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 capsulatus, and Rhodopseudomonas viridis has been investigated with transient optical and magnetic resonance techniques. In RCs from all species, the electrochromic response of the bacteriopheophytin cofactors associated with QA -QB → QAQB - electron transfer is slowed in the presence of Cu2+. This slowing is similar to the metal ion effect observed for RCs from Rb. sphaeroides where Zn2+ was bound to a specific site on the surface of the RC [Utschig et al. (1998) Biochemistry 37, 8278]. The coordination environments of the Cu2+ sites were probed with electron paramagnetic resonance (EPR) spectroscopy, providing the first direct spectroscopic evidence for the existence 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 EPR spectra. Electron spin echo envelope modulation (ESEEM) spectral results indicate multiple weakly hyperfine coupled 14N 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. Thus, RCs from Rb. sphaeroides, Rb. capsulatus, and Rps. viridis each have a structurally analogous Cu2+ binding site that is involved in modulating the QA -QB → QAQB - electron-transfer process. Inspection of the Rps. viridis crystal structure reveals four potential histidine ligands from three different subunits (M16, H178, H72, and L211) located beneath the QB binding pocket. The location of these histidines is surprisingly similar to the 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 protein motions coupled with electron transfer.