In simulations of the isolated MCR-1 catalytic domain in the di-zinc form, the Zn2 site, formed in crystal structures by His395 and His478, is unstable with bound zinc generally dissociating from unmodified (i.e. with free Thr285) enzyme over the course of 300 ns MD simulations . In contrast, in equivalent simulations with full-length MCR-1, a second zinc ion is retained over the complete duration of all three 500 ns simulations reported here. However, across the three replicate simulations Glu116 from the transmembrane domain is the only protein ligand that retains interaction with the second zinc equivalent, with interactions with the "gating" histidine residues, in particular His395, being lost in two of the three repeats. This behavior is replicated when Thr285 is modified through addition of PEtN; i.e. from a starting point close to the position of the Zn2 ion in the crystal structure of the dizinc catalytic domain (PDB 5LRM ) the second zinc equivalent migrates away from the MCR-1 active center towards the solvent-exposed periphery of the protein, but is prevented from dissociating altogether by persistent contact with Glu116. Thus, despite the potential for residues from the transmembrane domain of MCR-1, in particular Glu116, to contribute to a more stable, physiological Zn2 site, our simulations do not support existence of stable di-zinc forms of MCR-1 in either the "resting" ground state or after modification of Thr285 through addition of PEtN.