The observation that 6 M-urea denatures horse ferricytochrome c in the pH range 4-6, but not horse ferrocytochrome c, has been exploited to determine the denaturation-induced proton uptake of ferricytochrome c. This is related to the pKa values of ionizable groups buried within the native protein. The data indicate that one of the haem propionic acid substituents of ferricytochrome c has a pKa of less than 4.5, whereas the other has a pKa of greater than 9.
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Hartshorn et al. (1989) studied this question.
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