Some thermodynamic and kinetic properties of the primary photochemical reactants in a complex from a green photosynthetic bacterium (original) (raw)
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Primary photochemistry in the facultative green photosynthetic bacterium Chloroflexus aurantiacus
Journal of Cellular Biochemistry, 1983
The mechanism of primary photochemistry has been investigated in purified cytoplasmic membranes and isolated reaction centers of Chloroflexus aurantiacus. Redox titrations on the cytoplasmic membranes indicate that the midpoint redox potential of P870, the primary electron donor bacteriochlorophyll, is t-362 mV. An early electron acceptor, presumably menaquinone has Em 8.1 =-50 mV, and a tightly bound photooxidizable cytochrome c554 has Em 8.1 = +245 mV. The isolated reaction center has a bacteriochlorophyI1 to bacteriopheophytin ratio of 0.94: 1. A two-quinone acceptor system is present, and is inhibited by o-phenanthroline. Picosecond transient absorption and kinetic measurements indicate the bacteriopheophytin and bacteriochlorophyll form an earlier electron acceptor complex.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 2011
The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L-and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q A and Q B ), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285 K, the lifetimes of the excited singlet state, P*, and the charge-separated state P + H A − (where H A is the photoactive BPh) were found to be 3.2 ±0.3 ps and 200 ± 20 ps, respectively. Overall charge separation P* →→ P + Q A − occurred with ≥ 90% yield at 285 K. At 77 K, the P* lifetime was somewhat shorter and the P + H A − lifetime was essentially unchanged. Poteniometric titrations gave a P 865 /P 865 + midpoint potential of +390 mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265 mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P + suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.
Biochimica et Biophysica Acta (BBA) - Bioenergetics, 1978
The majority of photosynthetic bacteria possess bacteriochlorophyll a, but two species, Rhodopseudomonas viridis and Thiocapsa pfennigii, possess bacteriochlorophyll b, which absorbs at significantly longer wavelengths. The reaction center of Rps. viridis has been extensively studied; this paper presents studies on the reaction center of T. pfennigii, with the following results. (i) The "primary donor" bacteriochlorophyll dimer of T. pfennigii has a midpoint redox potential of +490 mV at pH 7, and the oxidized form has a gaussian EPR signal centered close to g = 2.0025 with a peak-to-peak width of 13 G. The zero field splitting characteristics of the light-induced triplet state of the bacteriochlorophyll dimer, seen when the "primary acceptor" is reduced before illumination, are D = 158 • 10-4 cm-1, E = 39 • 10-4 cm-1. These EPR properties are very similar to those of the bacteriochlorophyll dimer of Rps. viridis, and are quite distinct from those of bacteriochlorophyll a-containing species. (ii) The intermediary carrier, I, which is probably a bacteriopheophytin b molecule in close association with the "primary acceptor", can be trapped in its reduced form by the illumination of appropriately poised samples at 200 K. The EPR signals associated with I-are very similar to those of Chromatium vinosum, with the majority of the spins being observed in the form of a broad signal centered close to g = 2.003, and split by 68 G; the magnitude of the splitting of this signal is only half that seen in Rps. viridis. We have been unable to measure the E m of the I/I-couple in T. pfennigii. (iii) The "primary acceptor" of T. pfennigii is very similar to that of the Abbreviations: BChl a, bacteriochlorophyll a; BChl b, bacteriochlorophyll b; (BChl)2, bacteriochlorophyll dimer. majority of other purple bacterial species, being characterized by an EPR signal at g = 1.82 and g = 1.62. The Em of the redox couple is pH-dependent below pH 6.5, and the E m of the unprotonated couple, which is believed to function in electron flow, is-130 mV. (iv) Four cytochrome hemes can apparently donate electrons to the photooxidized bacteriochlorophyll dimer free radical cation, two of cytochrome c-555 (Em7 = 340 mV) and two of cytochrome c-550 (Era7 = 0). If all are reduced prior to illumination, the latter are preferentially oxidized. (v) The carotenoid bandshift of T. pfennigii can be interpreted as indicating that the bacteriochlorophyll dimer is located near the middle of the membrane dielectric, with the high potential cytochrome c-555 closer to the inside, and quinone • iron closer to the outside of the chromatophore membrane. The low potential cytochrome c-550 seems to be nearer the outside of the membrane than the bacteriochlorophyll dimer.
