Conformational changes in E. coli adenylate kinase (AK) on binding substrate. (a) The unliganded enzyme. (b) The enzyme with the bound substrate analog Ap₅A. The Ap₅A is drawn in stick form colored according to atom type (C green, N blue, O red, P orange). The protein's cyan and blue domains undergo extensive conformational changes on ligand binding, whereas the remainder of the protein (magenta), whose orientation is the same in Parts a and b, largely maintains its conformation. Clicking the "Backbones" button shows the two structures superimposed in worm form with the unliganded enzyme gray and the liganded enzyme blue. Clicking the "Unliganded transparent" button shows the superimposed structures as in Parts a and b but with the unliganded enzyme transparent. [Based on X-ray structures by Georg Schulz, Institut fur Organische Chemie und Biochemie, Freiburg, Germany. PDBids (a) 4AKE and (b) 1AKE.]