The transfer of a proton across a hydrogen bond can be influenced by a number of factors, including H-bond length, intramolecular angles, and the presence of neighboring groups. The ability of different factors to push a proton across the H-bond is examined for a specific and very important pair of catalytic groups, Ser-195 and His-57, within the context of a serine proteinase enzyme. The influence of residue Asp-102 is considered for different charge states, as is the nature of the surrounding medium. Also examined are the perturbations introduced by the substrate and external ions and dipoles.