Bicarbonate-controlled reduction of oxygen by the Q _A semiquinone in Photosystem II in membranes

Abstract

Photosystem II (PSII), the water/plastoquinone photo-oxidoreductase, plays a key energy input role in the biosphere. QA•−, the reduced semiquinone form of the nonexchangeable quinone, is often considered capable of a side reaction with O2, forming superoxide, but this reaction has not yet been demonstrated experimentally. Here, using chlorophyll fluorescence in plant PSII membranes, we show that O2 does oxidize QA•− at physiological O2 concentrations with a t1/2 of 10 s. Superoxide is formed stoichiometrically, and the reaction kinetics are controlled by the accessibility of O2 to a binding site near QA•−, with an apparent dissociation constant of 70 ± 20 µM. Unexpectedly, QA•− could only reduce O2 when bicarbonate was absent from its binding site on the nonheme iron (Fe2+) and the addition of bicarbonate or formate blocked the O2-dependant decay of QA•−. These results, together with molecular dynamics simulations and hybrid quantum mechanics/molecular mechanics calculations, indicate that electron transfer from QA•− to O2 occurs when the O2 is bound to the empty bicarbonate site on Fe2+. A protective role for bicarbonate in PSII was recently reported, involving long-lived QA•− triggering bicarbonate dissociation from Fe2+ [Brinkert et al., Proc. Natl. Acad. Sci. U.S.A. 113, 12144–12149 (2016)]. The present findings extend this mechanism by showing that bicarbonate release allows O2 to bind to Fe2+ and to oxidize QA•−. This could be beneficial by oxidizing QA•− and by producing superoxide, a chemical signal for the overreduced state of the electron transfer chain.

Transcriptional regulation of photoprotection in dark-to-light transition—More than just a matter of excess light energy

“…In addition to RS production by PSI, it has been recently reported that PSII isolated from plants can generate superoxide and that this production is inhibited by bicarbonate ( 59 ). Therefore, our results could also be explained by higher superoxide production in PSII in dark-acclimated cells than in those preacclimated in LL.…”

Functional Water Networks in Fully Hydrated Photosystem II

“…Based on the MD simulations and previous results from various experimental and computational studies, the Q B pocket waters can play two roles: (a) mediate proton transfer from the bicarbonate or bulk to Q B , and (b) mediate proton transfer to the deprotonated D1-His215. In addition to the role of the water channels in proton transfer, a recent work by Fantuzzi et al 122 demonstrated the role of the water channels in dioxygen diffusion toward the non-heme site. Assuming that D1-His215 transfers the second proton to Q B and becomes doubly deprotonated, the Q B pocket water molecules can transfer a proton to D1-His215.…”

Tyr244 of the D2 Protein Is Required for Correct Assembly and Operation of the Quinone-Iron-Bicarbonate Acceptor Complex of Photosystem II

“…In addition, disruption of the bicarbonate-and Q A -binding sites may impact cell signaling and ROS-induced damage by altering O 2 − and 1 O 2 levels. 69 Interestingly, similar tandem duplications in the vicinity of Tyr246 of D1 were found to restore photoautotrophic growth to mutants carrying deletions in the DE loop of D1. 70 We have observed, however, that a D1-Y246A mutant and a D1-E244A:Y246A double mutant lost their ability to grow photoautotrophically due to incorporation of spontaneous secondary mutations, including a deletion in the codon for D1-Leu91 in D1-Y246A cells and an 8 bp insertion in the double mutant that both led to frame-shift mutations.…”

“…Our result therefore suggests that the correct stabilization of bicarbonate binding to the NHI by Tyr244 is a critical function of the loop. In addition, disruption of the bicarbonate- and Q A -binding sites may impact cell signaling and ROS-induced damage by altering O 2 – and 1 O 2 levels . Interestingly, similar tandem duplications in the vicinity of Tyr246 of D1 were found to restore photoautotrophic growth to mutants carrying deletions in the DE loop of D1 .…”