Figure 7.

Reaction mechanism of the excision of misincorporated deoxyuridinefrom DNA by the uracil-DNA glycosylase UDG. a) Schematic diagram. Electron transfers are indicated in red, hydrogen bonds in green and enzyme residues in blue. The dashed line to C157 indicates a Cα Hα···O4 hydrogen bond. b) Adiabatic potential energy surface as a function of r_{C1′ N1} and r_{C1′ OH2}. In the region r_{C1′ N1} ≤ 2.20 Å and r_{C1′ OH2} ≤ 2.00 Å, the points above 32 kcal/mol are not shown for clarity. Red arrows follow the lowest energy pathway (stepwise dissociative); green arrows follow a perfect associative pathway; and yellow arrows follow a concerted pathway starting from the reactant structure. The states indicated are reactant (R), product (P), transition states (TS₁ and TS₂) and the oxocorbenium cation/anion intermediate (I₁). (From Dinner et al.⁶³⁷).