Directing electron transfer within Photosystem I by breaking H-bonds in the cofactor branches

Abstract

Photosystem I has two branches of cofactors down which lightdriven electron transfer (ET) could potentially proceed, each consisting of a pair of chlorophylls (Chls) and a phylloquinone (PhQ). Forward ET from PhQ to the next ET cofactor (F X) is described by two kinetic components with decay times of Ϸ20 and Ϸ200 ns, which have been proposed to represent ET from PhQ B and PhQA, respectively. Immediately preceding each quinone is a Chl (ec3), which receives a H-bond from a nearby tyrosine. To decrease the reduction potential of each of these Chls, and thus modify the relative yield of ET within the targeted branch, this H-bond was removed by conversion of each Tyr to Phe in the green alga Chlamydomonas reinhardtii. Together, transient optical absorption spectroscopy performed in vivo and transient electron paramagnetic resonance data from thylakoid membranes showed that the mutations affect the relative amplitudes, but not the lifetimes, of the two kinetic components representing ET from PhQ to F X. The mutation near ec3 A increases the fraction of the faster component at the expense of the slower component, with the opposite effect seen in the ec3B mutant. We interpret this result as a decrease in the relative use of the targeted branch. This finding suggests that in Photosystem I, unlike type II reaction centers, the relative efficiency of the two branches is extremely sensitive to the energetics of the embedded redox cofactors.Chlamydomonas ͉ directionality ͉ photosynthetic reaction center ͉ pump-probe spectroscopy ͉ transient EPR P hotosynthetic reaction centers (RCs) are the membrane proteins responsible for the capture and storage of light energy in photosynthetic organisms. As with all of the RCs, Photosystem I (PS1) has a C2 symmetrical structure with two virtually identical branches of redox cofactors extending across the membrane (see Fig. 1). The x-ray crystal structure of PS1 from Thermosynechococcus elongatus (1) has allowed identification of amino acid residues interacting with the electron transfer (ET) cofactors, permitting the use of site-directed mutagenesis to investigate to what extent ET occurs in the two branches. Many of these studies (2-10) point toward the possibility that ET takes place in both branches of cofactors (A-branch and Bbranch; see Fig. 1). The data consistently show that the major fraction occurs in the A-branch and that the slow phase of ET from phylloquinone (PhQ) to F x is associated with this branch. The involvement of the B-branch is less clear, but there is mounting evidence to support the assignment of the fast component of PhQ to F x ET to this branch. If ET does indeed occur in both branches in PS1, this behavior would be remarkably different from the type II RCs. In purple bacterial RCs, for example, it is well known that initial ET is biased almost exclusively toward the A-branch, probably because the B-branch quinone is a mobile electron and proton carrier in this type of RC. The strong biasing of the ET in the type II RCs makes it difficult to investigate the fac...

Independent initiation of primary electron transfer in the two branches of the photosystem I reaction center

“…Based on these ultrafast measurements, the A-branch is the faster one in primary CS and in tertiary ET. However, it is known from the nanosecond measurements (10,32) that the subsequent ET step to the F X cluster is about 10-fold slower in the A-branch (τ ∼ 200 ns vs. ∼20 ns). These facts are likely explained by the different ways in which the two branches divide up the available free energy in going from the Ant/RC* state to the P 700 þ F X state, if one assumes that the differences in rates are primarily due to differences in driving force.…”

“…We obtain a B/A ratio of 0.61 in WT, which increases to 1.44 in PsaA-Y696F and drops to 0.35 in PsaB-Y676F (Table S2). Similar B/A branching ratios (WT-0.67, PsaA-Y696F-1.77, and PsaB-Y676F-0.21) were estimated from the amplitudes of the kinetic components representing reduction of F X by PhQ A or PhQ B (32).…”

“…Construction of the PsaA-Y696F and PsaB-Y676F mutants in C. reinhardtii was described in ref. 32. PSI particles were purified from P71 FuD7 transformants, which lack PSII and have a low light-harvesting complex (LHC) content (reduced antenna size).…”

A Combined Genetic, Biochemical, and Biophysical Analysis of the A1 Phylloquinone Binding Site of Photosystem I from Green Algae

“…Combining mutations that alter directionality of ET within PS1 with those that slow reoxidation of PhQ A (hypothesis 1 & aim 6): We combined two of the mutants that slow ET from PhQ A (PsaA-W697F and PsaA-S692A) with one that redirects ET from the A-branch to the B-branch (PsaA-Y696F; Li et al, 2006). The primary reason for doing this was to test our interpretation of the ET kinetics even further.…”

A Combined Genetic, Biochemical, and Biophysical Analysis of the A1 Phylloquinone Binding Site of Photosystem I from Green Algae

“…Combining mutations that alter directionality of ET within PS1 with those that slow reoxidation of PhQ A (hypothesis 1 & aim 6): We have combined two of the mutants that slow ET from PhQ A (PsaA-W697F and PsaA-S692A) with one that redirects ET from the A-branch to the B-branch (PsaA-Y696F; Li et al, 2006). The primary reason for doing this was to test our interpretation of the ET kinetics even further.…”