Figure 3.

Catalytic residues within and around the active center of PPOs. The crystal structure of TYR from Juglans regia (JrPPO1, PDB ID: 5CE9) is used as a representative example.[ ⁵¹ , ⁵² ] The six conserved histidines HisA₁, HisA₂, and HisA₃ of CuA and HisB₁, HisB₂, and HisB₃ of CuB are depicted as sticks (carbon: white and nitrogen: blue). The seventh His, HisB₃‐1 (light green), waterkeeper residue (orange), gatekeeper residue (yellow), and the two activity controllers, HisB₁+1 (gray) and HisB₂+1 (purple) are highlighted. The thioether bridge (pink) connects the second conserved histidine of CuA (HisA₂) to an adjacent cysteine residue, whereas the two disulfide bonds are presented in red.