Ubiquinone and photochemical activity in Rhodospirillum rubrum

Studies on the singlet oxygen scavenging mechanism of human macular pigment

It is thought that direct quenching of singlet oxygen and scavenging free radicals by macular pigment carotenoids is a major mechanism for their beneficial effects against light-induced oxidative stress. Corresponding data from human tissue remains unavailable, however. In the studies reported here, electron paramagnetic resonance (EPR) spectroscopy was used to measure light-induced singlet oxygen generation in post-mortem human macula and retinal pigment epithelium/choroid (RPE/choroid). Under white-light illumination, production of singlet oxygen was detected in RPE/choroid but not in macular tissue, and we show that exogenously added macular carotenoids can quench RPE/choroid singlet oxygen. When the singlet oxygen quenching ability of the macular carotenoids was investigated in solution, it was shown that a mixture of meso-zeaxanthin, zeaxanthin, and lutein in a ratio of 1:1:1 can quench more singlet oxygen than the individual carotenoids at the same total concentration.

Lipidic cubic phase crystal structure of the photosynthetic reaction centre from Rhodobacter sphaeroides at 2.35A resolution

Well-ordered crystals of the bacterial photosynthetic reaction centre from Rhodobacter sphaeroides were grown from a lipidic cubic phase. Here, we report the type I crystal packing that results from this crystallisation medium, for which 3D crystals grow as stacked 2D crystals, and the reaction centre X-ray structure is refined to 2.35A resolution. In this crystal form, the location of the membrane bilayer could be assigned with confidence. A cardiolipin-binding site is found at the protein-protein interface within the membrane-spanning region, shedding light on the formation of crystal contacts within the membrane. A chloride-binding site was identified in the membrane-spanning region, which suggests a putative site for interaction with the light-harvesting complex I, the cytochrome bc(1) complex or PufX. Comparisons with the X-ray structures of this reaction centre deriving from detergent-based crystals are drawn, indicating that a slight compression occurs in this lipid-rich environment.

Role of the core region of the PufX protein in inhibition of reconstitution of the core light-harvesting complexes of Rhodobacter sphaeroides and Rhodobacter capsulatus

PufX, the protein encoded by the pufX gene of Rhodobacter capsulatus and Rhodobacter sphaeroides, has been further characterized. The mature forms of these proteins contain 9 and 12 fewer amino acids, respectively, at the C-terminal end of the protein than are encoded by their pufX genes. To identify the portion of PufX responsible for inhibition of LH1 formation in reconstitution experiments, different regions (N-terminus and several core regions containing different lengths of the C-terminus) of Rb. sphaeroides and Rb. capsulatus PufX were chemically synthesized. Neither the N- nor C-terminal polypeptides of Rb. sphaeroides were inhibitory to LH1 reconstitution. However, all core segments were active, causing 50% inhibition at a concentration ratio of between 3:1 and 6:1 relative to the LH1 alpha-polypeptides whose concentrations were 3-4 microM. CD measurements indicated that the core segment containing 39 amino acids of Rb. sphaeroides PufX exhibited 47% alpha-helix in trifluoroethanol while the core segment containing 43 amino acids of Rb. capsulatus PufX exhibited 59 and 55% alpha-helix in trifluoroethanol and in 0.80% octylglucoside in water, respectively. Approximately 50% alpha-helix was also indicated by a PHD (Burkhard-Rost) structure prediction. Binding of bacteriochlorophyll to these PufX core segments is implicated.

Is there a conserved interaction between cardiolipin and the type II bacterial reaction center?

