Recent Understanding on the Photosystem of Purple Photosynthetic Bacteria

Abstract

Bacterial photosynthesis provides a simplified model system ideally for studying the basic mechanism of light-energy harvest and conversion. The early events in this process are carried out by two distinct components, the light-harvesting (LH)Light-harvesting (LH) complexes and the reaction center (RC)Reaction center (RC) . The LH complexes in purple photosynthetic bacteria are classified into two major types, the core LH1 complex that surrounds the RC and the peripheral LH2 complex that exists around the LH1. In addition to light-harvesting, the LH1 also plays a role in quinone (Q)Quinone (Q) transport between the RC and quinone pool in the cell membrane. While several high-resolution structures are known for the RC and LH2, the structures of LH1 remained at low resolutions. Here, the crystal structure of a LH1-RC complex from thermophilic purple sulfur bacterium Thermochromatium tepidum is described. This complex is characterized by an enhanced thermostability and an absorption maximum at 915 nm for the LH1. These properties have been shown to be regulated by Ca2+ ions. The structure reveals a closed arrangement of LH1 complex around the RC, and the LH1 BChl a molecules form a partially overlapping ring with a shorter Mg–Mg spacing compared with that of B850 in LH2. Structural evidence is for the first time provided for the possible ubiquinone pathway in the closed LH1 complex. The Ca2+-binding sites are identified. Molecular mechanisms of quinone transport, Ca2+-regulation and interaction between LH1 and RC are discussed.