Figure 7

Hypothetical model for auto-ADP-ribosylation. (A) Formation of the diffusible strained NAD⁺ intermediate. At pH 7.5, the negatively charged carboxylate of the catalytic residue interacting with the 2′ hydroxyl group induced partial negative charges at the 2′hydroxyl position and formation of pseudo ribose diol to cause pre-protonation at C1′. This pre-protonated form of NAD⁺ is usually stabilized by the solvent molecules in its surroundings. We propose a mechanism in which this pre-protonated NAD⁺ intermediate gets released from the catalytic site and diffuses out of the NAD⁺ binding pocket prior to dissociation of nicotinamide when enzyme is busy in processing high concentration of NAD⁺. (B) Transfer of ADP-ribosyl moiety from strained NAD⁺ to arginine residue. This pre-protonated NAD⁺ intermediate approaches arginine residue through the positive charged side chain of arginine and interacts with the partially negatively charged pseudo ribose diol (step 1). Guanidinium group of arginine side chain contains three nitrogens. When one nitrogen is positively charged to interact with the ribose diol group, the induced partially negatively charged nitrogen (nucleophilic atom) interacts with the electrophilic C1 center of the N-ribose to cause dissociation of nicotinamide and formation of oxocarbenium ion which then immediately reacts with the partially negatively charged nitrogen on the guanidinium group of arginine to facilitate the transfer of ADP-ribose to the target arginine residue (step 2 and 3).