C&EN: NEWS OF THE WEEK - PHOTOSYSTEM I STRUCTURE

REBECCA RAWLS

After more than 10 years of effort, a team of German chemists and crystallographers has worked out the three-dimensional structure of photosystem I, the larger of the two huge protein-cofactor complexes where the initial steps of photosynthesis take place in plants, green algae, and cyanobacteria. It is, in the words of Graham R. Fleming, a chemistry professor at the University of California, Berkeley, and director of the Physical Biosciences Division of Lawrence Berkeley National Laboratory, "an absolutely spectacular piece of work."


	SUN CATCHER Each lobe of clover-leaf-shaped crystal of photosystem I contains 12 different proteins, shown here in different colors with only the protein backbone of each depicted. Amid the proteins are 96 chlorophylls (pale yellow) as well as other cofactors (gray).

	"When we see this structure, we see a lot of things that nobody expected." Petra Fromme, Technical University of Berlin

The photosystem, which the researchers crystallized in its trimeric form, contains 12 different proteins in each monomer, along with 96 chlorophylls and more than 30 other cofactors. For the first time, the structure allows them to exactly locate each of the chlorophylls within the protein complex and to begin to figure out how these molecules work together as a system to gather solar energy and then transfer that energy to the center of the complex, where electron-transfer reactions convert it to the chemical energy that drives almost all life on Earth.

The work is a long-term collaboration between a team of biophysical chemists at the Technical University of Berlin led by Petra Fromme and Horst T. Witt and a team of crystallographers at the Free University of Berlin led by Norbert Krauss and Wolfram Saenger [Nature, 411, 909 (2001)].

The structure has revealed some remarkable surprises. For example, the magnesium ion at the center of the chlorophyll molecule that serves as the primary acceptor of electrons in the photosystem has as a ligand a sulfur atom from a nearby methionine residue. Fromme believes this is the first example in all of inorganic chemistry of sulfur ligating to magnesium.

"That's not just weird," Fleming explains, "it's likely to be important. This particular chlorophyll is the strongest reducing agent in nature, and nobody has had any clue why. I suspect that it has something to do with that sulfur ligand."

In addition to providing a close-up view of an important biochemical complex, the work is also a crystallographic tour de force. High-resolution crystal structures have been determined for fewer than 20 membrane-bound protein complexes, compared with some 4,000 soluble proteins. In addition, the most complex of the earlier structures contained fewer than a dozen cofactors, not the 127 found in each unit of this structure.

No crystallization agents were added to this protein complex to help it crystallize, Fromme says. Instead the chemists used small crystals to work out the entire phase diagram for the complex, mapping its response to changes in salt concentration, pH, and other physical chemical parameters. They then used a sophisticated seeding technique in which small crystals were used to grow medium-sized ones, and then, by finely adjusting the salt concentration, the medium-sized crystals were converted into one large crystal. This crystallization, Fromme says, "is more science than art."