Direct Charge Recombination from D+QAQB- to DQAQB in Bacterial Reaction Centers from Rhodobacter sphaeroides

Labahn, Andreas; Paddock, Mark L.; McPherson, P.H.; Okamura, M. Y.; Fehér, G.

doi:10.1021/j100064a024

Cited by 54 publications

(45 citation statements)

References 11 publications

(18 reference statements)

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“…In L213DN RCs, however, the effect was to make the one-electron equilibrium constant very large and the recombination reaction was suggested to proceed directly from QB to P+ so that the decay kinetics did not reflect the value of KAB (8). The direct nature of the P+QB recombination in this mutant has since been confirmed (30). As discussed (8), therefore, it is possible that the pK shift in the L213DN mutant RCs was from -9.5 to as low as 5.0 (ApK = 4.5).…”

Section: Discussionmentioning

confidence: 91%

Potentiation of proton transfer function by electrostatic interactions in photosynthetic reaction centers from Rhodobacter sphaeroides: First results from site-directed mutation of the H subunit.

Takahashi

Wraight

1996

Proc. Natl. Acad. Sci. U.S.A.

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The x-ray crystallographic structure of the photosynthetic reaction center (RC) has proven critical in understanding biological electron transfer processes. By contrast, understanding of intraprotein proton transfer is easily lost in the immense richness of the details. In the RC of Rhodobacter (Rb.) sphaeroides, the secondary quinone (QB) is surrounded by amino acid residues of the L subunit and some buried water molecules, with M-and H-subunit residues also close by. Studies on RCs altered by site-directed mutagenesis strongly implicate two ionizable residues in functional proton transfer to the QB binding site in Rb. sphaeroides-Glu and Asp at positions 212 and 213, respectively, of the L subunit (6-9).These are the two ionizable residues nearest to the QB head group (Fig. 1). Alteration of AspL213 to Asn had severe effects, including a-very large increase in the QAQB = QAQB equilibrium in favor of QB reduction and modification of the pH dependence of the equilibrium, especially at acid pH (7-9).Also, transfer of the second electron to QB was almost completely blocked, due to failure in the transfer of the first proton necessary for the double reduction of QB: QAQB + H+X--QAQBH-. Alteration of GluL212 to Gln also had significant effects on the quinone reactions (6,8

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Section: Discussionmentioning

confidence: 91%

Potentiation of proton transfer function by electrostatic interactions in photosynthetic reaction centers from Rhodobacter sphaeroides: First results from site-directed mutation of the H subunit.

Takahashi

Wraight

1996

Proc. Natl. Acad. Sci. U.S.A.

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“…sphaeroides RCs. 22,31 The charge recombination pathway for an electron localized on Q B in membrane-bound RCs should be the same as for isolated RCs under the normal physiological conditions. The charge recombination occurs in a indirect way via the primary quinone acceptor Q A .…”

Section: Discussionmentioning

confidence: 99%

“…In accord with the Marcus theory, such a lowering of the P + Q A Q B -energy causes a simultaneous decrease in the rate constant for both the indirect and direct ET pathway at a given reorganization energy. 32 Several studies demonstrated that the ET pathway can be switched from an indirect route to the direct one either in mutant RCs, in which the reorganization energy of P + Q A Q B -state is modified by a protein environment mutation, 31 or by using a ubiquinone substitution in the Q A site with a lower potential. 23,33 Neither of these two possibilities applies to the case of a charge-separation induced increase of ∆G AB 0 during multiple, consecutive turnover events of an RC.…”

Section: Discussionmentioning

confidence: 99%

Light-Induced Equilibration Kinetics in Membrane-Bound Photosynthetic Reaction Centers: Nonlinear Dynamic Effects in Multiple Scattering Media

et al. 2004

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Nonequilibrium dynamic effects in membrane-bound reaction centers (RCs) from photosynthetic bacteria are studied for the first time. We show that the accumulation of slow conformational changes triggered by chargeseparation events in the RCs control system dynamics and depend on illumination conditions in a way similar to that for isolated RCs. The light-induced, transient kinetics of membrane-bound RCs are described using a model of electron-conformational transitions governed by system diffusion along the surface of a doublewell, effective adiabatic potential. The light-triggered conformational transitions in the chromatophores are estimated to occur at an actinic light intensity at least 10 times lower than that needed for conformational transitions in isolated RCs. This finding is attributed to efficient, multiple-scattering effects that occur in sample with membranes. Related optical properties of the chromatophores as a disordered system with multiple light scattering are characterized for the first time.

