Crystal structure of AQN (4DZT) with residues contributing to enzyme stabilization. (a) The residues contributing to the salt bridges Arg12-Asp183, Asp17-Arg259, Arg31-Glu237 and Arg43-Asp212 and the disulfide bridges Cys67-Cys99 and Cys163-Cys199 are indicated and are represented as stick models. Calcium ions are represented as pale green spheres. The catalytic residues are represented as blue stick models. (b) The structure is rotated approximately 90 degrees horizontally from (a).