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To probe the danger of individual DNA lesions to cells, chemists have had to figure out a way to synthesize DNA containing specific, often chemically unstable modifications. "In the whole field of DNA damage, the chemical synthesis of lesions is a major stumbling block," notes Gregory L. Verdine, a professor of chemical biology at Harvard University. The availability of synthetic stretches of DNA containing single 8-oxoguanine lesions "changed everything," Verdine tells C&EN. It allowed chemists to probe how the lesion affects the structure of the surrounding DNA. It made it possible to measure just how mutagenic this particular lesion is. And it permitted Verdine and others to study how repair proteins interact with the lesion. Verdine's lab is using X-ray crystallography to determine how Ogg1 hunts for and repairs 8-oxoguanine lesions in a vast sea of undamaged DNA. Their structure of Ogg1 bound to an oligonucleotide containing 8-oxoguanine shows that the damaged base is thrust out of the DNA helix and into Ogg1's active site [Nature, 403, 859 (2000)]. "Amazingly, Ogg1 doesn't interact with 8-oxoguanine's most distinctive feature, its extra oxygen atom," Verdine says. Instead, he says, the enzyme differentiates between guanine and 8-oxoguanine by way of a single hydrogen bond to the damaged base's N7 proton. But because the critical N7 proton is tucked out of reach in the DNA helix, Verdine and others have remained puzzled about how Ogg1 hunts for 8-oxoguanine lesions. Some have suggested that the enzyme may flip each and every base out of the helix in its search for 8-oxoguanine lesions. Others have proposed that Ogg1 might probe the helix in other, less drastic ways. Ogg1's search mechanism remains mysterious, but how it removes 8-oxoguanine is better understood. Like other DNA glycosylases, Ogg1 catalyzes removal of the damaged base via a transient Schiff base intermediate. Recently, Verdine has shown that Ogg1 uses this strategy to clip 8-oxoguanine from the DNA backbone. The enzyme uses the 8-oxoguanine it produces as a cofactor in a subsequent catalytic step that severs the backbone [Nat. Struct. Biol., 10, 204 (2003); C&EN, Feb. 24, page 8]. |
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