Kinetics of Electron Transfer between Redox Cofactors in Photosystem I Measured by High-Frequency EPR Spectroscopy

The kinetics of the primary electron donor P700+ and the quinone acceptor A1- redox transitions were simultaneously studied for the first time in the time range of 200 μs-10 ms using high-frequency pulse Q-band EPR spectroscopy at cryogenic temperatures in various complexes of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803. In the A1-core PSI complexes that lack 4Fe4S clusters, the kinetics of the A1- and P700+ signals disappearance at 100 K were similar and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the P700+A1A- ion-radical pair in the A branch of redox cofactors. The kinetics of the backward electron transfer from A1B- to P700+ in the B branch of redox cofactors with τ < 100 μs could not be resolved due to time limitations of the method. In the native PSI complexes with a full set of redox cofactors and in the FX-core complexes, containing the 4Fe4S cluster FX, the kinetics of the A1- signal was significantly faster than that of the P700+ signal. The disappearance of the A1- signal had a characteristic time of 280-350 μs; it was suggested that, in addition to the backward electron transfer from A1A- to P700+ with τ ≈ 500 μs, its kinetics also includes the forward electron transfer from A1A- to the 4Fe4S cluster FX, which had slowed down to 150-200 μs. In the kinetics of P700+ reduction, it was possible to distinguish components caused by the backward electron transfer from A1- (τ ≈ 500 μs) and from 4Fe4S clusters (τ = 1 ms for the FX-core and τ > 5 ms for native complexes). These results are in qualitative agreement with the data on the kinetics of P700+ reduction obtained previously using pulse absorption spectrometry at cryogenic temperatures.

Changes in the Electron Transfer Symmetry in the Photosystem I Reaction Centers upon Removal of Iron-Sulfur Clusters

In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P₇₀₀⁺ and phylloquinone acceptors A_1A^- and A_1B^- in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe-4S clusters, F_X, F_A, and F_B, and the PsaC protein subunit (the so-called P₇₀₀-A₁ core), in which phylloquinone molecules A_1A and A_1B serve as the terminal electron acceptors. It was shown that in the P₇₀₀-A₁ core preparations, the average distance between the centers of the P₇₀₀⁺A₁^- ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is ~25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe-4S clusters F_X, F_A, and F_B are discussed.

Control of electron transfer by protein dynamics in photosynthetic reaction centers

Trehalose and glycerol are low molecular mass sugars/polyols that have found widespread use in the protection of native protein states, in both short- and long-term storage of biological materials, and as a means of understanding protein dynamics. These myriad uses are often attributed to their ability to form an amorphous glassy matrix. In glycerol, the glass is formed only at cryogenic temperatures, while in trehalose, the glass is formed at room temperature, but only upon dehydration of the sample. While much work has been carried out to elucidate a mechanistic view of how each of these matrices interact with proteins to provide stability, rarely have the effects of these two independent systems been directly compared to each other. This review aims to compile decades of research on how different glassy matrices affect two types of photosynthetic proteins: (i) the Type II bacterial reaction center from Rhodobacter sphaeroides and (ii) the Type I Photosystem I reaction center from cyanobacteria. By comparing aggregate data on electron transfer, protein structure, and protein dynamics, it appears that the effects of these two distinct matrices are remarkably similar. Both seem to cause a "tightening" of the solvation shell when in a glassy state, resulting in severely restricted conformational mobility of the protein and associated water molecules. Thus, trehalose appears to be able to mimic, at room temperature, nearly all of the effects on protein dynamics observed in low temperature glycerol glasses.