Trapped Tyrosyl Radical Populations in Modified Reaction Centers from Rhodobacter sphaeroides

Electronic structure of the Mn-cofactor of modified bacterial reaction centers measured by electron paramagnetic resonance and electron spin echo envelope modulation spectroscopies

The electronic structure of a Mn(II) ion bound to highly oxidizing reaction centers of Rhodobacter sphaeroides was studied in a mutant modified to possess a metal binding site at a location comparable to the Mn4Ca cluster of photosystem II. The Mn-binding site of the previously described mutant, M2, contains three carboxylates and one His at the binding site (Thielges et al., Biochemistry 44:389-7394, 2005). The redox-active Mn-cofactor was characterized using electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM) spectroscopies. In the light without bound metal, the Mn-binding mutants showed an EPR spectrum characteristic of the oxidized bacteriochlorophyll dimer and reduced quinone whose intensity was significantly reduced due to the diminished quantum yield of charge separation in the mutant compared to wild type. In the presence of the metal and in the dark, the EPR spectrum measured at the X-band frequency of 9.4 GHz showed a distinctive spin 5/2 Mn(II) signal consisting of 16 lines associated with both allowed and forbidden transitions. Upon illumination, the amplitude of the spectrum is decreased by over 80 % due to oxidation of the metal upon electron transfer to the oxidized bacteriochlorophyll dimer. The EPR spectrum of the Mn-cofactor was also measured at the Q-band frequency of 34 GHz and was better resolved as the signal was composed of the six allowed electronic transitions with only minor contributions from other transitions. A fit of the Q-band EPR spectrum shows that the Mn-cofactor is a high spin Mn(II) species (S = 5/2) that is six-coordinated with an isotropic g-value of 2.0006, a weak zero-field splitting and E/D ratio of approximately 1/3. The ESEEM experiments showed the presence of one (14)N coordinating the Mn-cofactor. The nitrogen atom is assigned to a His by comparing our ESEEM results to those previously reported for Mn(II) ions bound to other proteins and on the basis of the X-ray structure of the M2 mutant that shows the presence of only one His, residue M193, that can coordinate the Mn-cofactor. Together, the data allow the electronic structure and coordination environment of the designed Mn-cofactor in the modified reaction centers to be characterized in detail and compared to those observed in other proteins with Mn-cofactors.

The evolutionary pathway from anoxygenic to oxygenic photosynthesis examined by comparison of the properties of photosystem II and bacterial reaction centers

In photosynthetic organisms, such as purple bacteria, cyanobacteria, and plants, light is captured and converted into energy to create energy-rich compounds. The primary process of energy conversion involves the transfer of electrons from an excited donor molecule to a series of electron acceptors in pigment-protein complexes. Two of these complexes, the bacterial reaction center and photosystem II, are evolutionarily related and structurally similar. However, only photosystem II is capable of performing the unique reaction of water oxidation. An understanding of the evolutionary process that lead to the development of oxygenic photosynthesis can be found by comparison of these two complexes. In this review, we summarize how insight is being gained by examination of the differences in critical functional properties of these complexes and by experimental efforts to alter pigment-protein interactions of the bacterial reaction center in order to enable it to perform reactions, such as amino acid and metal oxidation, observable in photosystem II.

Effect of anions on the binding and oxidation of divalent manganese and iron in modified bacterial reaction centers

The influence of different anions on the binding and oxidation of manganous and ferrous cations was studied in four mutants of bacterial reaction centers that can bind and oxidize these metal ions. Light-minus-dark difference optical and electron paramagnetic resonance spectroscopies were applied to monitor electron transfer from bound divalent metal ions to the photo-oxidized bacteriochlorophyll dimer in the presence of five different anions. At pH 7, bicarbonate was found to be the most effective for both manganese and iron binding, with dissociation constants around 1 muM in three of the mutants. The pH dependence of the dissociation constants for manganese revealed that only bicarbonate and acetate were able to facilitate the binding and oxidation of the metal ion between pH 6 and 8 where the tight binding in their absence could not otherwise be established. The data are consistent with two molecules of bicarbonate or one molecule of acetate binding to the metal binding site. For ferrous ion, the binding and oxidation was facilitated not only by bicarbonate and acetate, but also by citrate. Electron paramagnetic resonance spectra suggest differences in the arrangement of the iron ligands in the presence of the various anions.

Comparison of bacterial reaction centers and photosystem II

In photosynthetic organisms, the utilization of solar energy to drive electron and proton transfer reactions across membranes is performed by pigment-protein complexes including bacterial reaction centers (BRCs) and photosystem II. The well-characterized BRC has served as a structural and functional model for the evolutionarily-related photosystem II for many years. Even though these complexes transfer electrons and protons across cell membranes in analogous manners, they utilize different secondary electron donors. Photosystem II has the unique ability to abstract electrons from water, while BRCs use molecules with much lower potentials as electron donors. This article compares the two complexes and reviews the factors that give rise to the functional differences. Also discussed are the modifications that have been performed on BRCs so that they perform reactions, such as amino acid and metal oxidation, which occur in photosystem II.