Figure 4.

Low-magnesium EntC structure. (A) The overall topology of the EntC structures presented here is not changed from the previously determined structure. A cartoon of the fold is shown in pale green, with the catalytic magnesium ion shown as a gray sphere. Isochorismate (the product) is shown in pale cyan, whereas chorismate (the substrate) is shown in deep teal. (B) A closeup of the active site for the low-magnesium structure is shown, with colors as in (A). In addition, the general acid (E197) and general base (K147) are shown as yellow sticks, and the magnesium-ligand residues (E241 and E376) are shown in pale-green sticks. Two water molecules also act as ligands of the magnesium (red spheres), and one of the residues that holds these waters in place is visible in this view (D238). A simulated-annealing omit map contoured at 3σ surrounds the magnesium ion, chorismate, and isochorismate (the parts of the structure omitted during the calculation).