In a recent publication, the structural details of an interaction between the Rhodobacter sphaeroides reaction center and the anionic phospholipid diphosphatidyl glycerol (cardiolipin) were described (K. E. McAuley, P. K. Fyfe, J. P. Ridge, N. W. Isaacs, R. J. Cogdell, and M. R. Jones, 1999, Proc. Natl. Acad. Sci. U.S.A. 96:14706-14711). This was the first crystallographic description of an interaction between this biologically important lipid and an integral membrane protein and was also the first piece of evidence that the reaction center has a specific interaction with cardiolipin. We have examined the extent to which the residues that interact with the cardiolipin are conserved in other species of photosynthetic bacteria with this type of reaction center and discuss the possibility that this cardiolipin binding site is a conserved feature of these reaction centers. We look at how sequence variations that would affect the shape of the cardiolipin binding site might affect the protein-cardiolipin interaction, by modeling the binding of cardiolipin to the reaction center from Rhodopseudomonas viridis.

Photooxidase system of Rhodospirillum rubrum III. The role of rhodoquinone and ubiquinone in the activity of preparations of chromatophores and photoreaction centers

The role of rhodoquinone and ubiquinone in the oxygen photoreducing (photooxidase) activity of Rhodospirillum rubrum was investigated. The sole addition of purified rhodoquinone restored photooxidase activity in isolated chromatophores which had been extracted with organic solvents and which were apparently free of secondary acceptor ubiquinone. Rhodoquinone also enhanced photooxidase activity in photoreaction center preparations from which secondary ubiquinone seemed to have been removed. Those results suggest that rhodoquinone accepts electrons directly from primary ubiquinone during chromatophore photooxidase activity. In contrast, rhodoquinone does not participate in the basal activity of photoreaction center preparations, which seems to result from the autooxidation of both primary and secondary ubiquinone.

Electron acceptors of bacterial photosynthetic reaction centers. II. H+ binding coupled to secondary electron transfer in the quinone acceptor complex

The photoreduction of ubiquinone in the electron acceptor complex (QIQII) of photosynthetic reaction centers from Rhodopseudomonas sphaeroides, R26, was studied in a series of short, saturating flashes. The specific involvement of H+ in the reduction was revealed by the pH dependence of the electron transfer events and by net H+ binding during the formation of ubiquinol, which requires two turnovers of the photochemical act. On the first flash QII receives an electron via QI to form a stable ubisemiquinone anion (QII-); the second flash generates QI-. At low pH the two semiquinones rapidly disproportionate with the uptake of 2 H+, to produce QIIH2. This yields out-of-phase binary oscillations for the formation of anionic semiquinone and for H+ uptake. Above pH 6 there is a progressive increase in H+ binding on the first flash and an equivalent decrease in binding on the second flash until, at about pH 9.5, the extent of H+ binding is the same on all flashes. The semiquinone oscillations, however, are undiminished up to pH 9. It is suggested that a non-chromophoric, acid-base group undergoes a pK shift in response to the appearance of the anionic semiquinone and that this group is the site of protonation on the first flash. The acid-base group, which may be in the reaction center protein, appears to be subsequently involved in the protonation events leading to fully reduced ubiquinol. The other proton in the two electron reduction of ubiquinone is always taken up on the second flash and is bound directly to QII-. At pH values above 8.0, it is rate limiting for the disproportionation and the kinetics, which are diffusion controlled, are properly responsive to the prevailing pH. Below pH 8, however, a further step in the reaction mechanism was shown to be rate limiting for both H+ binding electron transfer following the second flash.

Two regimens of electrogenic cyclic redox chain operation in chromatophores of non-sulfur purple bacteria. A study using antimycin A

Antimycin A causes a biphasic suppression of the light-induced membrane potential generation in Rhodospirillum rubrum and Rhodopseudomonas sphaeroides chromatophores incubated anerobically. The first phase is observed at low antibiotic concentrations and is apparently due to its action as a cyclic electron transfer inhibitor. The second phase is manifested at concentrations which are greater than 1--2 muM and is due to uncoupling that may be connected with an antibiotic-induced dissipation of the electrochemical H+ gradient across the chromatophore membrane. The inhibitory effect of antimycin added at low concentrations under aerobic conditions is removed by succinate to a large extent. It is expected that the electrogenic cyclic redox chain in the bacterial chromatophores incubed under conditions of continuous illumination may function at two regimes: (1) as a complete chain involving all the redox components, and (2) as a shortened chain involving only the P-870 photoreaction center, ubiquinone and cytochrome c2.

