Investigation of the Flipping Dynamics of 1, N6-Ethenoadenine in Alkyladenine DNA Glycosylase

Alkyladenine DNA glycosylase (AAG) is an essential enzyme responsible for maintaining genome integrity by repairing several DNA lesions damaged by alkylation or deamination. Understanding how it can recognize and excise the lesions thus lays the foundation for therapeutic treatment against lesion-associated diseases or cancers. However, the molecular details of how the lesion can be distinguished from the matched base by AAG and how it enters the cleavage site, ready for excision, are not fully elucidated. In this study, we have revealed the molecular details of the flipping dynamics of 1, N6-ethenoadenine (epsilon A) not only in the form of free double-stranded DNA (dsDNA) but also in the form of the AAG-dsDNA complex. Our MD simulations and PMF calculations have shown that the flipping of epsilon A and dA is thermodynamically disfavored in the free dsDNA, even though epsilon A has a lower flipping energy barrier than dA. By sharp contrast, the flipping of epsilon A is thermodynamically favored in AAG with an obvious free energy drop, while dA is equally stabilized before and after the flipping. Moreover, a comparison of the PMFs in the forms of free dsDNA and the AAG-dsDNA complex has pinpointed the role of AAG in discriminating epsilon A against dA and facilitating the flipping of epsilon A. Besides, the flipping process is simulated along the major and minor grooves, and our results have additionally demonstrated that the flipping is not directional in the free dsDNA while flipping along the major groove is kinetically more favorable than the minor groove in the AAG-dsDNA complex. Overall, our study has offered molecular insights into the flipping dynamics of epsilon A and revealed its discrimination mechanism by AAG, which is expected to guide further enzyme engineering for therapeutic applications.