Quinone-dependent proton transfer pathways in the photosynthetic cytochrome b ₆ f complex

Abstract

As much as two-thirds of the proton gradient used for transmembrane free energy storage in oxygenic photosynthesis is generated by the cytochrome b 6 f complex. The proton uptake pathway from the electrochemically negative (n) aqueous phase to the n-side quinone binding site of the complex, and a probable route for proton exit to the positive phase resulting from quinol oxidation, are defined in a 2.70-Å crystal structure and in structures with quinone analog inhibitors at 3.07 Å (tridecyl-stigmatellin) and 3.25-Å (2-nonyl-4-hydroxyquinoline N-oxide) resolution. The simplest n-side proton pathway extends from the aqueous phase via Asp20 and Arg207 (cytochrome b 6 subunit) to quinone bound axially to heme c n . On the positive side, the heme-proximal Glu78 (subunit IV), which accepts protons from plastosemiquinone, defines a route for H + transfer to the aqueous phase. These pathways provide a structure-based description of the quinone-mediated proton transfer responsible for generation of the transmembrane electrochemical potential gradient in oxygenic photosynthesis.

Dynamic Localization of the Cyanobacterial Circadian Clock Proteins

“…The regulation of clock components by metabolites of photosynthesis that change with the light-dark cycle – KaiA and CikA by oxidized quinone [15, 16] and KaiC by the ratio of ATP:ADP [17] – provides a rationale for enrichment of clock protein complexes at a specific membrane location at night. The interaction of KaiA and CikA with quinones, in particular, would require close proximity to the thylakoid, where a quinone-binding site of the photosynthetic cytochrome b6f complex is located close to the membrane-water interface, where protons are transferred to the aqueous phase [45]. Within the thylakoid membrane system two key respiratory electron donors move from an even distribution in high light to discrete patches with an overrepresentation at the poles in low light conditions [30].…”

Why chloroplasts and mitochondria retain their own genomes and genetic systems: Colocation for redox regulation of gene expression

“…These results supported the conclusion (40,42) that the redox state of the Rieske iron-sulfur center governs transcription of mitochondrial and chloroplast DNA, with an oxidized center resulting in RNA synthesis and a reduced center inhibiting it. The Rieske ironsulfur center is a component of cytochrome b-c 1 and b 6 -f complexes, the sites of oxidation of the ubiquinone and plastoquinone pools in the proton-motive Q-cycle (43)(44)(45). Acting as a transcriptional redox sensor, the Rieske protein will respond to imbalance in electron flow into, and out of, the Q-cycle.…”

The Non-Photochemical Quenching of the Electronically Excited State of Chlorophyll a in Plants: Definitions, Timelines, Viewpoints, Open Questions

“…(Source of the figure: Stirbet and Govindjee (2012), as modified by A. Stirbet and Govindjee (unpublished); it also includes information from Govindjee (2011), Cramer andZhang (2006), Baniulis et al (2008), and Hasan et al (2013). periodic and aperiodic, fluctuations.…”

“…At 77 K, state 1 (also referred to as state I) and state 2 (also referred to as state II) can be recognized by the characteristic emission spectra: in state 1, one observes higher PS II (F686 and F696) emission bands, and a lower PS I (F730) emission band. For further background on these emission bands, see chapters in Govindjee et al (1986) and Papageorgiou and Govindjee (2004); for LHCII subunits, see Kargul and Barber (2008), Iwai et al (2008Iwai et al ( , 2010a, and Minagawa (2011); for the Cyt b 6 f complex, see Cramer and Zhang (2006), Baniulis et al (2008), Kallas (2012), and Hasan et al (2013); for the PBS, see Allen and Mullineaux (2004).…”