Structure-function relationship in type II cytochrome c(3) from Desulfovibrio africanus: a novel function in a familiar heme core.
NMR and visible spectroscopy were used to characterize the type II tetraheme cytochrome c(3) isolated from the periplasmic space of Desulfovibrio africanus, a sulfate-reducing bacterium. Although structurally similar to other cytochromes c(3), 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 c(3) 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 c(3). 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 c(3) having a physiological function that is different from that of type I cytochromes c(3).