The potential energy surface (PES) for guanine formation from 4-aminoimidazole-5-carbonitrile (AICN) after adding H2O and cyanogen (NCCN) was determined from quantum chemical calculations using the CBS-QB3 method. Its activation energy was too high for the reaction to occur in the interstellar medium. The PES for the formation of a protonated guanine from AICN by adding H3O+ and NCCN was also determined. Although its overall activation energy was zero, its occurrence in the interstellar gas phase is not likely due to unstable intermediates. When H2O was added to some reaction steps as a catalyst, their activation barriers were significantly reduced.