In the various simulations of POPE-bound MCR-1, one of the two POPE acyl tails spends most of the simulation duration packed against helix α3 of the transmembrane domain (Figure , residues Tyr74 -Thr94), with frequent additional interactions with helix α4 (Ala123 -Val143). Interactions with the MCR-1 active site are however more variable. In the "resting" (mono-zinc) state, where Thr285 is unmodified by PEtN, the POPE PEtN head group interacts with the conserved "gating" histidine residues His395 and (less commonly) His478, that bind the second zinc equivalent (Zn2) in crystal structures of the MCR-1 catalytic domain in the di-zinc form. These interactions are mediated by the terminal primary amine of the PEtN head group. The PEtN phosphate group can then variably contact any of three charged residues, Glu116, Asn108, and Tyr97, on the periplasmic face of the transmembrane domain. In one of the three replicate simulations, however, the POPE head group reorients to a position where it more deeply penetrates the catalytic domain, with the phosphate group approaching the terminal hydroxyl of the PEtN acceptor Thr285. After 500 ns this simulation has reached an equilibrium state (Figure ) with many features of a potentially reactive conformation, with a distance of 3.7 Å between the nucleophile (Thr285 Oγ) and electrophile (POPE phosphorous) of the first (PEtN transfer) step of the lipid A modification reaction.