Electrogenic reactions and dielectric properties of photosystem II

Gorbikova

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

et al. 2010

Cytochrome c oxidase is the terminal enzyme of the respiratory chain that is responsible for biological energy conversion in mitochondria and aerobic bacteria. The membrane-bound enzyme converts free energy from oxygen reduction to an electrochemical proton gradient by functioning as a redox-coupled proton pump. Although the 3D structure and functional studies have revealed proton conducting pathways in the enzyme interior, the location of proton donor and acceptor groups are not fully identified. We show here by time-resolved optical and FTIR spectroscopy combined with time-resolved electrometry that some mutant enzymes incapable of proton pumping nevertheless initiate catalysis by proton transfer to a proton-loading site. A conserved tyrosine in the so-called D-channel is identified as a potential proton donor that determines the efficiency of this reaction.cell respiration | cytochrome aa3 | proton transfer | time-resolved electrometry T he electrochemical proton gradient across phospholipid membranes is the primary product of biological energy transduction during oxidation of foodstuffs by oxygen, followed by the synthesis of ATP by use of this gradient. The reduction of oxygen catalyzed by cytochrome c oxidase (CcO) may be summarized as follows. Electrons from cytochrome c, located on the positively charged P-side of the membrane, are transferred via the Cu A center at the membrane surface, via heme a, to a binuclear heme/ copper (a 3 ∕Cu B ) center (BNC) at a dielectric distance d about ⅓ into the membrane ( Fig. 1A; for reviews, see refs. 1-4). O 2 binding to the reduced BNC yields the oxygen adduct ferrousoxy intermediate (A) (Fig. 1B), and scission of the O-O bond in A yields the P ("peroxy") intermediate. If the reaction with O 2 is started with fully reduced enzyme (R), as in this study, the P state is denoted as P R . The scission of the O-O bond requires simultaneous transfer of four electrons to O 2 . Three of these electrons are taken from the BNC itself, while the fourth is supplied by heme a (5, 6). Protonation of the BNC in the P R state converts it into intermediate F (ferryl), and uptake of another electron and another proton yields the oxidized state O (see Fig. 1B) (7,8). Two more electron and proton transfer steps convert the O state back to the reduced form, which can react with the next O 2 molecule.Each electron transfer into the BNC is thus associated with uptake of a proton into this site from the negatively charged N-side of the membrane and is in addition coupled to translocation (pumping) of one more proton across the entire membrane (9). Proton uptake for formation of the equivalent of water at the BNC takes place via two different pathways, the so-called D-and K-channels, whereas proton uptake for pumping is restricted to the D-channel (Fig. 1A) (2, 3, 10).The dual role of the D-channel has raised much experimental and computational interest (see ref. 11), but its intriguing properties are not yet fully understood. It starts with a conserved aspartate (D124) at the entrance and e...

Section: Methodsmentioning

confidence: 56%

Initiation of the proton pump of cytochrome c oxidase

Gorbikova

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

et al. 2010

“…Direct electrometry is a convenient method for detecting transmembrane transfer of charged species by proteins operating as generators of transmembrane electric potential (Δψ). This approach has been successfully used in studies of bacterial photosynthetic reaction centers and cytochrome bc 1 -complex 13 14 15 , pigment–protein complexes of photosystems II and I 16 17 , terminal oxidases 18 19 20 21 , bacteriorhodopsin 22 23 , and halorhodopsin 24 . In the present paper, we used this method to study the catalytic cycle of the recently described NaR.…”

mentioning

confidence: 99%

Real-time kinetics of electrogenic Na+ transport by rhodopsin from the marine flavobacterium Dokdonia sp. PRO95

Bogachev

Bertsova

Verkhovskaya

et al. 2016

Sci Rep

Discovery of the light-driven sodium-motive pump Na+-rhodopsin (NaR) has initiated studies of the molecular mechanism of this novel membrane-linked energy transducer. In this paper, we investigated the photocycle of NaR from the marine flavobacterium Dokdonia sp. PRO95 and identified electrogenic and Na+-dependent steps of this cycle. We found that the NaR photocycle is composed of at least four steps: NaR519 + hv → K585 → (L450↔M495) → O585 → NaR519. The third step is the only step that depends on the Na+ concentration inside right-side-out NaR-containing proteoliposomes, indicating that this step is coupled with Na+ binding to NaR. For steps 2, 3, and 4, the values of the rate constants are 4×104 s–1, 4.7 × 103 M–1 s–1, and 150 s–1, respectively. These steps contributed 15, 15, and 70% of the total membrane electric potential (Δψ ~ 200 mV) generated by a single turnover of NaR incorporated into liposomes and attached to phospholipid-impregnated collodion film. On the basis of these observations, a mechanism of light-driven Na+ pumping by NaR is suggested.

“…Only following the second flash, when the formation of a doubly-reduced quinone species Q B H 2 occurs, trapping of two protons takes place and an additional electrogenic phase of a ∆Ψ with an amplitude corresponding to ∼11% of the fast phase (τ∼0.85 ms at pH 7.5) is formed (41). The following observations such as the sensitivity of this phase to diuron, an inhibitor of electron transfer between Q A and Q B , the flash-number dependence of its amplitude and the decrease of its risetime and amplitude with decreasing pH indicate that this electrogenic reaction is associated with protonation of Q B 2- (7). The fact that the amplitude of the phase related to proton-coupled electron transfer between Q A and Fe 3+ is higher than the phase related to protonation of Q B 2could be explained by different distances between the non-heme iron/Q B -binding sites and protein-water interface (3).…”

Section: -mentioning

confidence: 93%

“…The asymmetrical transmembrane organization of RCs makes charge separation reactions electrogenic. ∆Ψ can be monitored by a variety of methods, namely: microelectrodes, electroluminescence, transient absorbance (electrochromic response of pigments in the lightharvesting complexes), as well as electrometric techniques (see (7) and references therein). The time-resolved measurements of the voltage generated by PS II upon its single-turnover is a valuable tool to obtain information on the nature and mechanisms of electrogenic reactions and dielectric properties of this pigment-protein complex.…”

Section: Introductionmentioning

confidence: 99%

Photosysem II: where does the light-induced voltage come from?

Mamedov¹

2010

Front Biosci

Self Cite

Photosystem II (PS II) is a biological energy transducer. The enzyme catalyses the light-driven oxidation of water and reduction of plastoquinone. The aim of this work was to review the mechanisms of electrical events in PS II. The major contribution to the total photoelectric response is due to the charge-separation between the primary chlorophyll donor P680 and quinone acceptor QA accompanied by re-reduction of P680+ by tyrosine residue YZ. The remaining part of the membrane potential is believed to be associated mainly with electron and proton transfer events due to the S-state transitions of the oxygen-evolving complex and proton uptake associated with protonation of the doubly reduced secondary quinone acceptor QB. Under certain non-physiological conditions, some other electrogenic reactions are observed, namely: proton-coupled electron transfer between QA and non-heme Fe3+ and electron transfer from the protein-water interface to the YZ radical in the presence of artificial electron donors. These data may provide a good platform for further development of artificial photosynthetic constructs and bio-inspired catalysts.