Homology model of the TrkA TK domain. The model was obtained using rat MuSK kinase structure 1luf . Activation loop (orange) and catalytic loop (blue) are indicated in accordance to de Pablo et al. . The kinase insert fragment of kinase domain constitutes a loop joining two alpha-helices. Most of its residues are missing in the crystal structure presumably due to the loop's high conformational flexibility and therefore not present in homology model (depicted as green beads). The His604 residue (red) is located at the end of structurally determined helical region, distant from ATPase active site (arrow). It may, however, be involved in interactions with residues of the unstructured loop region. Although the function of this loop is not yet known, it may be speculated that it is involved in specific for TrkA interactions with secondary messengers or regulatory proteins. Homology model was visualized with VMD program .