Crystal structure of Y. pestis OppA protein. The OppA protein (green) is bound to tri-lysine peptide (red). PDB code: 2Z23. a The OppA protein is formed by two lobes with secondary structural βαβ units and a flexible hinge between the two lobes where the substrate-binding cleft is located. The ligand is completely enclosed in the hydrophobic cleft. b Key binding-site residues are labeled (residues Glu32, Val34, Tyr109, Asn246, Asn247, Arg404, His405, Arg413, Ala415, Trp416, Cys417, Asp419, and Thr438).