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Subunit L of bacterial photosynthetic reaction center Bacterial photosynthetic reaction center (RC) complex, subunit L. The bacterial photosynthetic reaction center couples light-induced electron transfer with pumping protons across the membrane using reactions involving a quinone molecule (QB) that binds two electrons and two protons at the active site. The reaction center consists of three membrane-bound subunits, designated L, M, and H, plus an additional extracellular cytochrome subunit. The L and M subunits are arranged around an axis of 2-fold rotational symmetry perpendicular to the membrane, forming a scaffold that maintains the cofactors in a precise configuration. The L and M subunits have both sequence and structural similarity, suggesting a common evolutionary origin. The L and M subunits bind noncovalently to the nine cofactors in 2-fold symmetric branches: four bacteriochlorophylls (Bchl), two bacteriopheophytins (Bphe), two ubiquinone molecules (QA and QB), and a non-heme iron. Two Bchls on the periplasmic side of the membrane form the 'special pair' or dimer which is the primary electron donor for the photosynthetic reactions. The electron transfer reaction proceeds from the dimer to an intermediate acceptor (PA), a primary quinone (QA), and a secondary quinone (QB). Protons are translocated from the bacterial cytoplasm to the periplasmic space, generating an electrochemical gradient of protons (the protonmotive force) that can be used to power reactions such as ATP synthesis. The RC complex is found in photosynthetic bacteria, such as purple bacteria and other proteobacteria species.
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