Electron Transfer in Cyanobacterial Photosystem I

Abstract

The Photosystem I (PS I) reaction center contains two branches of nearly symmetric cofactors bound to the PsaA and PsaB heterodimer. From the x-ray crystal structure it is known that Trp A 1؊ radical pair show that none of the mutations causes a significant change in the orientation of the measured phylloquinone. Pulsed ENDOR spectra reveal that the W697F PsaA mutation leads to about a 5% increase in the hyperfine coupling of the methyl group on the phylloquinone ring, whereas the S692C PsaA mutation causes a similar decrease in this coupling. The changes in the methyl hyperfine coupling are also reflected in the transient EPR spectra of P 700 ؉ A 1 ؊ and the CW EPR spectra of photoaccumulated A 1 ؊ . We conclude that: (i) the transient EPR spectra at room temperature are predominantly from radical pairs in the PsaA branch of cofactors; (ii) at low temperature the electron cycle involving P 700 and A 1 similarly occurs along the PsaA branch of cofactors; and (iii) mutation of amino acids in close contact with the PsaA side quinone leads to changes in the spin density distribution of the reduced quinone observed by EPR. Photosynthetic reaction centers (RCs)1 are classified into two general types depending on the identity and function of the terminal electron acceptors. Those RCs that incorporate ironsulfur clusters are classified as "Type I," and those that incorporate a mobile (secondary) quinone are classified as "Type II." Type I RCs include Photosystem I (PS I) of cyanobacteria and plants and those in heliobacteria and green sulfur bacteria. Type II RCs include Photosystem II of cyanobacteria and plants and those in green non-sulfur bacteria and purple bacteria. Despite the difference in the identity of the terminal electron acceptors, Type I and Type II RCs share a common motif in terms of polypeptide arrangement and cofactor composition (1).The primary cofactors are bound to proteins that are present as dimers in the membrane. This results in a set of electron transfer cofactors that are arranged (pseudo)symmetrically (2). In PS I, these cofactors include a special pair of chlorophyll a/aЈ molecules as the primary donor, two bridging chlorophyll a molecules, and two chlorophyll a molecules, at least one of which functions as the primary acceptor (3). In the purple bacterial reaction center, the cofactors include a special pair of bacteriochlorophyll a molecules as the primary donor, two bridging bacteriochlorophyll a molecules, and two pheophytin molecules, one of which functions as the primary acceptor (4). PS I and the purple bacterial reaction center contain two quinones; in the latter, one quinone is rather immobile (Q A ) and the other is mobile (Q B ), whereas in PS I both quinones (Q K -A and Q K -B) are, to the best of our knowledge, immobile in their normal function.In Type II reaction centers, a single turnover results in the reduction of Q B , to a semiquinone and a second turnover results in the further reduction (and protonation) of Q B to a hydroquinone. The stability of Q B Ϫ requires that t...

Electron Transfer in Cyanobacterial Photosystem I

“…2) could be well fitted with two exponential decay phases of ϭ 10.6 and ϭ 240 ns at an amplitude ratio of about 1:3 (see Table II), in line with a previous report (8). 2 Compared with the measurements on 2 A slight bleaching present at the end of the depicted time scale is presumably from the state P 700 ϩ (F A /F B ) Ϫ and/or a small number of antenna chlorophyll triplets. whole cells from the wild type at 390 and 400 nm (see Table I), the lifetimes of the two phases are rather similar, but the relative amplitude of the faster phase appears to be smaller in isolated PS I (but only at 380 nm, not at 480 nm).…”

“…PCC 6803 (this work), and this is the same quinone that is observed by EPR in photoaccumulation experiments (29) and in low temperature spin-polarized EPR and electron nuclear double resonance experiments (2,34).…”

“…Mutants and Photosystem I Complexes-The mutant strains containing the W697F PsaA , W677F PsaB , S692C PsaA , S672C PsaB , R694A PsaA , and R674A PsaB substitutions have been described previously (2). Isolation of membranes and purification of PS I complexes was performed according to previously published procedures (2).…”

“…Isolation of membranes and purification of PS I complexes was performed according to previously published procedures (2).…”

“…As discussed in the previous paper (2), the use of point mutants and time-resolved EPR and optical spectroscopy allows this issue to be addressed in PS I. The initial electron transfer steps are difficult to observe because the trapping of the excitation from the antenna masks these early events.…”