Computational QM/MM Study of the Reaction Mechanism of Human Glutathione S-Transferase A3-3

Human Glutathione S‐Transferase A3‐3(hGSTA3‐3) is the most efficient human steroid double‐bond isomerase enzyme. It catalyzes the double bond isomerization of Δ5 ‐androstene–3,17–dione ( Δ5 ‐AD) and Δ5 ‐pregnene–3,20–dione ( Δ5 ‐PD). The isomerization products are the precursors of the steroid hormones testosterone and progesterone. We have carried out a QM/MM study to elucidate some interesting aspects of the enzyme catalytic mechanism. In particular, we have analyzed either a concerted or a stepwise reaction path. Moreover, we have attempted to rationalize the electrostatic effects on the catalytic activity of the residues surrounding the active site. Specifically, we have performed a “finger print” analysis to determine the electrostatic contribution of each aminoacid residue to the global electrostatic term, thus ranking the effect of the various aminoacids in the course of the reaction. In this way, we have highlighted the most important terms affecting the stabilization‐destabilization of the enzyme.