Characterisation of reaction centers and their phospholipids from Rhodospirillum rubrum

1. Reaction centers from Rhodospirillum rubrum have been extracted with the zwitterionic detergent lauryl dimethyl amine oxide. Subsequent purification has been achieved by gel filtration and ion-exchange chromatography. The pure reaction centers are composed of three protein subunits (L, M, H), bacteriocholorophyll and bacteriopheophytin in the ratio 2 : 1 and phospholipids. 2. The phospholipid composition has been found to be similar to that of whole chromatophore membrane, except that diphosphatidyl glycerol is present in higher amount in the isolated complex. When the detergent treatment of the chromatophore membrane is done in the presence of NaCl, a lower phospholipid content in isolated reaction centers has been found together with a lower stability in the association among the protein subunits. In this complex, the largest subunit H is easily split off and a LM complex is obtained. It is concluded that the phospholipids play an important role in the stability of reaction center complexes.

Ubiquinone in Rhodopseudomonas sphaeroides. Some thermodynamic properties

In Rhodopseudomonas sphaeroides chromatophores there are 25 +/- 3 ubiquinone (Q) molecules/reaction center protein. They comprise several thermodynamically and functionally different ubiquinone complements. There are approx. 19 ubiquinones (Em7 = 90 mV) in the main ubiquinone complement which, within experimental resolution, appears thermodynamically homogenous and follows the redox reaction Q + 2e + 2H+ in equilibrium with QH2 from pH 5--9. A method which takes advantage of the 2H+ bound/molecule of Q reduced is described for measuring the time course of light-activated reaction center-driven reduction and oxidation of the 19 Q complement. No stable semiquinones were detected in the constitutents of the 19 Q complement. There are approx. 6 ubiquinones of lower Em which are currently unaccounted for, although one or possibly two of these can be assigned to the quinones of the reaction center protein. The remainder may be associated with the NADH-ubiquinone oxidoreductase.

The involvement of iron and ubiquinone in electron transfer reactions mediated by reaction centers from photosynthetic bacteria

Reaction centers from Rhodopseudomonas sphaeroides strain R-26 were prepared with varying Fe and ubiquinone (Q) contents. The photooxidation of P-870 to P-870+ was found to occur with the same quantum yield in Fe-depleted reaction centers as in control samples. The kinetics of electron transfer from the initial electron acceptor (I) to Q also were unchanged upon Fe removal. We conclude that Fe has no measurable role in the primary photochemical reaction. The extent of secondary reaction from the first quinone acceptor (QA) to the second quinone acceptor (QB) was monitored by the decay kinetics of P-870+ after excitation of reaction centers with single flashes in the absence of electron donors, and by the amount of P-870 photooxidation that occurred on the second flash in the presence of electron donors. In reaction centers with nearly one iron and between 1 and 2 ubiquinones per reaction center, the amount of secondary electron transfer is proportional to the ubiquinone content above one per reaction center. In reaction centers treated with LiClO4 and o-phenanthroline to remove Fe, the amount of secondary reaction is decreased and is proportional to Fe content. Fe seems to be required for the secondary reaction. In reaction centers depleted of Fe by treatment with SDS and EDTA, the correlation between Fe content and secondary activity is not as good as that found using LiClO4. This is probably due in part to a loss of primary photochemical activity in samples treated with SDS; but the correlation is still not perfect after correction for this effect. The nature of the back reaction between P-870+ and Q-B was investigated using stopped flow techniques. Reaction centers in the P-870+ Q-B state decay with a 1-s half-time in both the presence and absence of o-phenanthroline, an inhibitor of electron transfer between Q-B and QB. This indicates that the back reaction between P-870+ and Q-A is direct, rather than proceeding via thermal repopulation of Q-A. The P-870+ Q-B state is calculated to lie at least 100 mV in free energy below the P-870+ Q-A state.