Modeling of the Passive Permeation of Mercury and Methylmercury Complexes Through a Bacterial Cytoplasmic Membrane

Cellular uptake and export are important steps in the biotransformation
of mercury (Hg) by microorganisms. However, the mechanisms of transport
across biological membranes remain unclear. Membrane-bound transporters
are known to be relevant, but passive permeation may also be involved.
Inorganic HgII and methylmercury ([CH3HgII]+) are commonly complexed with thiolate ligands.
Here, we have performed extensive molecular dynamics simulations of
the passive permeation of HgII and [CH3HgII]+ complexes with thiolate ligands through a model
bacterial cytoplasmic membrane. We find that the differences in free
energy between the individual complexes in bulk water and at their
most favorable position within the membrane are ∼2 kcal mol–1. We provide a detailed description of the molecular
interactions that drive the membrane crossing process. Favorable interactions
with carbonyl and tail groups of phospholipids stabilize Hg-containing
solutes in the tail–head interface region of the membrane.
The calculated permeability coefficients for the neutral compounds
CH3S–HgII–SCH3 and
CH3HgII–SCH3 are on the order
of 10–5 cm s–1. We conclude that
small, nonionized Hg-containing species can permeate readily through
cytoplasmic membranes.