We report a hybrid quantum mechanics/molecular mechanics theoretical study on the reaction mechanism of mammalian histidine decarboxylase that allows us to obtain valuable insights on the structure of the cofactor-substrate adduct (external aldimine) in the active site of rat histidine decarboxylase. By means of molecular dynamics simulations, we traced the potential of mean force corresponding to the decarboxylation reaction of the adduct both in the active site of the enzyme and in aqueous solution. By comparing this process in both media, we have identified the key electrostatic interactions that explain the lowering of the free energy barrier in the enzyme. Our analysis also offers a validation of Dunathan's hypothesis (Dunathan, H. (1966) Proc. Natl. Acad. Sci. U. S. A. 55, 712-716) regarding the role of stereoelectronic effects in the enzymatic decarboxylation process.