The redox potential of the primary quinone Q_A of bacterial photosynthesis is independent of the divalent metal ion

The function of the ferrous ion which links the primary and secondary quinone electron acceptors, Q_A and Q_B, via histidine ligands from the reaction center protein of the photosynthetic bacterium Rbodobacter sphaeroids (Y) has been investigated by substitution with Mn(II), Cu(II) and Zn(II), using biosynthetic incorporation of the metal ion. The midpoint potential and pH dependence for reduction of Q_A in chromatophore membranes were found to be indistinguishable for all four divalent ions, and in agreement with earlier results on the R26 and Ga strains of R. sphaeroides containing only iron. We therefore conclude that the divalent ion contributes no functionally significant covalent coupling to the primary acceptor and that these metals have no differential electrostatic influence. Furthermore, inhibition of photoreduction of Q_B by the herbicide ametryne was found to be the same for all four samples. Thus, these four divalent ions exhibit no differential influence on the binding properties of this herbicide in the Q_B site.