The properties of the primary electron acceptor in the Photosystem I reaction centre of spinach chloroplasts and its interaction with P700 and the bound ferredoxin in various oxidation-reduction states

Abstract

The properties of the component ‘X’ identified as the primary electron acceptor of Photosystem I in spinach was investigated by electron-paramagnetic-resonance spectroscopy and the complete spectrum obtained for the first time. Component ‘X’ has gx = 1.78, gy = 1.88 and gz = 2.08; it can be observed only at very low temperatures (8-13K) and high microwave powers. Component X was identified in Photosystem I particles prepared with the French press or with Triton X-100. In samples reduced with ascorbate, illumination at low temperatures results in the photo-oxidation of P700 and reduction of a bound iron-sulphur protein; this is irreversible at low temperature. In samples in which the iron-sulphur proteins are reduced by sodium dithionite, illumination at low temperature results in the oxidation of P700 and the reduction of component ‘X’; this is reversible at low temperature. The light-induced P700 signal is the same size with either ascorbate or dithionite as reducing agent, showing that all of the P700 involved in reduction of bound ferredoxin also functions in the reduction of component ‘X’.

Introduction of a [4Fe‐4S (S‐cys)4]+1,+2 iron‐sulfur center into a four‐α helix protein using design parameters from the domain of the Fx cluster in the Photosystem I reaction center

“…Interestingly, the spectrum of bona fide Fx is somewhat atypical with regard to these parameters, so several differences exist between the Fx spectrum and that of a4-FeS. The line,widths of the Fx spectrum are broad and the g anisotropy is distributed over an unusually large range, as shown by the g values of 2.08, 1.88, and 1.78 (Evans et al, 1976), whereas a4-FeS has relatively narrow line widths and apparent g values of 2.053, 1.936, and 1.910. The temperature optimum of the Fx spectrum is 10 K (Evans et al, 1976), compared to I 8 K for the a4-FeS spectrum here.…”

“…The line,widths of the Fx spectrum are broad and the g anisotropy is distributed over an unusually large range, as shown by the g values of 2.08, 1.88, and 1.78 (Evans et al, 1976), whereas a4-FeS has relatively narrow line widths and apparent g values of 2.053, 1.936, and 1.910. The temperature optimum of the Fx spectrum is 10 K (Evans et al, 1976), compared to I 8 K for the a4-FeS spectrum here. These differences indicate that the EPR properties of Fx are influenced by factors other than the sequence of the two loops containing the ligands to the cluster.…”

Electron paramagnetic resonance spectrometry of photosystem I mutants in barley

“…The spectrum is expanded two times compared to those in Figure 4A and 4B and indicates a content of centre A and B which is only 20-25 % of that of the wildtype level. Optimal conditions for the detection of the two acceptors Al and X are obtained by lowering the temperature of the samples to 8K (6). The spectrum of centre X has g-values at !.78, 1.88 and 2.08 (8).…”

Electron paramagnetic resonance studies of photosynthetic electron transport: Photoreduction of ferredoxin and membrane-bound iron-sulfur centers

“…The chemical nature of component X is unknown; its EPR spectrum differs from that of known iron-sulfur proteins. McIntosh and Bolton (15) and Evans et al (16) regarded component X as the primary acceptor of electrons released by the concurrent photooxidation of P700, the reaction center chlorophyll of photosystem I; photoreduced component X is thought to reduce centers A and B. Bearden and Malkin (17)(18)(19)(20)(21)(22) assigned the role of primary electron acceptor in photosystem I to center A which, unlike the more electronegative center B, undergoes a photoreduction at cryogenic temperatures concurrently with the photooxidation of P700. There is as yet no evidence bearing on the role of center B in photosystem I although it appears that center B may be interacting with center A under certain experimental conditions.…”

“…EPR spectroscopy at cryogenic temperatures was also used to identify in chloroplasts and algal preparations another photoreducible component (component X) with resonances at g = 1.78, 1.88, and 2.08 (12)(13)(14)(15)(16). The photoreduction of component X was detectable only after prior reduction of centers A and B with dithionite.…”