Overall structure. Colour code: L-TYR main core, blue; C-terminal domain, turquoise; A-TYR, purple; sodium ions, yellow; copper ions, bronze; POM (TEW) oxygen, red; tellurium, grey; positive electrostatic potential of coulombic surface, blue; negative electrostatic potential of coulombic surface, red. (a) Overall structure of asymmetric unit; the crystallographic heterodimer of abPPO4. L-TYR is shown on the left and A-TYR is depicted on the right. (b) Illustration of the activation process. By the proteolytic removal of the C-terminal domain the active site becomes solvent-exposed and substrates (e.g. L-tyrosine) are able to approach. (c) Different viewing angle of the single monomer of L-TYR to illustrate the β-barrel-shaped C-terminal domain, the proteolytic cleavage site and the putative CXXC motif-containing copper grappler. (d) Coulombic surface illustration of the L-TYR main core with the C-terminal domain (cartoon) attached. The solvent-accessible groove, exhibiting a strong negative electrostatic potential, is indicated by the dashed arrow. Phe454 is shown to protrude into the active site (transparent surface around the active site).