The enzyme cleaves Lys-Arg and Arg-Ser bonds. It activates, in a reciprocal reaction, factor XII after its binding to a negatively charged surface. It also releases bradykinin from HMW kininogen and may also play a role in the renin-angiotensin system by converting prorenin into renin. Cleaves selectively Arg-|-Xaa and Lys-|-Xaa bonds, including Lys-|-Arg and Arg-|-Ser bonds in (human) kininogen to release bradykinin. Inhibited by SERPINA5. Forms a heterodimer with SERPINA5. The zymogen is activated by factor XIIa, which cleaves the molecule into a light chain, which contains the active site, and a heavy chain, which associates with HMW kininogen. These chains are linked by one or more disulfide bonds (By similarity). Belongs to the peptidase S1 family. Plasma kallikrein subfamily.