Cytochrome b6f complex (Qi-like site) (Cyt b6f (Qi site))

The photosynthetic cytochrome b6f complex is a multi-subunit integral membrane protein found in the thylakoid membranes of plants, algae, and cyanobacteria (UniProt: P00124). It plays a critical role in photosynthesis by mediating the transfer of electrons between Photosystem II and Photosystem I while simultaneously pumping protons into the thylakoid lumen to drive ATP synthesis via the Q-cycle (Baniulis et al., 2008, PubMed: 18635140). The Qi-like site, also known as the Qn site, is the quinone-reducing pocket of the complex where plastoquinone is reduced back to plastoquinol (Wikipedia: Cytochrome b6f complex). This site is a specific target for various inhibitors, such as NQNO, which disrupt the energy conversion process in plants (PubChem: CID 104844). Because this complex is unique to photosynthetic organisms and lacks a direct human ortholog, it serves as a selective target for agricultural herbicides. Inhibition of the Qi site effectively halts the photosynthetic electron transport chain, leading to a cessation of carbon fixation and eventual plant death. Understanding the structural biology of the Qi site is essential for developing more effective and environmentally sustainable herbicides.

Other names

Plastoquinol—plastocyanin reductaseCytochrome b6f complex Qn siteCytochrome bf complexPlastoquinol-plastocyanin oxidoreductaseQuinone reduction site of cytochrome b6f

Inhibition of the quinone reduction site (Qi/Qn) within the cytochrome b6f complex, which blocks the Q-cycle and prevents the generation of a proton gradient across the thylakoid membrane (Baniulis et al., 2008, PubMed: 18635140).