5.B.4 The Plant Photosystem I Supercomplex (PSI) Family
Oxygenic photosynthesis is the principal producer of both oxygen and organic matter on Earth. Water, the electron donor for this process, is oxidized to O2 and four protons by PSII. The electrons that have been extracted from water are shuttled through a quinone pool and the cytochrome b6f complex to plastocyanin&151;a small, soluble, copper-containing protein. Solar energy that has been absorbed by PSI induces the translocation of an electron from plastocyanin at the inner face of the membrane (thylakoid lumen) to ferredoxin on the opposite side (stroma). PSI generates the most negative redox potential in nature (-1 V), and thus largely determines the global amount of enthalpy in living systems. The structures of three of the four complexes that catalyse oxygenic photosynthesis in cyanobacteria have been solved at relatively high resolution, and the position of most of their amino acids and prosthetic groups has been defined. Thus, the architecture of oxygenic photosynthesis in cyanobacteria has largely been determined. The structure of the cytochrome b6f complex from chloroplasts of the algae Chlamydomonas reinhardtii has also been solved at high resolution, and has remarkable similarity to the cyanobacterial complex. Two high-resolution structures of light-harvesting complexes of PSII from higher plants have also been published.
All higher organisms on Earth receive energy directly or indirectly from oxygenic photosynthesis performed by plants, green algae and cyanobacteria. Photosystem I (PSI) is a supercomplex of reaction centre and light-harvesting complexes. It generates the most negative redox potential in nature. The structure of plant PSI has been solved at 3.4 Å resolution, revealing 17 protein subunits. The crystal structure of PSI provides a picture at near atomic detail of 11 out of 12 protein subunits of the reaction centre. At this level, 168 chlorophylls (65 assigned with orientations for Qx and Qy transition dipole moments), 2 phylloquinones, 3 Fe4S4 clusters and 5 carotenoids are described. This structural information extends the understanding of the most efficient nano-photochemical machine in nature. (Amunts et al., 2007).