Multiple crystal structures have been determined for the isolated soluble periplasmic catalytic domain of MCR-1 . These include a range of metalation states (i.e. zinc equivalents) and with the PEtN acceptor residue Thr285 in unmodified and phosphorylated forms. On the basis of these structures we have used MD simulations and QM cluster model calculations to propose that, in the isolated catalytic domain, the Zn1 ion is stably coordinated while the Zn2 site can retain its metal ion only in the presence of the lipid A acceptor, i.e. during the final step of the PEtN transfer reaction from Thr285 to the lipid A phosphate. However, such simulations necessarily neglect the possible involvement of residues from the transmembrane domain in zinc coordination. Accordingly, we used our MD simulations to study the stability and coordination of both zinc ions in full-length MCR-1, in the absence and presence of PEtN bound to Thr285. As in our previous studies of the isolated catalytic domain , in mono-zinc MCR-1 the zinc ion remains stably bound in the Zn1 site for the full 500 ns of our simulations. As before, in these simulations Zn1 favored 6 co-ordinate geometry as a consequence of our use of the Lennard-Jones 12-6 parameters to treat the metal ions, with this change in coordination achievable through a move to bidentate coordination by the two carboxylate ligands Glu246 and Asp465. The main change in active site geometry observed in these simulations is a reorientation of the transmembrane domain residue Glu116 in 2 out of 3 replicate simulations, that facilitates H-bonding to the side chain of His395 (Figure ). Similarly, when Thr285 is modified by addition of PEtN, while Zn1 coordination now involves one of the phosphoryl oxygen atoms in place of the Thr hydroxyl group, the metal center is otherwise stable, with reorganization of the system over the time scale of the simulation involving the PEtN moiety sampling alternative orientations through fluctuating H-bonds to His395, His478 and Glu116 (Figure ).