Structure of serpin polymers and other inactive conformers. (a) Schematic diagram of domain swapping in serpins; the RCL of one molecule (magenta loop), is docked into β-sheet A (black lines) of the next (only four strands of β-sheet A are shown). (b) Structure of a cleaved serpin polymer (PDB code 1D5S) , showing the promiscuous nature of the RCL. Cleavage at the P5/P6 position has resulted in RCL (magenta) insertion into β-sheet A; the 'gap' at the bottom of β-sheet A is filled with the P5-P1 portion (pale pink) from an RCL from another molecule. (c) The structure of an alternative confirmation of SERPINA3 -δ-SERPINA3 (PDB code 1QMN) . Four residues of the RCL (magenta) are inserted into the top of β-sheet A. The F-helix (green) has partially unwound and filled the bottom half of β-sheet A. (d) Serpins can accept a peptide with the sequence of the RCL (pale pink) into β-sheet A (PDB code 1BR8) .