Biological incorporation of alternative quinones into photosystem I

All well-characterized photosynthetic reaction centers contain quinone molecules. Type II reaction centers, such as Photosystem II (PS II), contain two quinones, one that functions as a bound one-electron acceptor, and the other that functions as a mobile two-electron (and two-proton) accumulator. In PS II, plastoquinone-9 (PQ-9) serves the role of the mobile quinone, which shuttles electrons to Photosystem I (PS I) via the cytochrome b f complex. Type I reaction centers, such as PS I of cyanobacteria and green plants, contain two bound phylloquinones (PhQ - vitamin K, 2-methyl-3-phytyl-1,4-naphthoquinone).