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STRUCTURE ELUCIDATION
Group photos of the precisely ordered cluster of proteins and pigments in photosynthetic systems are hard to come by. Two new reports of two crystal structures--plant photosystem I (PSI) and purple bacteria's reaction center/light-harvesting 1 complex--reveal important details about the organization, energy transfer, and evolution of photosynthetic systems. Early photosynthesis evolved from an anaerobic, one-reaction-center process in purple bacteria to a water-splitting, oxygen-releasing, two-photosystem mechanism in cyanobacteria. Because the first plant chloroplast was actually an ancient cyanobacterium that made its home inside a eukaryotic cell, cyanobacterial and plant photosynthesis are much more similar to each other than they are to anaerobic photosynthesis used by purple bacteria.
The Glasgow group's most unusual finding is a break in the ellipse. One protein that they've named W is heavier than the rest and sits at the gap like a doorman at a portal. "There's always been this problem that the light-harvesting complex, if completed, would form this wall around the reaction center," Isaacs says, which would block the electron acceptor ubiquinone from transporting electrons outside the reaction center to cytochrome b/c1. "There had to be some way for the quinone to pass the electrons out," Isaacs says. The placement of this gap protein strongly suggests that it plays a role in ubiquinone's handoff. The Glasgow chemists are now trying to identify the gene that codes for W. The crystal structure of plant photosystem I, obtained from Pisum sativum var. alaska, is best compared to cyanobacteria's PSI, which was crystallized in 2001 by a group led by Petra Fromme and Norbert Krauss in Germany [Nature, 411, 909 (2001); C&EN, June 25, 2001, page 9]. The plant system has some unique aspects, including an especially dense outer band of light-harvesting chlorophylls. But according to lead author Nathan Nelson, professor of biochemistry at Tel Aviv University, given that "the two photosystems diverged about 1.2 billion years ago, and during this time a lot of mutations occurred in all the subunits comprising these complexes," it is remarkable how well the overall structure is conserved--in particular, the placement and the angle of 167 chlorophylls [Nature, 426, 630 (2003)]. In addition, the plant PSI is a monomer, as opposed to the trimer seen in marine cyanobacteria. Nelson and coworkers also reveal details that show how this more flexible state of plant PSI allows it to temporarily receive electrons from light-harvesting chlorophylls in plant photosystem II at high light intensities, reducing damage to the more sensitive PSII. Plants, Nelson says, likely evolved the spillover option because of the variable and more intense light they encountered on land. Both crystal structures are the first for their respective systems--but hopefully not the last. The resolution--4.4 Å for plant and 4.8 Å for purple bacteria--is only high enough to make out large structural features and not side chains or other atomic details. |
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