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 double resonance (ENDOR).
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(A)Q(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(A)Q(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 degrees ) 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.
Utschig
Lisa M
LM
Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, USA. utschig@anl.gov
Thurnauer
Marion C
MC
Tiede
David M
DM
Poluektov
Oleg G
OG
D001665:Binding Sites; D003080:Cold Temperature; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D004579:Electron Transport; D005609:Free Radicals; D041921:Hyphomicrobiaceae / Q000737:chemistry*; D007501:Iron / Q000737:chemistry; D007553:Isotope Labeling; D007700:Kinetics; D010777:Photochemistry; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry
D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones; D007501:Iron; D015032:Zinc