Shedding light on photosynthesis.

Shedding light on photosynthesis

Scientists have succeeded in using genetic engineering to study the first steps of photosynthesis, an achievement that should speed understanding of how plants and bacteria change light energy to chemical energy.

The researchers used recombinant DNA technology to change a specific amino acid in a protein that forms part of the "reaction center" where photosynthesis begins in bacteria. The method allows scientists to tinker with a few key parts of the reaction center and then use ultrafast lasers to analyze how these changes affect photosynthesis, they report in the October PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol.85, No.20).

The reaction center for photosynthesis is an elaborate protein cage holding other molecules, including chlorophyll, in such a way that lightenergized electrons pass along these molecules, known as prosthetic groups. Usually, incoming light energizes a pair of chlorophyll molecules so that they pass on an electron to a close relative of chlorophyll called a pheophytin, which passes the electron to a molecule called a quinone. The movement of the negatively charged electron causes a separation of positive and negative charges in the reaction center, creating an electrochemical potential that can power reactions in other cell parts.

In the engineered protein, the researchers altered the primary link between the protein and one of the chlorophylls so the chlorophyll lost a magnesium atom and became a pheophytin. This changed the chemistry of the reaction center, but it still functioned at about half its original efficiency, report Douglas C. Youvan and Edward J. Bylina of the Massachusetts Institute of Technology and Christine Kirmaier and Dewey Holten of Washington University in St. Louis.

The genetic engineering method "supplies scientists with the material to study the effect of specific perturbations [in the reaction center] instead of the random changes provided by classical genetics," Youvan says. The work got a big boost by the discovery of the structure of the bacterial reaction center, for which three German scientists won this year's Nobel in chemistry (SN: 10/29/88, p.282). "When the crystal structure came out it was marvelous," Youvan says. "It's now possible to look at the structure on a computer screen and choose amino acids that seem critical for either prosthetic group binding or helping the electron transfer reaction."

The amino acid they changed seems to take part in both actions, because it anchors the prosthetic group and contributes to the electron transfer process itself, Youvan says.

Because the engineered reaction center differs so much from that found in nature, "it's a surprise it works at all," Kirmaier says. The observation that the changed reaction center does work "suggests someone should look for similar compounds for artificial photosynthesis," Youvan says. At some point, he adds, scientists and engineers may be able to use knowledge about bacterial reaction centers to construct compounds that efficiently synthesize chemical energy from sunlight without the help of biological molecules.