The protein C pathway plays an important role in the control and regulation of the blood coagulation cascade and prevents the propagation of the clotting process on the endothelium surface. In physiological systems, protein C activation is initiated by the thrombin-thrombomodulin complex. The protein C activator from Agkistrodon contortrix contortrix (Protac), a glycosylated single-chain serine proteinase, activates protein C without relying on thrombomodulin. Suitable crystals of Protac were obtained from a solution containing 2 M ammonium sulfate as the precipitant and soaking with inhibitors was carried out. The crystal structures of native and inhibited Agkistrodon contortrix contortrix protein C activator determined at 1.65 and 1.54 Å resolutions, respectively, indicate the pivotal roles played by the positively charged belt and the strategic positioning of the three carbohydrate moieties surrounding the catalytic site in protein C recognition, binding, and activation. Structural changes in the benzamidine-inhibited enzyme suggest a probable function in allosteric regulation for the anion-binding site located in the C-terminal extension, which is fully conserved in snake venom serine proteinases.