Figure 4.

(a) The tetramer of PaADH. Subunits A, B, C, and D are colored purple, green, orange, and blue, respectively. The catalytic and structural Zn²⁺ ions are shown in yellow. (b) The ion-pair network at the interface of three subunits of PaADH. The long ion-pair network consists of Lys 161^A-Asp 297^A-Lys 164^A-Glu 300^A and Arg 169^D (superscripts denote the different subunits). The short ion pair includes Arg 258^D and Glu 102^C. The ion pairs are shown as thick dotted green lines. Subunit assignment and coloring are as in the legend to Figure 4a, above. (c) The interface of subunits A and B. At the dimer interface, a distortion of the noncrystallographic twofold symmetry is due to the asymmetrical, donor–acceptor nature of the intersubunit Thr 275^A–Thr 275^B hydrogen bond. The hydrogen bonds are shown in green dashed lines; the 4.6 Å distance between the Thr 275^A hydroxyl and the Phe 273^A carbonyl is shown in red (dashed line).