Photosynthesis Research, 2008
The purpose of the review is to show that the tetrameric (bacterio)chlorophyll ((B)Chl) structures in reaction centers of photosystem II (PSII) of green plants and in bacterial reaction centers (BRCs) are similar and play a key role in the primary charge separation. The Stark effect measurements on PSII reaction centers have revealed an increased dipole moment for the transition at ~730 nm (Frese et al., Biochemistry 42:9205–9213, 2003). It was found (Heber and Shuvalov, Photosynth Res 84:84–91, 2005) that two fluorescent bands at 685 and 720 nm are observed in different organisms. These two forms are registered in the action spectrum of QA photoreduction. Similar results were obtained in core complexes of PSII at low temperature (Hughes et al., Biochim Biophys Acta 1757: 841–851, 2006). In all cases the far-red absorption and emission can be interpreted as indication of the state with charge transfer character in which the chlorophyll monomer plays a role of an electron donor. The role of bacteriochlorophyll monomers (BA and BB) in BRCs can be revealed by different mutations of axial ligand for Mg central atoms. RCs with substitution of histidine L153 by tyrosine or leucine and of histidine M182 by leucine (double mutant) are not stable in isolated state. They were studied in antennaless membrane by different kinds of spectroscopy including one with femtosecond time resolution. It was found that the single mutation (L153HY) was accompanied by disappearance of BA molecule absorption near 802 nm and by 14-fold decrease of photochemical activity measured with ms time resolution. The lifetime of P870* increased up to ~200 ps in agreement with very low rate of the electron transfer to A-branch. In the double mutant L153HY + M182HL, the BA appears to be lost and BB is replaced by bacteriopheophytin ΦB with the absence of any absorption near 800 nm. Femtosecond measurements have revealed the electron transfer to B-branch with a time constant of ~2 ps. These results are discussed in terms of obligatory role of BA and ΦB molecules located near P for efficient electron transfer from P*.
The Journal of Physical Chemistry B, 2015
The heliobacteria are a family of strictly anaerobic, Gram-positive, photoheterotrophs in the Firmicutes. They make use of a homodimeric type I reaction center (RC) that contains ∼20 antenna bacteriochlorophyll (BChl) g molecules, a special pair of BChl g′ molecules (P 800 ), two 8 1 -OH-Chl a F molecules (A 0 ), a [4Fe−4S] iron−sulfur cluster (F X ), and a carotenoid (4,4′-diaponeurosporene). It is known that in the presence of light and oxygen BChl g is converted to a species with an absorption spectrum identical to that of Chl a. Here, we show that main product of the conversion is 8 1 -OH-Chl a F . Smaller amounts of two other oxidized Chl a F species are also produced. In the presence of light and oxygen, the kinetics of the conversion are monophasic and temperature dependent, with an activation energy of 66 ± 2 kJ mol −1 . In the presence of oxygen in the dark, the conversion occurs in two temperature-dependent kinetic phases: a slow phase followed by a fast phase with an activation energy of 53 ± 1 kJ mol −1 . The loss of BChl g′ occurs at the same rate as the loss of Bchl g; hence, the special pair converts at the same rate as the antenna Chl's. However, the loss of P 800 photooxidiation and flavodoxin reduction is not linear with the loss of BChl g. In anaerobic RCs, the charge recombination between P 800 + and F X − at 80 K is monophasic with a lifetime of 4.2 ms, but after exposure to oxygen, an additional phase with a lifetime of 0.3 ms is observed. Transient EPR data show that the line width of P 800 + increases as BChl g is converted to Chl a F and the rate of electron transfer from A 0 to F X , as estimated from the net polarization generated by singlet− triplet mixing during the lifetime of P 800 + A 0 − , is unchanged. The transient EPR data also show that conversion of the BChl g results in increased formation of triplet states of both BChl g and Chl a F . The nonlinear loss of P 800 photooxidiation and flavodoxin reduction, the biphasic backreaction kinetics, and the increased EPR line width of P 800 + are all consistent with a model in which the BChl g′/BChl g′ and BChl g′/Chl a F ′ special pairs are functional but the Chl a F ′/Chl a F ′ special pair is not.
Function of Quinones in Energy Conserving Systems, 1982
Light-induced oxidation of the primary electron donor P and of the secondary donor cytochrome c2 was studied in whole cells of Rhodospirillum rubrum in the presence of myxothiazole to slow down their reduction. 1. The primary and secondary electron donors are close to thermodynamic equilibrium during continuous illumination when the rate of the electron transfer is light-limited. This implies a long-range thermodynamic equilibration involving the diffusible cytochrome c2. A different behavior is observed with Rhodobacter sphaeroides R26 whole cells, in which the cytochrome c2 remains trapped within a supercomplex 'P.J. was supported by a contract CEA/CollSge de France (no. V.2491.002); A.J. was supported by a contract CEA/CNRS (no. V.2491.001).
Proceedings of the National Academy of Sciences of the United States of America, 1968
Chromatophores, or membrane fragments, prepared from photosynthetic bacteria exhibit a variety of properties which demonstrate that they are able to carry out the quantum conversion steps characteristic of the intact bacterial cells.", 2 The pigment molecules consist of two distinct functional types: an antenna, which includes over 90 per cent of the bacteriochlorophyll molecules, and a smaller fraction incorporated into reaction centers. The primary function of the antenna is to absorb incident photons and to transfer the resulting electronic excitation energy to the reaction center bacteriochlorophyll molecules. It is in these reaction centers that the actual transformation to chemical energy is initiated.