Reduction and protonation of the secondary quinone acceptor of Rhodobacter sphaeroides photosynthetic reaction center: kinetic model based on a comparison of wild-type chromatophores with mutants carrying Arg→Ile substitution at sites 207 and 217 in the L-subunit

Cherepanov, Dmitry A.; Bibikov, Sergei I.; Bibikova, Marina; Bloch, Dmitry A.; Drachev, Lel A.; Gopta, Oksana A.; Oesterhelt, Dieter; Semenov, Alexey Yu.

doi:10.1016/s0005-2728(00)00110-9

Cited by 32 publications

(50 citation statements)

References 73 publications

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“…A proposed mechanistic model to explain the amplitude of proton uptake in the RCs takes into account the movement of Q B from the distal position in its neutral state to the proximal position in the Q B Ϫ state (46,47). According to this hypothesis, Q B in the proximal position is bound via hydrogen bond to GluL212, and stabilizes thereby the protonated form of the latter, whereas the Q B in the distal position is likely to favor the anionic form of GluL212 as suggested by the calculations (28).…”

Section: Protonation Events Triggered By the Q Bmentioning

confidence: 99%

Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

Tandori

Maróti

Alexov

et al. 2002

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

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T he biological role of bacterial reaction center (RC) membrane proteins is to convert light energy into chemical free energy. The sequential absorption of two photons by the system results into the production of the doubly reduced and doubly protonated form of the ultimate electron acceptor of the complex, a ubiquinone (Q B ). The formed Q B H 2 molecule then delivers its reducing power to the cytochrome bc1 complex, resulting in the release of protons on the periplasmic side of the membrane. The resulting transmembrane proton gradient drives ATP synthesis through the ATP synthase. The reduction of Q B coupled to the uptake of protons from the bulk is an important step shared by many systems involved either in photosynthesis or in respiratory chains (1).The three-dimensional structure of the reaction center from the purple photosynthetic bacterium Rhodobacter (Rb.) sphaeroides is known at atomic resolution (2-4). Three subunits with a total molecular weight of about 100 kDa compose these RCs. The transmembrane L and M subunits carry the nine pigments and cofactors: four bacteriochlorophylls, two bacteriopheophytins, two ubiquinones 10, and one non-heme iron atom. The third subunit, H, caps the reaction center on the cytoplasmic side of the membrane. The initial photochemical event induced by the absorption of a photon is the creation of the singlet excited state of a dimer of bacteriochlorophylls (P3P*), which constitutes the primary electron donor. P* is a strong reducing species that initiates the electron transfer reaction through the protein. In about 200 ps, the charge separation occurs between P and the first quinone electron acceptor, Q A , situated on the cytoplasmic side of the complex. The electron is then transferred from Q A Ϫ to a secondary quinone Q B within 10-100 s (5-7). Both Q A and Q B are deeply buried within the reaction center protein. The role of the protein in stabilizing the redox species is essential to ensure high forward electron transfer rates and to prevent charge recombinations to occur. Although chemically identical, Q A and Q B behave differently. Q A , bound to the M subunit in a relatively hydrophobic pocket, functions as one electron acceptor and is never protonated. At variance, Q B , bound to the L subunit, is surrounded by charged and polar residues and behaves as a two-electron gate, accepting sequentially two electrons from Q A and two protons from the cytoplasm. In chromatophores, the semiquinone Q B Ϫ can bind a proton below pH 6.8 (8). However, in isolated RCs, the semiquinones are not directly protonated but induce the shift of the pKas of ionizable interacting residues, which results in substoichiometric proton uptake by the protein (9, 10). The proton uptake may occur through a number of water molecules and protonatable amino acid residues situated between Q B and the cytoplasmic surface. Of main interest is to identify the dynamical and structural role of the protein that contributes to the stability of the Q A Ϫ and Q B Ϫ states and to the energetic and functiona...

