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SOMETHING NEW UNDER THE SUN By Sean Henahan, Access Excellence LA JOLLA, Ca (MAY 8, 1997 ) New atomic details of the biochemical machinery of photosynthesis are helping explain what happens when photons of light first strike living photoreceptors. Using innovative techqniques, a team of biophysicists at the University of California, San Diego and the California Institute of Technology were able to capture the moment in time when light strikes a membrane protein called the reaction center, the site where the primary events of photosynthesis takes place. Photons are captured in the reaction center by molecules of chlorophyll. When these molecules are excited by light, they release electrons which are shuttled from one electron acceptor molecule to another, the last two being quinone molecules. These processes ultimately set in motion a charge separation across the membrane which the organism uses to synthesize adenosine triphosphate (ATP). "This work helps answer some basic questions about photosynthesis," said George Feher, research professor of physics at UCSD. The problems the planet faces have to do with famine and energy, and both of these are related to photosynthesis," said Feher. "So it's a very basic process. People who are worried about relevance, shouldn't be in this case." The new research created the first dynamic visualization of this process at the atomic level. The researchers exposed a reaction center crystal to a light source, and then immediately plunging the crystal into liquid nitrogen at temperatures of 90 degrees above absolute zero. In this manner, the activity was literally frozen in time. Afterward, the researchers compared the neutral structure with the light-adapted structure. In some cases, the changes were quite dramatic. "We found significant structural changes in the light-adapted structure," Feher said. Among other things, the secondary quinone in the light-adapted structure was visibly shifted from its position in the neutral structure. Also, a region called the "headgroup" appeared to be twisted about 180 degrees from its former position, as if turned a half revolution by a propeller. These structural changes help explain the kinetic behavior of the electron transfer reactions inside the reaction center after it is exposed to light. This has never been determined so precisely before. This enabled the researchers to locate water molecules within several channels leading to the secondary acceptor quinone. Water is a natural conveyor of protons, which when combined with the shuttled electrons at the secondary acceptor quinone, create dihydroquinone. This latter chemical leaves the reaction center, carrying the protons that trigger a proton gradient across the membrane that results in ATP production. While these results were obtained in studies of bacterial photosynthesis, they ar likely to be applicable to the similar process in green plants, he notes: "Nature usually doesn't invent things twice completely differently." Dr. Feher's research has focused on certain photosynthetic bacteria called Rhodobacter spheroides. Bacteriax are simpler to study than green plants since bacterial photosynthesis is a one-step process that does not result in oxygen evolution. The research appeared in the May 2, 1997 issue of the journal Science.
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