Mercury, From an Organic Point-of-View

Methylmercury cation
Mercury is most harmful to humans in the form of methylmercury.  Methylmercury is a an organometallic cation in the form of H3C-Hg+.
 The positive charge on methylmercury ion warrants the molecule to interact with other negatively charged ions.  Methylmercury has a high affinity for binding to proteins that contain the amino acid cysteine.

Methylmercury-cysteine complex
After ingestion of food containing methylmercury, such as fish, the cation is absorbed into the blood stream through the gastrointestinal tract and transported throughout the body.  While traveling in the blood, methylmercury is free to interact with other proteins and is often found bound to cysteine containing proteins.  The combination of methylmercury and cysteine creates a compound that is similar in structure to methionine.  Amino acid transport proteins are embedded in the blood-brain barrier and placenta.  The transport protein that is responsible for moving methionine across the blood-brain barrier and the placenta are fooled by the methylmercury-cysteine complex and shuttle it into the brain and placenta where it can then cause serious health complications.