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Photosystem I
Look around. Just about everywhere that you go, you will see something green. Plants cover the Earth, and their smaller cousins, algae and photosynthetic bacteria, can be found in nearly every corner. Everywhere, they are busy converting carbon dioxide into sugar, creating living organic molecules out of air using the energy of sunlight as power. This process, termed photosynthesis, provides the material foundation on which all life rests.
Capturing Light
At the center of photosynthesis is a class of proteins termed photosynthetic reaction centers. These proteins capture individual light photons and use them to provide power for building sugar. The example shown here is photosystem I (PDB entry 1jb0), one of the two large reaction centers used in cyanobacteria, algae and plants. Photosystem I is a trimeric complex that forms a large disk. In cells, the complex floats in a membrane (the membrane is indicated by the two red lines in the lower picture) with the large flat faces exposed above and below the membrane.Colorful Cofactors
Each of the three subunits of photosystem I is a complex of a dozen proteins, which together support and position over a hundred cofactors. Some of these cofactors, shown here in green and orange, are exposed around the edge of the complex and many others are buried inside. Cofactors are small organic molecules that are used to perform chemical tasks that are beyond the capabilities of pure protein molecules. The cofactors in photosystem I include many small, brightly-colored molecules such as chlorophyll, which is bright green, and carotenoids, which are orange. The colors are, in fact, the reason that these molecules are useful: the colors are an indication that the cofactors absorb other colors strongly. For instance, chlorophyll absorbs blue and red light, leaving the beautiful greens for us to see. The energy from these absorbed colors is then captured to perform photosynthesis.Last changed by: A.Honegger,