(A) Alignment of the EnSerp1 sequence with four of the most similar serpin sequences of other platyhelminths: 1, EnSerp1 from Eudiplozoon nipponicum (GenBank: MF288891.1); 2, Echinococcus multilocularis (GenBank: CDS35969.1); 3, Schistosoma haematobium (GenBank: XP_012797533.1); 4, Echinococcus granulosus (GenBank: CDS22753.1); 5, Taenia solium (GenBank: ATG83400.1). Conserved motifs characteristic for serpins are highlighted. The serpin motif (E³⁴² – E³⁴⁶) shown in red is part of the reactive centre loop (RCL, A³⁴⁷ – N³⁶⁵), shown in green. Immediately after RCL, follows serpin signature (F³⁶⁶ – I³⁷⁶) in yellow. Scissile bond is situated within the RCL between P1 (F³⁵⁸) and P1’ (C³⁵⁹) residue, shown in magenta. (B) Predicted 3D structure of EnSerp1. Coloured areas of the molecule correspond to the sequence highlighted in Figure 1A. (C) RCL and β-sheet A. After the peptidase cleaves the scissile bond within RCL (in orange), the residual part of RCL is incorporated as a new strand into β-sheet A (in cyan).