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“…The direct electron transfer from Q B -to P + is much slower. The latter mechanism only becomes important when the free energy of either Q A or Q B is altered by quinone replacement (50)(51)(52) or by mutation (49,50,53) to make electron transfer via the indirect route slower than the direct route. The direct electron transfer from Q B -to P + occurs at 0.12-0.19 s -1 at room temperature (50)(51)(52)(53).…”

Section: The Charge Recombination Rate In the Light Conformation Is Cmentioning

confidence: 99%

Trapping Conformational Intermediate States in the Reaction Center Protein from Photosynthetic Bacteria

Gunner

2001

Biochemistry

108

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In protein, conformational changes are often crucial for function but not easy to observe. Two functionally relevant conformational intermediate states of photosynthetic reaction center protein (RCs) are trapped and characterized at low temperature. RCs frozen in the dark do not allow electron transfer from the reduced primary quinone, Q A -, to the secondary quinone, Q B . In contrast, RCs frozen under illumination in the product (P + Q A Q B -) state, with the oxidized electron donor, P + , and reduced Q B -, return to the ground state at cryogenic temperature in a conformation that allows a high yield of Q B reduction. Thus, RCs frozen under illumination are found to be trapped above the ground state in a conformation that allows product formation. When the temperature is raised above 120 K, the protein relaxes to an inactive conformation which is different from the RCs frozen in the dark. The activation energy for this change is 87 ( 8 meV, and the active and inactive states differ in energy by only 16 ( 3 meV. Thus, there are several conformational substates along the reaction coordinate with different transition temperatures. The ground state spectra of the RCs in active and inactive conformations report differences in the intraprotein electrostatic field, demonstrating that the dipole or charge distribution has changed. In addition, the electrochromic shift associated with the Q A -to Q B electron transfer at low temperature was characterized. The electron-transfer rate from Q B -to P + was measured at cryogenic temperature and is similar to the rate at room temperature, as expected for an exothermic, electron tunneling reaction in RCs.Conformational flexibility is important for the function of proteins (1, 2). At room temperature, proteins fluctuate among many conformational states. At low temperature, proteins will become trapped in harmonic motions near the conformation it was frozen into (3-9). Intraprotein reactions are inhibited if this conformation cannot access the transition state. When the temperature is raised, relaxation between conformational substates becomes possible. Elastic incoherent neutron scattering and X-ray crystallography measurements of bacteriorhodopsin (10) and myoglobin (4) show that different parts of the protein relax at different temperatures (11).Bacterial photosynthetic reaction center protein (RCs) 1 has a number of physiological electron tunneling reactions that occur even at cryogenic temperatures. This system can therefore be used to characterize conformational substates of physiologically important reactions. Previous studies of RCs have characterized the protein at cryogenic temperatures. RCs can be trapped in a distribution of substates which exhibit a wider distribution of reaction rates (12,13). Other studies have characterized unrelaxed product states (14). In addition, reactions that normally do not occur at low temperatures have been observed when the protein is frozen into appropriate conformational states (12).The bacterial photosynthetic reaction c...

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Direct Charge Recombination from D+QAQB- to DQAQB in Bacterial Reaction Centers from Rhodobacter sphaeroides

Cited by 54 publications

References 11 publications

Potentiation of proton transfer function by electrostatic interactions in photosynthetic reaction centers from Rhodobacter sphaeroides: First results from site-directed mutation of the H subunit.

Potentiation of proton transfer function by electrostatic interactions in photosynthetic reaction centers from Rhodobacter sphaeroides: First results from site-directed mutation of the H subunit.

Light-Induced Equilibration Kinetics in Membrane-Bound Photosynthetic Reaction Centers: Nonlinear Dynamic Effects in Multiple Scattering Media

Trapping Conformational Intermediate States in the Reaction Center Protein from Photosynthetic Bacteria

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