Alteration of the Axial Met Ligand to Electron Acceptor A₀ in Photosystem I: An Investigation of Electron Transfer at Different Temperatures by Multifrequency Time-Resolved and CW EPR

Abstract

The ambient temperature and low-temperature electron transfer properties of Photosystem I (PS I) from the M688N_PsaA and M668N_PsaB mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 are studied using transient electron paramagnetic resonance (EPR) and continuous-wave (CW) EPR. The two mutations are expected to alter the midpoint potentials of, and the reorganization energies around, the primary electron chlorophyll acceptors A_0A and A_0B, which should lead to a change in the yield and/or rate of electron transfer to the phylloquinone acceptors A_1A and A_1B, respectively. At ambient temperature it is known that both quinone acceptors are active in electron transfer. At low temperature there are at least two fractions that undergo either reversible or irreversible electron transfer. The EPR data of the two PS I variants are used to investigate the relationship between these low-temperature fractions and the ambient temperature electron transfer pathway. The results show that mutation in the PsaA-branch increases the rate of ${\text{A}}_{{1{\text{A}}}}^{. - }$ to F_X electron transfer at ambient temperature, while the corresponding mutation in the PsaB-branch has no effect on the electron transfer rate observable by transient EPR. An analysis of the complete time/field datasets from both variants suggests that the yield of electron transfer in the branch carrying the mutation is reduced. The mutations have no effect on the low-temperature CW EPR spectra of the iron–sulfur clusters if the samples are frozen under illumination but they both cause a decrease in the yield of reduced F_A and F_B if the samples are frozen in the dark and then illuminated. The PsaA-branch mutation greatly reduces the intensity and changes the polarization pattern of the radical pair ${\text{P}}_{700}^{ + } {\text{A}}_{1}^{. - }$. Possible causes of the changes in the polarization pattern are discussed and it is suggested that the mutations introduce structural heterogeneity in the vicinity of the A₀ binding site. No clear correlation between the yield of electron transfer in a particular branch and the yield of stable charge separation is found.