Quinone (Q(B)) binding site and protein stuctural changes in photosynthetic reaction center mutants at Pro-L209 revealed by vibrational spectroscopy

Quantum Sensing of Electron Transfer Pathways in Natural Photosynthesis Using Time-Resolved High-Field Electron Paramagnetic Resonance/Electron-Nuclear Double Resonance Spectroscopy

Photosynthetic integral membrane reaction center (RC) proteins capture and convert sunlight into chemical energy via efficient charge separation achieved through a series of rapid, photoinitiated electron transfer steps. These fast electron transfers create an entangled spin qubit (radical) pair that contains detailed information about the weak magnetic interactions, structure, and dynamics of localized protein environments involved in charge separation events. Herein, we investigate how these entangled electron spin qubits interact with nuclear spins of the protein environment using the high spectral resolution of 130 GHz electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR). Spectroscopic interrogation enabled the observation and probing of protons located in the electron transfer pathway between the membrane-spanning electron pair P⁺Q_A^- (where P⁺ is the primary donor, a special pair of bacteriochlorophylls, and Q_A is the primary quinone acceptor) in the bacterial RC. Spectroscopic analysis reveals hydrogen-bonding interactions involved in regulating the route that light-induced electrons travel through the RC protein during charge separation.

M234Glu is a component of the proton sponge in the reaction center from photosynthetic bacteria

Bacterial reaction centers use light energy to couple the uptake of protons to the successive semi-reduction of two quinones, namely Q(A) and Q(B). These molecules are situated symmetrically in regard to a non-heme iron atom. Four histidines and one glutamic acid, M234Glu, constitute the five ligands of this atom. By flash-induced absorption spectroscopy and delayed fluorescence we have studied in the M234EH and M234EL variants the role played by this acidic residue on the energetic balance between the two quinones as well as in proton uptake. Delayed fluorescence from the P(+)Q(A)(-) state (P is the primary electron donor) and temperature dependence of the rate of P(+)Q(A)(-) charge recombination that are in good agreement show that in the two RC variants, both Q(A)(-) and Q(B)(-) are destabilized by about the same free energy amount: respectively approximately 100 +/- 5 meV and 90 +/- 5 meV for the M234EH and M234EL variants, as compared to the WT. Importantly, in the M234EH and M234EL variants we observe a collapse of the high pH band (present in the wild-type reaction center) of the proton uptake amplitudes associated with formation of Q(A)(-) and Q(B)(-). This band has recently been shown to be a signature of a collective behaviour of an extended, multi-entry, proton uptake network. M234Glu seems to play a central role in the proton sponge-like system formed by the RC protein.

Temperature and cryoprotectant influence secondary quinone binding position in bacterial reaction centers

We have determined the first de novo position of the secondary quinone QB in the Rhodobacter sphaeroides reaction center (RC) using phases derived by the single wavelength anomalous dispersion method from crystals with selenomethionine substitution. We found that in frozen RC crystals, QB occupies primarily the proximal binding site. In contrast, our room temperature structure showed that QB is largely in the distal position. Both data sets were collected in dark-adapted conditions. We estimate that the occupancy of the QB site is 80% with a proximal: distal ratio of 4:1 in frozen RC crystals. We could not separate the effect of freezing from the effect of the cryoprotectants ethylene glycol or glycerol. These results could have far-reaching implications in structure/function studies of electron transfer in the acceptor quinone complex because the above are the most commonly used cryoprotectants in spectroscopic experiments.

Characterization of the bonding interactions of QB upon photoreduction via A-branch or B-branch electron transfer in mutant reaction centers from Rhodobacter sphaeroides

In Rhodobacter sphaeroides reaction centers (RCs) containing the mutation Ala M260 to Trp (AM260W), transmembrane electron transfer along the full-length of the A-branch of cofactors is prevented by the loss of the Q(A) ubiquinone, but it is possible to generate the radical pair P(+)H(A)(-) by A-branch electron transfer or the radical pair P(+)Q(B)(-) by B-branch electron transfer. In the present study, FTIR spectroscopy was used to provide direct evidence for the complete absence of the Q(A) ubiquinone in mutant RCs with the AM260W mutation. Light-induced FTIR difference spectroscopy of isolated RCs was also used to probe the neutral Q(B) and the semiquinone Q(B)(-) states in two B-branch active mutants, a double AM260W-LM214H mutant, denoted WH, and a quadruple mutant, denoted WAAH, in which the AM260W, LM214H, and EL212A-DL213A mutations were combined. The data were compared to those obtained with wild-type (Wt) RCs and the double EL212A-DL213A (denoted AA) mutant which exhibit the usual A-branch electron transfer to Q(B). The Q(B)(-)/Q(B) spectrum of the WH mutant is very close to that of Wt RCs indicating similar bonding interactions of Q(B) and Q(B)(-) with the protein in both RCs. The Q(B)(-)/Q(B) spectra of the AA and WAAH mutants are also closely related to one another, but are very different to that of the Wt complex. Isotope-edited IR fingerprint spectra were obtained for the AA and WAAH mutants reconstituted with site-specific (13)C-labeled ubiquinone. Whilst perturbations of the interactions of the semiquinone Q(B)(-) with the protein are observed in the AA and WAAH mutants, the FTIR data show that the bonding interaction of neutral Q(B) in these two mutants are essentially the same as those for Wt RCs. Therefore, it is concluded that Q(B) occupies the same binding position proximal to the non-heme iron prior to reduction by either A-branch or B-branch electron transfer.

Time-resolved crystallographic studies of light-induced structural changes in the photosynthetic reaction center

Light-induced structural changes in the bacterial reaction center were studied by a time-resolved crystallographic experiment. Crystals of protein from Blastochloris viridis (formerly Rhodopseudomonas viridis) were reconstituted with ubiquinone and analyzed by monochromatic and Laue diffraction, in the dark and 3 ms after illuminating the crystal with a pulsed laser (630 nm, 3 mJ/pulse, 7 ns duration). Refinement of monochromatic data shows that ubiquinone binds only in the "proximal" Q(B) binding site. No significant structural difference was observed between the light and dark datasets; in particular, no quinone motion was detected. This result may be reconciled with previous studies by postulating equilibration of the "distal" and "proximal" binding sites upon extended dark adaption, and in which movement of ubiquinone is not the conformational gate for the first electron transfer between Q(A) and Q(B).