NMR and visible spectroscopy were used to characterize the type II tetraheme cytochrome c3 isolated from the periplasmic space of Desulfovibrio africanus, a sulfate-reducing bacterium. Although structurally similar to other cytochromes c3, this protein displays distinct functional properties. Proton NMR signals from the four hemes were assigned to the structure in the ferri- and ferrocytochromes using two-dimensional NMR experiments. The thermodynamic parameters of the hemes and of an acid-base center in the type II cytochrome c3 were determined using NMR and visible spectroscopies. The thermodynamic features indicate that electrostatic effects dominate all of the interactions between the centers and no positive cooperativity between hemes is observed. The redox-Bohr effect in this protein is associated with the acid-base equilibrium of a propionate of heme II instead of propionate 13 of heme I as is the case for all of the type I cytochromes c3. These novel functional properties are analyzed together with the redox-linked structural differences reported in the literature and reveal a mechanistic basis for type II cytochromes c3 having a physiological function that is different from that of type I cytochromes c3.
- Cytochrome c
- Electron transfer
- Paramagnetic NMR
- Redox-Bohr effect
- Structure-function relationship