However, even if the oxygens in the figure above were not colored, the stereochemical outcome of the reaction should tell you that the second step is not an S N 1 or S N 2 reaction. When the fosfomyci...However, even if the oxygens in the figure above were not colored, the stereochemical outcome of the reaction should tell you that the second step is not an S N 1 or S N 2 reaction. When the fosfomycin molecule (which is structurally a mimic of the phosphoenolpyruvate substrate) enters the active site of the enzyme, the cysteine thiol attacks one of the electrophilic carbons and opens the ring, leading to the irreversible alkylation of the active site.