This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The mechanism by which DNA glycosylases differentiate between specific damaged bases and the overwhelming excess of normal bases remains a mystery. Moreover, it is also unclear how these enzymes, despite spending so much time interrogating normal DNA, constrain themselves to cleaving only damaged bases. In particular, the feat of recognizing 8 oxoguanine lesions (oxoG) is impressive, because guanine and oxoG differ by merely two atoms, and the presence of oxoG in B-form DNA elicits no significant structural perturbation. Per our efforts to better understand these issues, we have used disulfide cross-linking technology to covalently trap a human 8-oxoguanine glycosylase (hOGG1) to an undamaged G:C base pair in a normal duplex of DNA. The x-ray structure of this complex reveals that the target guanine base has been swiveled out from the base stack and is fully inserted into the extrahelical active site pocket. Interestingly, though the enzyme is catalytically active, the target guanine remains uncleaved, an observation supported by biochemical analysis. The fact that even complete binding of guanine in the active site does not result in base cleavage demonstrates the existence of discrimination between G and oxoG at the level of catalysis.