Heavy metal ions such as mercury and lead are poisonous partly because they
- A. increase enzyme activity uncontrollably
- B. are converted into proteins by the body
- C. supply too much energy to the cell
- D. bind to sulphur containing groups in enzymes and denature them irreversibly
Explanation
These ions attack the sulphydryl side chains of cysteine, breaking disulphide bridges and destroying the tertiary structure, so the affected enzymes lose all activity permanently. Because the damage cannot be reversed by removing the metal, the effect is poisoning rather than simple inhibition. This is also why drinking milk or egg white gives some first aid benefit, as those proteins bind the metal first.
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