Covalent inhibition involves the chemical modification of the enzyme so that it is no longer active. For example, the compound diisopropylfluorophosphate reacts with many enzymes by adding a phosphate group to an essential serine hydroxyl group in the enzymes' active sites. When phosphorylated, the enzyme is totally inactive. Many useful pharmaceutical compounds work by covalent modification. Aspirin is a covalent modifier of enzymes involved in the inflammatory response. Penicillin covalently modifies enzymes required for bacterial cell‐wall synthesis, rendering them inactive. Because the cell wall is not able to protect the bacterial cell, the organism bursts easily and is killed.