Figure 3

Binuclear active site of tyrosinase proteins and placeholder residue blocking the entrance of the substrate. Stereo view of tyrosinase active site region with both Cu-binding sites is presented. Copper ions are depicted in green, Cu(A) is shown on the left and Cu(B) is shown on the right. The yellow sphere represents a dioxygen molecule. In each structure, the occupation and positioning of copper varies. The six copper-coordinating histidine ligands coordinating the structural conformation of the active site are shown in dark blue. In addition, the placeholder residue that reaches into the active site above Cu(A) and blocks the substrate-binding pocket and the entrance of the substrate into the active site is indicated in red. Differences in the orientation and size of the blocking residue are key to the enzymatic activity of tyrosinases. Representatives of three tyrosinase proteins from α- subclass are shown. A. Mollusc tyrosinase, B. Nematode tyrosinase and C. Cnidarian tyrosinase.