A new charge-relay motif for serine proteases. Part of the structure of the active site of PRCP as determined by Soisson et al.  is shown here. The classic catalytic triad comprises Ser179 (S179), His455 (H455) and Asp430 (D430), which are linked by hydrogen bonds (H bonds, dotted lines). In the PRCP active site, the catalytic H455 is also linked via S179 to H456, which is in turn H-bonded to Arg460 (R460). Thus, these five residues form a potential charge-relay system. A similar structure is present in DPP7 (PDB 3JYH).