Modulating the Redox Potential of the Stable Electron Acceptor, Q_B, in Mutagenized Photosystem II Reaction Centers

Abstract

One of the unique features of electron transfer processes in photosystem II (PSII) reaction centers (RC) is the exclusive transfer of electrons down only one of the two parallel cofactor branches. In contrast to the RC core polypeptides (psaA and psaB) of photosystem I (PSI), where electron transfer occurs down both parallel redox-active cofactor branches, there is greater protein-cofactor asymmetry between the PSII RC core polypeptides (D1 and D2). We have focused on the identification of protein-cofactor relationships that determine the branch along which primary charge separation occurs (P(680)(+)/pheophytin(-)(Pheo)). We have previously shown that mutagenesis of the strong hydrogen-bonding residue, D1-E130, to less polar residues (D1-E130Q,H,L) shifted the midpoint potential of the Pheo(D1)/Pheo(D1)(-) couple to more negative values, reducing the quantum yield of primary charge separation. We did not observe, however, electron transfer down the inactive branch in D1-E130 mutants. The protein residue corresponding to D1-E130 on the inactive branch is D2-Q129 which presumably has a reduced hydrogen-bonding interaction with Pheo(D2) relative to the D1-E130 residue with Pheo(D1). Analysis of the recent 2.9 Å cyanobacterial PSII crystal structure indicated, however, that the D2-Q129 residue was too distant from the Pheo(D2) headgroup to serve as a possible hydrogen bond donor and directly impact its midpoint potential as well as potentially determine the directionality of electron transfer. Our objective was to characterize the function of this highly conserved inactive branch residue by replacing it with a nonconservative leucine or a conservative histidine residue. Measurements of Chl fluorescence decay kinetics and thermoluminescence studies indicate that the mutagenesis of D2-Q129 decreases the redox gap between Q(A) and Q(B) due to a lowering of the redox potential of Q(B). The resulting increased yield of S(2)Q(B)(-) charge recombination in the D2-Q129 mutants leads to an increased susceptibility to photoinhibitory light presumably due to (3)P(680)-mediated oxidative damage. The results indicate that the D2-Q129 residue plays a critical role in stabilizing the charge-separated state in PSII and further documents the structural and functional asymmetry between the two cofactor branches in PSII.

Structure/Function/Dynamics of Photosystem II Plastoquinone Binding Sites

“…In contrast, the double reduced and protonated Q B moves away from its binding site and is promptly replaced by a new PQ molecule (as well as by artificial quinones or herbicides), leading to consider this molecule more as a substrate rather than a tightly bound cofactor. Although chemically identical, Q A and Q B demonstrate distinguishable redox potentials, necessary for the potentiation of PSII forward electron transfer and the fine tuning of the backward recombination reactions [24, 25]. The different functional and redox-properties of the PSII plastoquinone molecules are largely determined by the interactions with the surrounding environment (H-bonding, hydrophobic interfaces, π-stacking), and the conformation of the individual binding sites and phytyl chains [4-6].…”

Automated analysis of food-borne pathogens using a novel microbial cell culture, sensing and classification system

“…Nevertheless, the oxygen peak presented different behavior over time which may suggest that thylakoids found inside the chloroplast of the cells can generate a redox reaction. [17][18][19] The RVC materials used in this work are biocompatible with three dimensional structures that allow microorganisms to grow well. 20 In that respect, we used SEM to observe the formation of biofilm on the RVCs (Fig.…”

Noise-assisted quantum electron transfer in photosynthetic complexes

“…The characteristic time-scales of the electron dynamics vary from a few picoseconds to milliseconds. The primary charge separation occurs on a very short time-scale, of a few picoseconds [1][2][3][4][5]. Because this time-scale is so short, even the room-temperature fluctuations of the protein environment do not destroy the quantum coherent effects, which were recently discovered in these complexes [5][6][7][8][9][10][11][12][13].…”