Separation-of-Function Mutants Unravel the Dual-Reaction Mode of Human 8-Oxoguanine DNA Glycosylase

7,8-Dihydro-8-oxoguanine (8oxoG) is a major mutagenic base lesion formed when reactive oxygen species react with guanine in DNA. The human 8oxoG DNA glycosylase (hOgg1) recognizes and initiates repair of 8oxoG. hOgg1 is acknowledged as a bifunctional DNA glycosylase catalyzing removal of the damaged base followed by cleavage of the backbone of the intermediate abasic DNA (AP lyase/6-elimination). When acting on 8oxoG-containing DNA, these two steps in the hOgg1 catalysis are considered coupled, with Lys249 implicated as a key residue. However, several lines of evidence point to a concurrent and independent monofunctional hydrolysis of the N-glycosylic bond being the in vivo relevant reaction mode of hOgg1. Here, we present biochemical and structural evidence for the monofunctional mode of hOgg1 by design of separation-of-function mutants. Asp268 is identified as the catalytic residue, while Lys249 appears critical for the specific recognition and final alignment of 8oxoG during the hydrolysis reaction.