Wednesday, January 31, 2007

Nobel Laureates: Deisenhofer, Huber, and Michel

 
The Nobel Prize in Chemistry 1988.

"for the determination of the three-dimensional structure of a photosynthetic reaction centre"

Johann Deisenhofer, Robert Huber, and Hartmut Michel received the Nobel Prize in 1988 for working out the structure of the first photosystem—the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis. We now know that this is a Photosystem II-type complex with a type II reaction center. Its chlorophyll molecules absorb a photon of light and catalyze the transfer of electrons from an electron donor (usually cytochrome c) to quinone.

The photosystem structure was one of the most complex structures ever solved by X-ray crystallography. Even today there are only a handful of solved structures that are as complicated as this one.

The complex is normally embedded in a lipid bilayer that surrounds the vertical α-helices shown in the figure. The large gray space-filling molecules in the middle are the chlorophyll molecules that absorb light. Excited electrons are released from the chlorophylls and transferred down toward the bottom of the molecule to reduce a bound quinone near the iron atom (brown dot).

The cytoplasm on the inside of the cell is at the bottom of this picture and the intermembrane space between the inner and outer bacterial membranes is at the top.

The reaction center chlorophylls need to be resupplied with electrons and these come from a type c-like cytochrome (purple) that's attached to the top of the photosystem. This particular cytochrome is unusual since it has multiple heme groups. In most other species the electron donor is cytochrome c.

As noted in the presentation speech, by solving the structure of a bacterial photosystem Deisenhofer, Huber, and Michel not only contributed to our understanding of photosynthesis but also to our understanding of all membrane proteins and of electron transfer reactions in general.
The structural determination awarded has led to a giant leap in our understanding of fundamental reactions in photosynthesis, the most important chemical reaction in the biosphere of our earth. But it has also consequences far outside the field of photosynthesis research. Not only photosynthesis and respiration are associated with membrane-bound proteins but also many other central biological functions, e. g. the transport of nutrients into cells, hormone action or nerve impulses. Proteins participating in these processes must span biological membranes, and the structure of the reaction center has delineated the structural principles for such proteins. Michel's methodological contribution has, in addition, the consequence that there is now hope that we can determine detailed structures also for many other membrane proteins. Not least important is the fact that the reaction center structure has given theoretical chemists an indispensable tool in their efforts to understand how biologic electron transfer over very large distances on a molecular scale can occur as rapidly as in one billionth (American English, trillionth) of a second. In a longer perspective it is possible that such research can lead to important energy technology in the form of artificial photosynthesis.

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