Figure 4.

Transition-state model for the ADP-ribosyltransferase reaction. (A) The imidazole ring of the diphthamide residue (Diph; dark green; N3 nucleophile, cyan) migrates towards the nicotinamide ribose as the TMA group of the diphthamide moves between L1 of the toxin and the NAD⁺ phosphates. The original diphthamide position from the eEF2–ExoA_c(R551H)–NAD⁺ structure is shown in light brown with Gly 701, Gly 702 and Gly 703 in red. Active-site residues of ExoA_c are shown as light blue ball and sticks, with the position of Glu 546 (before roamer rotation) and Arg 551 shown in brown. ADP-ribose and nicotinamide carbons are shown in grey (C1 electrophile, dark grey). Known hydrogen bonds are shown in purple, whereas potential interactions during transition-state formation are indicated in yellow. (B) The electrostatic molecular surface of the binding pocket of the eEF2–ExoA_c(R551H)–NAD⁺ complex. Charges were assigned to ExoA_c during electrostatic calculations and the position of NAD⁺ (grey) is shown as a ball-and-stick representation. Asp 461 and Glu 546 (blue), and the diphthamide (white) are shown as a ball-and-stick representation. ExoA, exotoxin A; eEF2, elongation factor 2; L, loop; TMA, trimethyl ammonium.