Figure 7. Proposed mechanisms for the new toxins by homology-based transfer.

The S_N1 alleviated-strain mechanism, developed for Iota toxin, is likely widely applicable throughout the CT group ADPRTs [34], given high structure similarity and consistent NAD⁺ conformation in the active site. Therefore, we use a 3DLOGO-based method to propose a homology-based mechanism for the new ADPRTs. First, the universally conserved region 3 catalytic Glu (which H-bonds to the N-ribose) and the universally conserved region 1 Arg (which creates phosphate electrostatic interactions) hold the NAD⁺ in a conformation that favors oxacarbenium ion formation. Then, we invoke a Phe as well as the Tyr that induces a rotation of the oxacarbenium ion about the O_5D-P_N bond of the N-ribose to relieve the strained NAD⁺ conformation and help reduce the nucleophile-electrophile distance. (Previous work has shown the Tyr to Phe substitution in Iota toxin is still active [34].) The electrophile and nucleophile may migrate by an unknown mechanism that further reduces the distance between them. Finally, a target Glu or Asp stabilizes the N-ribose, the region 3 Glu or Gln stabilizes the target Arg, Asn or Cys; Asn, Gln or Cys attacks the oxacarbenium ion, for region 3 QXE toxins, or an Arg attacks the oxacarbenium ion for region 3 EXE toxins.