The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers
نویسندگان
چکیده
Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. 13C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQA and SQB, were compared with DFT calculations of the 13C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔEm) between QA and QB of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔEm of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.
منابع مشابه
Energetics of electron-transfer and protonation reactions of the quinones in the photosynthetic reaction center of Rhodopseudomonas viridis.
The electron-transfer reactions involving the quinones in the bacterial photosynthetic reaction center (bRC) are coupled to a proton uptake by the bRC. In this study, we calculated the energies of the different states of the bRC occurring during these electron-transfer and protonation reactions by an electrostatic model. We considered the possibility that titratable groups of the bRC can change...
متن کاملRedox potential tuning through differential quinone binding in the photosynthetic reaction center of Rhodobacter sphaeroides.
Ubiquinone forms an integral part of the electron transport chain in cellular respiration and photosynthesis across a vast number of organisms. Prior experimental results have shown that the photosynthetic reaction center (RC) from Rhodobacter sphaeroides is only fully functional with a limited set of methoxy-bearing quinones, suggesting that specific interactions with this substituent are requ...
متن کاملCalculated protein and proton motions coupled to electron transfer: electron transfer from QA- to QB in bacterial photosynthetic reaction centers.
Reaction centers from Rhodobacter sphaeroides were subjected to Monte Carlo sampling to determine the Boltzmann distribution of side-chain ionization states and positions and buried water orientation and site occupancy. Changing the oxidation states of the bacteriochlorophyll dimer electron donor (P) and primary (QA) and secondary (QB) quinone electron acceptors allows preparation of the ground...
متن کاملLight-induced charge separation in Rhodopseudomonas viridis reaction centers monitored by Fourier-transform infrared difference spectroscopy: the quinone vibrations.
Static FTIR light-induced difference spectra have been recorded for reaction centers from Rhodopseudomonas viridis in the following charge-separated states: P+QA(-)-PQA, P+QB(-)-PQB, I(-)-I, I-QA(-)-IQA, and I-QA(2-)-IQA. A comparison of the I(-)-I difference spectra with the I-QA(-)-IQA difference spectra reveals new bands which can be assigned to QA- vibrations; these vibrations are also obse...
متن کاملAbsence of a bicarbonate-depletion effect in electron transfer between quinones in chromatophores and reaction centers of Rhodobacter sphaeroides.
Higher plants, algae, and cyanobacteria are known to require bicarbonate ions for electron flow from the first stable electron acceptor quinone QA to the second electron acceptor quinone QB, and to the intersystem quinone pool. It has been suggested that in Photosystem II of oxygenic photosynthesis, bicarbonate ion functions to maintain the reaction center in a proper conformation and, perhaps,...
متن کامل