Fig. 3: Falcilysin-peptide substrates interactions.

A Amino-acid sequence alignment of the hemoglobin α- and β-peptides. Residues that could be built in clear electron density are colored in orange and purple for the α- and β-peptide respectively. Vertical arrows indicate proteolytic cleavage site by FLN. Substrate positions having either conserved or chemically similar amino acids are shown in bold. Residues at P2, P1, and P1’ positions form a β strand. B, C Detailed view of the interactions between FLN and the hemoglobin α-peptide (B) and β-peptide (C). Hydrogen bonds are displayed as yellow dashes; cation–π and π–π interactions as black dashes; salt bridge: orange dash; zinc-coordination: purple dashes. Residues K40, T41, and Y42 of α-peptide (panel B) and residues Q39, R40, and F41 of β-peptide (panel C) form a β-strand antiparallel to strand β5 of FLN. D, E Magnified view of the active site with tetrahedral coordination of the zinc ion. The fourth coordinating atom is a water molecule (red sphere) in the α-peptide complex (panel D) and a carbonyl oxygen in the β-peptide complex (panel E). Distances in Å between the carbonyl carbon of the scissile bond and Q132 are labeled in gray. F Superimposition of complexes between the FLN and α-peptide and the FLN- β-peptide complex. The overlay was based on the C-half of FLN. N-half domains are shown as molecular surfaces colored in cyan (α-peptide complex) and gray (β-peptide complex). The linker and C-halves are shown as ribbons. FLN adopts a slightly more closed conformation when bound with the β-peptide (“C2”) than in the α-peptide complex (“C1”), see text. G Magnified view of the FLN active site. The slightly more closed conformation in the β-peptide complex is accompanied by a shift in the position of the zinc ion toward the β-peptide.