Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Abstract

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A 0A (M688N PsaA ) and A 0B (M668N PsaB ) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N PsaA mutant. In particular, the P 700 Á ? decay kinetics in the M688N PsaA mutant is significantly slower than in the WT or the M668N PsaB mutant. The analysis of the out-of-phase electron-electron dipolar electron spin echo envelope modulation shows that in the M668N PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P 700 Á ? A 1A Á -radical pair measured in the X-ray crystal structure. In the M688N PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7-0.3, which agree well with the 25.8 Å distance for the P 700 Á ? A 1A Á -radical

Directionality of Electron Transfer in Type I Reaction Center Proteins: High-Frequency EPR Study of PS I with Removed Iron–Sulfur Centers

“…The SCRP spectra in Figures 2Aa and 2Ac would be nearly identical and unresolvable with the use of either protonated RCs or lower microwave frequency EPR spectroscopies. 20,23,[34][35] The bidirectional nature of ET in PS I and blocking of the A-branch ET beyond phylloquinone acceptor A 1A at low temperature provides a straight-forward explanation of not only current data but also the suite of reported magnetic resonance and optical experimental data on native and mutant RCs at room and low temparatures. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][31][32] Note, that if [4Fe-4S] cluster removal is the root cause of the observed in this work equal probability for ET along both branches, then the assertion of unidirectional ET in native RCs would require many strong assumptions to explain reported observations.…”

“…One example of this are the well-known lightinduced EPR signals observed for dark-adapted PS I samples. 23 This work also sheds light on the regulation of ET in photosynthetic RCs. Two possible mechanisms could determine the directionality of ET after photoexcitation of PS I.…”

“…This definitive observation of bidirectionality was later confirmed by a number of mutagenesis studies of Type I RCs. [21][22][23][24][25][26][27][28] In mutagenesis experiments, one of the ET pathways is shut down by altering the protein/cofactor structure, which leads to redirection of the ET from one branch to the other branch. This approach makes it impossible to determine the extent of asymmetry of ET along Aand B-branches.…”

Species-dependent alteration of electron transfer in the early stages of charge stabilization in Photosystem I

“…this temperature, several fractions are present in wild type PS I with different behavior following flash excitation, [25][26][27]. Transient absorbance data show that P 700 + decays with lifetimes of~500 μs and~2 ms [26]. The latter phase can also be seen by time-resolved EPR and can be assigned unambiguously to recombination of P 700 + A 1 − .…”

“…As discussed above, the shorter lifetime of P 700 + A 1B − compared to P 700 + A 1A − means that there is less singlet-triplet mixing during B-branch electron transfer hence less net polarization is expected in the P 700 + (FeS) − spectrum from the M688H PsaA variant. this temperature, several fractions are present in wild type PS I with different behavior following flash excitation, [25][26][27]. Transient absorbance data show that P 700 + decays with lifetimes of~500 μs and~2 ms [26].…”