Enzymatic catalysis and photoinduced excited-state dynamics of DNA nucleobases have attracted considerable interest over the past decades. Using ab initio QM/MM MD simulations, we have investigated the structural features of zinc enzymes, nucleoside hydrolases, and deaminases and plausible enzymatic mechanisms. Theoretically, the electronic structure calculations on the decay channels and conical intersections in DNA nucleobases and their analogues have identified the main internal conversion pathways, and further dynamics simulations may provide information about the relative efficiency of related decay channels. Here we performed a series of QM and QM/MM simulations on the excited-state dynamics of three organic conjugated molecules including two base analogues in the gas phase and in solution. At the ab initio level of theory, their excited-state lifetimes and possible decay pathways were explored in detail. Based on the hybrid QM/MM MD simulations with surface hopping, the influence of solvent on their nonadiabatic dynamics properties was also discussed. This talk will cover our recent studies of the catalytic ring-opening of GlcN6P by SmuNagB and the ultrafast nonadiabatic decay of the nucleobase-related systems in the gas phase and in water.