Porphyrin cytochrome c. pH effects and interaction with cytochrome-c oxidase.

1. Porphyrin cytochrome c, the iron-free derivative of cytochrome c, has been used extensively as a fluorescent analog of cytochrome c. It appears as though its fluorescence intensity but not its relative quantum yield is affected by pH in the physiological range; an apparent pK of about 6.2 is found suggesting a histidine close to the porphyrin. 2. The fluorescence intensity of the porphyrin cytochrome c in the presence of cytochrome c oxidase is independent of pH; this suggests that the oxidase has the capacity to control the pK of whichever group is responsible for the pH sensitivity of the free porphyrin cytochrome c. The most likely candidate for this pH-sensitive group is histidine-18. The N-3 nitrogen of this residue forms one of the axial ligands to the iron in the intact cytochrome c but it is uncoordinated in the iron-free derivative.