Fig. 7.

Schematic illustrating the mechanism of ADP ribosylation. (A) SN1 mechanism in Ia: (i) NAD⁺-Ia-Actin as the prereaction state; (ii) nicotinamide cleavage occurs via an SN1 reaction induced by an NMN ring-like structure and the first oxocarbenium cation intermediate is formed with a strained conformation; (iii) the second cationic intermediate is induced through alleviation of the strained conformation mainly by O3-NP and NP-NO5 rotation, and then NC1 of N-ribose nucleophilically attacks Arg177 of actin; (iv) Ia-ADPR-actin as the postreaction state. (B) Successive structures during ADP ribosylation and the structure of each reaction: step 1 (apo-Ia-actin), step 2 [NAD⁺-Ia-actin (prereaction state)], step 3 [βTAD-Ia-actin (transition state)], and step 4 [Ia-ADPR-actin (postreaction state)].