and 1 heme propionic carboxylic, 1 imidazole, 1 phenolic and 18 .epsilon.-amino residues appear to titrate normally. Variations in the electrostatic interaction factor .omega. suggest conformational changes in the protein at the extremes of pH, although the relationship of the variations in .omega. to the magnitude of the conformational changes does not appear to be strictly quantitative for cytochrome c. The acid-base behavior of cytochrome c is complex in nature; the Lindenstrom-Lang model may not be adequate for cytochrome c.

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