Research Article

Structure of a symmetric photosynthetic reaction center–photosystem

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Science  27 Jul 2017:
DOI: 10.1126/science.aan5611

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Reaction centers are pigment-protein complexes that drive photosynthesis by converting light into chemical energy. It is believed that they arose once from a homodimeric protein. The symmetry of a homodimer is broken in heterodimeric reaction center structures, such as those reported previously. The 2.2-Å resolution X-ray structure of the homodimeric reaction center/photosystem from the phototroph, Heliobacterium modesticaldum, exhibits perfect C2 symmetry. The core polypeptide dimer and two small subunits coordinate 54 (bacterio)chlorophylls and 2 carotenoids that capture and transfer energy to the electron transfer chain at the center, which performs charge separation and consists of 6 (bacterio)chlorophylls and an iron-sulfur cluster; unlike other reaction centers, it lacks a bound quinone. This structure preserves characteristics of the ancestral reaction center, providing insight into evolution of photosynthesis.

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