Fig. 2.

(a) Crystal structure of Cps4B (stereo pair). The structure is shown in ribbon representation and colored with a gradient running from yellow (N-terminus) to red (C-terminus). The metal ions bound to the active site are shown as purple spheres; metal ligands, in ball-and-stick mode. The alternative conformation of the flexible loop connecting α6/βF is shown in blue. (b) Topology plot of Cps4B. Helices are shown as cylinders; strands, as arrows; and loops, as gray lines. The approximate positions of the metal ions, with respect to the secondary structure elements, are indicated by purple spheres.