Reaction profiling of a set of acrylamide-based human tissue transglutaminase inhibitors

The major function of the enzyme human tissue transglutaminase (TG2) is the crosslinking of proteins via a transamidation between the γ-carboxamide of a glutamine and the ε-amino group of a lysine. Overexpression of TG2 can lead to undesirable outcomes and has been linked to conditions such as fibrosis, celiac disease and neurodegenerative diseases. Accordingly, TG2 is a tempting drug target. The most effective TG2 inhibitors to date are small-molecule peptidomimetics featuring electrophilic warheads that irreversibly modify the active site catalytic cysteine (CYS277). In an effort to facilitate the design of such TG2 inhibitors, we undertook a quantum mechanical reaction profiling of the Michael reaction between a set of six acrylamide-based known TG2 inhibitors and the TG2 CYS277. The inhibitors were docked into the active site and the coordinates were refined by MD simulations prior to modelling the covalent modification of the CYS277 thiolate. The results of QM/MM MD umbrella sampling applied to reaction coordinates driving the Michael reaction are presented for two approximations of the Michael reaction: a concerted reaction (simultaneous thiolate attack onto the acrylamide warhead and pronation from the adjacent HIS335) and a two-stage reaction (consecutive thiolate attack and protonation). The two-stage approximation of the Michael reaction gave the better results for the evaluation of acrylamide-based potential TG2 inhibitors in silico. Good correlations were observed between the experimental TG2 IC₅₀ data and the calculated activation energies over the range 0.0061-6.3μM (three orders of magnitude) and we propose that this approach may be used to evaluate acrylamide-based potential TG2 inhibitors.