Scientists working with Werner Kühlbrandt at the Max Planck Institute of Biophysics in Frankfurt/Main have gained new insights into the molecular architecture of the key enzyme involved in photosynthesis in plants - namely photosystem II. In addition to this photosystem, there are two further known types of photosynthetic reaction centres, the bacterial reaction centre bRC and photosystem I. The photosynthetic reaction centre in purple bacteria was the first membrane protein which was successfully crystallized more than 10 years ago and which atomic structure could be clarified. The scientists involved received the 1988 Nobel Prize for their work. Numerous investigations have, in the past, given reason to believe that there is a close relationship between the bacterial photosynthetic reaction centre and that of higher plants. With their results published in the newest issue of the journal Nature (Vol. 396, No. 6708, 19 November 1998), the researchers give undoubted proof that there actually is a close evolutionary connection between these three reaction centres.
Photosystem II in plants is a so-called multi-enzyme complex comprising more than 25 different proteins which work together in a manner resembling a small factory. Among them are proteins D1, D2, CP43 and CP47, as well as a few smaller proteins including the a and b subunits of a cytochrome. The chlorophyll a molecules, which give plant leaves their green colour, belong to the D1/D2 complex. They act like light antennas and allow them to collect sunlight. Within the process of photosynthesis solar energy is converted into biochemical energy stored in the form of organic substance. The entire oxygen of our atmosphere results from this process as a waste product. Thus, photosystem II is not only an energy converter, it also produces the air we need to breath.
On the basis of electron-crystallographic investigations, the scientists were able to identify 16 of the 23 protein chains in photosystem II which helically span the cell membrane of green plant cells. The surprisingly high degree of structural similarity between the bacterial reaction centre and photosystem II implies that the 5-helix scaffold of the two reaction centres has been largely unchanged since the photosynthesis first evolved more than 3.5 billion years ago.
Journal
Nature