Figure 2
The asymmetric monomers. (a) Superimposition of monomer A (light and dark blue) and B (yellow and green) highlights their overall conformational differences especially the loop connecting α4 and β6 (Asp109 to the conserved Arg122). Residues Ala117 to Thr120 of monomer A fold into an α-helical turn, while they form an irregular loop in monomer B, thus affecting the geometry of the active site. (b) View of the residues lining the active site environment in monomer A. The α-helical turn is oriented such that the carbonyl oxygen atoms and the helical axis point towards CP. Part of strand β9 and Pro181 from monomer B are marked in yellow. (c) Hydrogen bonding network of monomer A and monomer C. The Cys45-O2-OH forms a H-bond (dotted line) through its Oδ2 atom with the oxygen (Oγ1) of Thr42, and through its Oδ1 atom an H-bond with the Nδ1 atom of His37. While Arg122 mainly engages in a salt-bridge with Glu48, it also forms a H-bond through its Nη1 to the Oδ1 of Cys-O2-OH. Downstream in the sequence but close in space lies Arg145 whose main chain nitrogen atom H-bonds to the Oε1 atom of Glu48 and its Nη2 atom forms a bifurcated H-bond with the carbonyl oxygen atoms of Ser36 and Arg122. In addition, the Nε and Nη1 of Arg145 bind one water molecule each. A third water molecule is held in place by the main-chain nitrogen atom of Val44 and does not directly H-bond with Cys45-O2-OH. A corresponding water molecule is found in many Prx structures. (d) The active site of monomer D. Here, the side chain of His37 flips away from CP and forms an H-bond with a water molecule. Arg122 forms a salt bridge with the oxygen atoms Oδ1 and Oδ3 of Cys45-O2-OH, and maintains a H-bond to Glu48. The side chain of Arg145 rotates into a position where its Nε and Nη2 atoms can form a new salt bridge with the Oε1 and Oε2 atoms of Glu48. The rearrangement leads to a shift of the CP-loop, which disrupts the H-bond between CP and Thr42. (e) The electron density (blue mesh) covering the active site residues of monomer B clearly suggests alternate conformations for Phe35, Ser36 and His37, which reflects the dynamics of these residues.
