Conformational changes in citrate synthase. (a) The open conformation. (b) The closed, substrate-binding conformation. In both forms, the homodimeric protein is viewed along its twofold axis with its polypeptide chains drawn in worm form colored in rainbow order from their N-termini (blue) to their C-termini (red), embedded in their transparent molecular surfaces (gray). The reaction product citrate, which is bound to both enzymatic forms, and coenzyme A, which is also bound to the closed form, are shown in space-filling form with citrate C cyan, CoA C green, N blue, O red, and P orange. The large conformational shift between the open and closed forms entails 18° rotations of the small domains (upper left and lower right) relative to the large domains resulting in relative interatomic movements of up to 15 Å. Clicking the "Show Animation" button shows an animation of the structural changes between the two conformations, initially with the protein shown in space-filling form with small domain C blue-gray, large domain C magenta, N blue, O red, and S yellow. Clicking on the "Backbone" button shows the animated protein in worm form colored in rainbow order from its N-terminus (blue) to its C terminus (red). The citrate and acetyl-CoA are seen by clicking the "Transparent" button. [Based on X-ray structures by James Remington and Robert Huber, Max-Planck-Institut für Biochemie, Martinsried, Germany. PDBids 1CTS and 2CTS.]