O-Chem in Real Life: The Epoxide Antibiotic Fosfomycin

Epoxides have a lot of strain energy, which makes them quiet reactive. When they are not protonated. we learned that nucleophiles can attack one of the carbons (the least substituted one) to open the epoxide. This reaction is the basis of a common broad spectrum antibiotic.

The antibiotic fosfomycin works by inhibiting the chemical reactions that form the bacterial cell walls. The biochemical reactions are too complicated to get into here, but we can understand its action as inhibiting the function of the enzyme MurA, which is required to form the peptidoglycans that usually coat bacterial cell walls.

The active site in MurA contains the amino acid cysteine, which in turn contains a thiol functional group, which is the sulfur equivalent to an alcohol. Alcohols are weak nucleophiles, but thiols are much stronger - REMEMBER, larger atoms tend to be better nucleophiles because they make stronger partial bonds in the transitions state in nucleophilic reactions, And so the thiol component of cysteine is a good nucleophile, good enough to react with an epoxide.

Reaction of the cysteine in the active site with the epoxide fosfomycin results in a new S-C bond that is formed irreversibly, i.e. the active site becomes alkylated and the enzyme doesn't work any more and the bacteria can't build cell walls.