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Section: Protonation Events Triggered By the Q Bmentioning

confidence: 99%

Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

Tandori

Maróti

Alexov

et al. 2002

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

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“…[34] Similar anionic semiquinone destabilization has been observed at the Q B site of the bacterial photosynthetic reaction center following mutation of arginine residues to non-polar isoleucines. [35] The small stability constant for SQ o is consistent with its function in the Q-cycle since a stabilized SQ would lack sufficient reducing power to push electrons against the electrochemical gradient to the low-potential cyt b . [8] Thermodynamically, an unstable SQ o should also be weakly bound in the Q o site since specific, tight binding through hydrogen bonds or salt bridges would stabilize the intermediate.…”

Section: Discussionmentioning

confidence: 84%

A Caged, Destabilized, Free Radical Intermediate in the Q‐Cycle

et al. 2013

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The Rieske/cytochrome b complexes, also known as cytochrome bc complexes, catalyze a unique oxidant-induced reduction reaction at their quinol oxidase sites (Qo), in which substrate hydroquinone reduces two distinct electron transfer chains, one through a series of high-potential electron carriers, the second through low-potential cytochrome b. This reaction is a critical step in energy storage by the Q-cycle. The semiquinone intermediate in this reaction can reduce O2 to produce deleterious superoxide. It is yet unknown how the enzyme controls this reaction, though numerous models are proposed. In previous work we trapped a Q-cycle semiquinone anion intermediate, termed SQo, in bacterial cyt bc1 by rapid freeze-quenching. In this work, we apply pulsed EPR techniques to determine the location and properties of SQo in the mitochondrial complex. In contrast to semiquinone intermediates in other enzymes, SQo is not thermodynamically stabilized, and may even be destabilized with respect to solution. It is trapped in the Qo at a site, which is distinct from previously described inhibitor-binding sites, yet sufficiently close to cytochrome bL to allow rapid electron transfer. The binding site and EPR analysis show that SQo is not stabilized by hydrogen bonds to proteins. The formation of SQo involves “stripping” of both substrate -OH protons during the initial oxidation step, as well as conformational changes of the semiquinone and Qo proteins. The resulting charged radical is kinetically trapped, rather than thermodynamically stabilized (as in most enzymatic semiquinone species), conserving redox energy to drive electron transfer to cytochrome bL, while minimizing certain Q-cycle bypass reactions including oxidation of pre-reduced cytochrome b and reduction of O2.

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“…Since a detailed description of the scheme can be found elsewhere [10,39,40], we focus here only on its main features. (1) In the ground state, the Q B ring is distributed between the distal and proximal sites.…”

Section: Pt (Proton Transfer) Reactions and The Mechanistic Scheme Ofmentioning

confidence: 99%

“…A high activation barrier of approx. 60 kJ/mol for the transfer of the second proton upon the Q B H − → Q B H 2 transition [37,39] was explained by its coupling to the relocation of the quinone ring into the distal site (see Figure 1E and [37,39]), where Q B H 2 is seen in the respective crystal structure (see Figure 1F and [47]). …”

Section: Pt (Proton Transfer) Reactions and The Mechanistic Scheme Ofmentioning

confidence: 99%

Ubiquinone reduction in the photosynthetic reaction centre of Rhodobacter sphaeroides: interplay between electron transfer, proton binding and flips of the quinone ring

Mulkidjanian

Kozlova

Cherepanov

2005

Biochemical Society Transactions

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This review is focused on reactions that gate (control) the electron transfer between the primary quinone Q(A) and secondary quinone Q(B) in the photosynthetic reaction centre of Rhodobacter sphaeroides. The results on electron and proton transfer are discussed in relation to structural information and to the steered molecular dynamics simulations of the Q(B) ring flip in its binding pocket. Depending on the initial position of Q(B) in the pocket and on certain conditions, the rate of electron transfer is suggested to be limited either by the quinone ring flip or by the charge-compensating proton equilibration between the surface and the buried Q(B) site.

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Reduction and protonation of the secondary quinone acceptor of Rhodobacter sphaeroides photosynthetic reaction center: kinetic model based on a comparison of wild-type chromatophores with mutants carrying Arg→Ile substitution at sites 207 and 217 in the L-subunit

Cited by 32 publications

References 73 publications

Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

Key role of proline L209 in connecting the distant quinone pockets in the reaction center of Rhodobacter sphaeroides

A Caged, Destabilized, Free Radical Intermediate in the Q‐Cycle

Ubiquinone reduction in the photosynthetic reaction centre of Rhodobacter sphaeroides: interplay between electron transfer, proton binding and flips of the quinone ring

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