SCIENCE & TECHNOLOGY
LIGHT SHED ON PHOTOLYASES X-ray structure reveals how these enzymes use light to repair DNA damaged by UV radiation AMANDA YARNELL, C&EN WASHINGTON
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teractions explain how these enzymes recognize CPD lesions and hint at how they catalyze the repair of such lesions, he adds. Previous work has suggested that photolyases' so-called antenna pigment (deazaflavin or methenyltetrahydrofolate) absorbs a photon of the appropriate wavelength and passes the energy to the enzyme's catalytic flavin cofactor. It's thought that this energy drives the transfer of an electron from the activated flavin cofactor to the C P D lesion, splitting its cyclobutane ring. The German team's 1.8-Â structure shows that the enzyme flips the CPD lesion into an active-site cavity right next to the catalytic flavin cofactor. This proximity suggests that the activated flavin passes its electron directly to the lesion, Carell says, adding that his group has unpublished calculations that strongly support direct electron transfer. He and Essen are now working to further clarify the electron transfer pathway
now provided this long-awaited picture ness light energy to repair D N A [Science, 306,1789 (2004)}. damaged by ultraviolet radia"This structure confirms predictions tion. The first X-ray structure of that the damaged dinucleotide would be one of these enzymes bound to flipped out of the D N A stack and into the its damaged D N A substrate was recently enzyme's active site," comments bioreported by German crystallographers. In chemist Aziz Sancar of the University of addition to confirming many suspicions N o r t h Carolina School of Medicine, about how these enzymes recognize and Chapel Hill. repair light-induced D N A damage, this "We knew that repair proteins can flip a long-awaited structure paints a picture of Perhaps the most surprising aspect of the single damaged base out of the helix for rethe enzyme midway through its work. structure is the fact that the initially intact pair," Carell says. "But can a repair enzyme cyclobutane dimer is split during X-ray Photolyases are thought to have been flip two bases out of the helix?" No one had structure determination. Although it's well one of nature's earliest solutions to the danbeen able to prove that this could happen, known that synchrotron radiation can progers caused by life under the sun. The sun's he says. "Our D N A photolyase structure duce solvated electrons in protein and DNA UV rays play havoc with genomic DNA, tells us that this is indeed possible." crystals, previous crystal structures of CPD causing a variety of toxic DNAlesions. The most common of these is the cis-syn cy- THE STRUCTURE also "reveals the exten- lesions have captured the intact cyclobutane ring, suggesting that synchrotron-genclobutane pyrimidine dimer (CPD), sive network of interactions the damaged erated electrons aren't themselves capable formed by [2 + 2] cycloaddition of two ad- dinucleotide makes with the enzyme's acof repairing CPD lesions. But in the presjacent pyrimidine bases (usually a pair of tive site," Sancar says. W h e n combined ence of photolyase, "the enzyme binds the thymines.) Such CPD lesions bring polywith the wealth of biochemical data that's dimer lesion in away that makes it more vulmerases to a standstill, eventually leading been gathered for the photolyases, these innerable to cleavage" by a synto cell death. chrotron-generated electron, Photolyases—which are Carell says. He suggests that found in modern-day prokaryhis team's structure mimics an otes; plants; and a variety of intermediate during lightanimals including frogs, fish, driven DNArepair, where the and snakes—use blue or nearcleaved thymines have not yet UV sunlight to drive the cleavbeen flipped back into the age of C P D lesions' cycloDNAhelix. butane ring. (Humans use another set of enzymes to reSo why did this team sucpair C P D lesions.) Decades' ceed where others failed? worth of biochemical data, Carell speculates that it's becomputer modeling, and cause they used a simple, structural studies have been more easily synthesized anainvested in figuring out how log of the C P D lesion. The these enzymes carry out CPD analog retains the stereorepair. But despite many atchemistry of the real thing tempts, no one had been able but replaces the real lesion's to obtain a structural picture intradimer phosphate linkof a photolyase bound to its age with a more sturdy formdamaged D N A substrate. A acetal linkage. This small German team led by chemchange allowed production of FLIP OUT A new X-ray structure reveals that the istry professors Thomas the pure DNAlesion in large cyclobutane pyrimidine dimer (red) is flipped out of the DNA Carell of Ludwig Maximilians quantities, he says. "When it helix (green) and into the active site of photolyase (gray University, in Munich, and comes to repair research, ribbon), where the catalytic flavin cofactor (yellow) awaits. Lars-Oliver Essen of Philipps making the DNAlesion is alThe light-harvesting antenna pigment is shown in blue. University, in Marburg, has ways the limiting step." • NA PHOTOLYASE ENZYMES HAR-
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