Characterization of the active site of DNA polymerase beta by molecular dynamics and quantum chemical calculation

It is well established that the fully formed polymerase active site of the DNA repair enzyme, polymerase beta (pol beta), including two bound Mg2+ cations and the nucleoside triphosphate (dNTP) substrate, exists at only one point in the catalytic cycle just prior to the chemical nucleotidyl transfer step. The structure of the active conformation has been the subject of much interest as it relates to the mechanism of the chemical step and also to the question of fidelity assurance. Although crystal structures of ternary pol beta-(primer-template) DNA-dNTP complexes have provided the main structural features of the active site, they are necessarily incomplete due to intentional alterations (e.g., removal of the 3'OH groups from primer and substrate) needed to obtain a structure from midcycle. Working from the crystal structure closest to the fully formed active site [Protein Data Bank (PDB) code: 1bpy], two molecular dynamics (MD) simulations of the solvated ternary complex were performed: one with the missing 3'OHs restored, via modeling, to the primer and substrate, and the other without restoration of the 3'OHs. The results of the simulations, together with ab initio optimizations on simplified active-site models, indicate that the missing primer 3'OH in the crystal structure is responsible for a significant perturbation in the coordination sphere of the catalytic cation and allow us to suggest several corrections and additions to the active-site structure as observed by crystallography. In addition, the calculations help to resolve questions raised regarding the protonation states of coordinating ligands.