Modeling the Chemical Step Utilized by Human Alkyladenine DNA Glycosylase: A Concerted Mechanism Aids in Selectively Excising Damaged Purines

Human alkyladenine DNA glycosylase (AAG) initiates the repair of a wide variety of (neutral or cationic) alkylated and deaminated purines by flipping damaged nucleotides out of the DNA helix and catalyzing the hydrolytic N-glycosidic bond cleavage. Unfortunately, the limited number of studies on the catalytic pathway has left many unanswered questions about the hydrolysis mechanism. Therefore, detailed ONIOM(M06-2X/6-31G(d):AMBER) reaction potential energy surface scans are used to gain the first atomistic perspective of the repair pathway used by AAG. The lowest barrier for neutral 1,N(6)-ethenoadenine (epsilon A) and cationic N(3)-methyladenine (3MeA) excision corresponds to a concerted (A(N)D(N)) mechanism, where our calculated Delta G double dagger = 87.3 kJ mol(-1) for epsilon A cleavage is consistent with recent kinetic data. The use of a concerted mechanism supports previous speculations that AAG uses a nonspecific strategy to excise both neutral (epsilon A) and cationic (3MeA) lesions. We find that AAG uses nonspecific active site DNA-protein pi-pi interactions to catalyze the removal of inherently more difficult to excise neutral lesions, and strongly bind to cationic lesions, which comes at the expense of raising the excision barrier for cationic substrates. Although proton transfer from the recently proposed general acid (protein-bound water) to neutral substrates does not occur, hydrogen-bond donation lowers the catalytic barrier, which clarifies the role of a general acid in the excision of neutral lesions. Finally, our work shows that the natural base adenine (A) is further inserted into the AAG active site than the damaged substrates, which results in the loss of a hydrogen bond with Y127 and misaligns the general base (E125) and water nucleophile to lead to poor nucleophile activation. Therefore, our work proposes how AAG discriminates against the natural purines in the chemical step and may also explain why some damaged pyrimidines are bound but are not excised by this enzyme.