Cytochrome b6f complex, PetB, PetC, PetD, PetM, Hcf164. The dimeric photosynthetic cytochrome b6f complex, a 16-mer of eight distinct subunits and 26 transmembrane helices, catalyzes transmembrane proton-coupled electron transfer for energy storage. Using a 2.5 Å crystal structure of the dimeric complex, Hasan and Cramer 2014 identified 23 distinct lipid-binding sites per monomer. Annular lipids provide a connection for super-complex formation with the photosystem-I reaction center and the LHCII kinase for transmembrane signaling. Internal lipids mediate crosslinking to stabilize the domain-swapped iron-sulfur protein subunit, dielectric heterogeneity within intermonomer and intramonomer electron transfer pathways, and dimer stabilization through lipid-mediated intermonomer interactions.  In the cytochrome b6f complex with the quinol analog, stigmatellin, which partitions in the Qp portal of the bc1 complex, but not of b6f, the Qp portal is partially occluded in the b6f complex relative to bc1. Occlusion of the Qp portal is attributed to the presence of the chlorophyll phytyl tail, which increases the quinone residence time within the Qp portal and is inferred to be a cause of enhanced superoxide production (Hasan et al. 2014).  PetD subunit integration into the thylakoid membrane is a post-translational and an SRP-dependent process that requires the formation of a cpSRP-cpFtsY-ALB3-PetD complex (Króliczewski et al. 2017). The b₆f complex plays a role in trans-membrane signal transduction from reductant. The effect of the p-side of the electron transport chain on the regulation of light energy to the two photosystems by trans-side phosphorylation of the light-harvesting chlorophyll protein has been discussed (Cramer 2018). The cryo-EM structure of the spinach cytochrome b6 f complex has been solved at 3.6 A resolution (Malone et al. 2019). The cytb6f complex links electron transfer between photosystems I and II and converting solar energy into a pmf for ATP synthesis. Electron transfer within cytb6 f occurs via the quinol (Q) cycle, which catalyses the oxidation of plastoquinol (PQH₂) and the reduction of both plastocyanin (PC) and plastoquinone (PQ) at two separate sites via electron bifurcation. In higher plants, cytb6 f also acts as a redox-sensing hub, pivotal to the regulation of light harvesting and cyclic electron transfer that protect against metabolic and environmental stresses. Malone et al. 2019 presented a 3.6 Å resolution cryo-EM structure of the dimeric cytb6 f complex from spinach, which reveals the structural basis for operation of the Q cycle and its redox-sensing function. The complex contains up to three natively bound PQ molecules. The first, PQ1, is located in one cytb6 f monomer near the PQ oxidation site (Qp), adjacent to haem bp and chlorophyll a. Two conformations of the chlorophyll a phytyl tail were resolved, one that prevents access to the Qp site and another that permits it, supporting a gating function for the chlorophyll a involved in redox sensing. PQ2 straddles the intermonomer cavity, partially obstructing the PQ reduction site (Qn) on the PQ1 side and committing the electron transfer network to turnover at the occupied Qn site in the neighbouring monomer. A conformational switch involving the haem cn propionate promotes two-electron, two-proton reduction at the Qn site and avoids formation of the reactive intermediate semiquinone. The location of a tentatively assigned third PQ molecule is consistent with a transition between the Qp and Qn sites in opposite monomers during the Q cycle. The spinach cytb6 f structure therefore provides new insights into how the complex fulfils its catalytic and regulatory roles in photosynthesis (Malone et al. 2019). Cytochrome b6f (cytb6f) lies at the heart of the light-dependent reactions of oxygenic photosynthesis, where it serves as a link between photosystem II (PSII) and photosystem I (PSI) through the oxidation and reduction of the electron carriers plastoquinol (PQH2) and plastocyanin (Pc). A mechanism of electron bifurcation, known as the Q-cycle, couples electron transfer to the generation of a transmembrane proton gradient for ATP synthesis. Cytb6f catalyses the rate-limiting step in linear electron transfer, is pivotal for cyclic electron transfer and plays a key role as a redox-sensing hub involved in the regulation of light-harvesting, electron transfer and photosynthetic gene expression (Malone et al. 2021).  Xiao et al. 2022 investigated the phytotoxicity of reduced graphene oxide (RGO), graphene oxide (GO) and amine-functionalized graphene (G-NH2) on Brassica napus L. RGO impaired photosynthesis mainly by decreasing the chlorophyll content and Rubisco activity. This effect of RGO could be due to its toxicity on sulfate transmembrane transporter and nitrogen metabolism, which ultimately led to nutrient imbalance. However, GO directly damaged the photosystem by disrupting the chloroplast structure, and a decrease in Rubisco activity indicated that GO also inhibits carbon fixation. Gene-level analysis demonstrated that GO has toxicity on the chloroplast membrane, photosystem, photosynthethic electron transport and F-type ATPase (Xiao et al. 2022). Regulation of the generation of reactive oxygen species during photosynthetic electron transport has been discussed (Krieger-Liszkay and Shimakawa 2022). Cyclic electron transfer and photoreduction of oxygen contribute to the size of the proton gradient. The yield of singlet oxygen production in photosystem II is regulated by changes in the midpoint potential of its primary quinone acceptor. In addition, numerous antioxidants inside the photosystems, the antenna and the thylakoid membrane quench or scavenge ROS (Krieger-Liszkay and Shimakawa 2022). Rieske FeS overexpression in tobacco provides increased abundance and activity of cytochrome b(6) f (Heyno et al. 2022).