Fig. 1. Cartoon representations of the crystal structures of the three serine proteases and their backbone superposition. (A) Psychrophilic VPR. (B) Mesophilic PRK. (C) Thermophilic AQN. (D) Backbone superposition of the three structures. In (A), (B), and (C), α-helices, β-strands, loops, substrate-binding segments (residues 100–104 and 132–136) are colored red, yellow, green, and purple, respectively; catalytic triads (D37-H70-S220, D39-H69-S224, and D39-H70-S222 in VPR, PRK, and AQN, respectively) and oxyanion holes (N157, N161, and N157 in VPR, PRK, and AQN, respectively) are represented as stick models, with carbon, oxygen, and nitrogen atoms colored cyan, red, and blue, respectively; disulfide bridges (S–S) are shown as stick models in orange; calcium ions are represented as blue spheres and numbered Ca1, Ca2, or Ca3 according to their numbering order in the crystal structures. In (D), the backbones of VPR, PRK, and AQN, as well as their contained calcium ions, are colored red, cyan, and blue, respectively.