Low-Temperature Interquinone Electron Transfer in Photosynthetic Reaction Centers from Rhodobacter sphaeroides and Blastochloris viridis: Characterization of Q_B~- States by High-Frequency Electron Paramagnetic Resonance (EPR) and Electron-Nuclear Do

摘要

High-frequency electron paramagnetic resonance (HF EPR) techniques have been employed to look for localized light-induced conformational changes in the protein environments around the reduced secondary quinone acceptor (Q_B~-) in Rhodobacter sphaeroides and Blastochloris viridis RCs.The Q_A~-and Q_B~- radical species in Fe-removed/Zn-replaced protonated RCs substituted with deuterated quinones are distinguishable with pulsed D-band (130 GHz) EPR and provide native probes of both the low-temperature Q_A~- Q_B -> Q_AQ_B~- electron-transfer event and the structure of trapped conformational substates.We report here the first spectroscopic evidence that cryogenically trapped,light-induced changes enable low-temperature Q_A ~-Q_B -> Q_AQ_B~- electron transfer in the B.viridis RC and the first observation of an inactive,trapped P~+Q_B~- state in both R.sphaeroides and B.viridis RCs that does not recombine at 20 K.The high resolution and orientational selectivity of HF electron-nuclear double resonance (ENDOR) allows us to directly probe protein environments around Q_B for distinct P~+Q_B~- kinetic RC states by spectrally selecting specific nuclei in isotopically labeled samples.No structural differences in the protein structure near Q_B~- or reorientation (within 5~°) of Q_B~- was observed with HF ENDOR spectra of two states of P~+Q_B~-: "active" and "inactive" states with regards to low-temperature electron transfer.These results reveal a remarkably enforced local protein environment for Q_B in its reduced semiquinone state and suggest that the conformational change that controls reactivity resides beyond the Q_B local environment.