Copper(II) is known to bind in the influenza virus His37 cluster in the homotetrameric M2 proton channel and block the proton current needed for uncoating. Copper complexes based on iminodiacetate also block the M2 proton channel and show reduced cytotoxicity and zebrafish-embryo toxicity. In voltage-clamp oocyte studies using the ubiquitous amantadine-insensitive M2 S31N variant, the current block showed fast and slow phases in contrast to the single phase found for amantadine block of WT M2. Here we evaluate the mechanism of block by copper adamantyl iminodiacitate (Cu(AMT-IDA)) and copper cyclooctyl iminodiacitate (Cu(CO-IDA)) complexes and address whether the complexes can coordinate with one or more of the His37 imidazoles. The current traces were fitted to parametrized master equations. The energetics of binding and the rate constants suggest that the first step is copper-complex binding within the channel and the slow step in the current block is the formation of a Cu-histidine coordination complex. Solution-phase isothermal titration calorimetry (ITC) and density functional theory (DFT) calculations indicate that imidazole binds to the copper complexes. Structural optimization using DFT reveals that the complexes fit inside the channel and project the Cu(II) towards the His37 cluster allowing one imidazole to form a covalent bond with the Cu(II). Electrophysiology and DFT studies also show that the complexes block the G34E amantadine-resistant mutant in spite of some crowding in the binding site by the glutamates.
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