Cu2+ site in photosynthetic bacterial reaction centers from Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodopseudomonas viridis

The interaction of metal ions with isolated photosynthetic reaction centers (RCs) from the purple bacteria Rhodobacter sphaeroides, Rhodobacter capsul atus, and Rhodopseudomonas viridis has been investigated with transient opt ical and magnetic resonance techniques. In RCs from all species, the electr ochromic response of the bacteriopheophytin cofactors associated with Q(A)- Q(B) --> Q(A)Q(B)(-) electron transfer is slowed in the presence of Cu2+. T his slowing is similar to the metal ion effect observed for RCs from Rb. sp haeroides where Zn2+ was bound to a specific site on the surface of the RC [Utschig et al. (1998) Biochemistry 37, 8278]. The coordination environment s of the Cu2+ sites were probed with electron paramagnetic resonance (EPR) spectroscopy, providing the first direct spectroscopic evidence for the exi stence of a second metal site in RCs from Rb, capsulatus and Rps. viridis. In the dark, RCs with Cu2+ bound to the surface exhibit axially symmetric E PR spectra. Electron spin echo envelope modulation (ESEEM) spectral results indicate multiple weakly hyperfine coupled N-14 nuclei in close proximity to Cu2+. These ESEEM spectra resemble those observed for Cu2+ RCS from Rb, sphaeroides [Utschig et al. (2000) Biochemistry 39, 2961] and indicate that two or more histidines ligate the Cu2+ at the surface site in each RC. Thu s, RCs from Rb. sphaeroides, Rb. capsulatus, and Rps. viridis each have a s tructurally analogous Cu2+ binding site that is involved in modulating the Q(A)(-)Q(B)-->Q(A)Q(B)(-) electron-transfer process. Inspection of the Rps, viridis crystal structure reveals four potential histidine ligands from th ree different subunits (M16, H178, H72, and L211) located beneath the QB bi nding pocket. The location of these histidines is surprisingly similar to t he grouping of four histidine residues (H68, H126, H128, and L211) observed in the Rb. sphaeroides RC crystal structure. Further elucidation of these Cu2+ sites will provide a means to investigate localized proton entry into the RCs of Rb, capsulatus and Rps. viridis as well as locate a site of prot ein motions coupled with electron transfer.