Fig. 5

A loop gating model for AgPPO8 activation. (a) The top view of Site II pocket with docked tyramine (green) and dopamine (yellow). The secondary structures of AgPPO8 are shown in gray. The flexible loop Y230-P235 (red) forms loose van der Waals contacts with the α-helix P101-D116 (red), controlling the entrance to the substrate binding Site II (cyan). The putative placeholder F99 and the catalytic residue E364 are shown as violet sticks. Inset shows the sequence alignment of the loop among different PPOs from insects and crustaceans. Ms, Manduca sexta; Dm, Drosophila melanogaster; Ag, Anopheles gambiae; Aa, Aedes aegypti; Tm, Tenebrio molitor; Bm, Bombyx mori; Mr, Macrobrachium rosenbergii; Pt, Portunus trituberculatus; Mj, Marsupenaeus japonicas. The distances from E364 to the putative placeholder F99 (magenta) and the docked tyramine (green)/dopamine (yellow) are indicated. (b) A cartoon illustration of the loop gated AgPPO8 activation. The side view of the Site II pocket is shown with the loop-helix controlled entrance located at the top. Left, closed state. Right, open state. Conformational changes at the molecular surface disrupt the loose contact between the loop Y230-P235 and the helix P101-D116, opening the gate and allowing the substrates to access the active site