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Antimetabolites are substances that kill organisms by interfering with normal metabolism. They are widely used in medicine because they can kill or inactivate microorganisms that cause disease. Terms such as antibacterials, antifungals, and antivirals are used to describe antimetabolites that act on bacteria, fungi, and viruses, respectively. In most cases, an anti-metabolite works by inhibiting the action of an enzyme that is crucial to the organisms metabolic processes. When the enzyme is immobilized, the series of reactions by which metabolism occurs is interrupted and the microorganism dies.

Classic examples of antimetabolite action include the effects of sulfa drugs, discovered in the 1930s, a group that includes sulfathiazole, sulfadiazine, and sulfacetamide. All sulfa drugs affect the metabolism of microorganisms in the same way. Under normal circumstances, a bacterium makes use of a compound known as para-aminobenzoic acid (PABA) to produce a second compound, folic acid, which is then used to manufacture nucleic acids in the bacterium.

Sulfa drugs chemical structures are very similar to that of PABA. When a bacterium ingests a sulfa compound, the microorganism attempts to make folic acid using the sulfa drug rather than PABA. The folic acidlike compound that is produced, however, can not be used to make nucleic acids. The bacteriums normal metabolism is interrupted, and it dies.

Antimetabolites may also work by interfering with protein synthesis in the cell of a microorganism or with the synthesis of a cell wall, causing it to break apart and die.

Today, a wide variety of antimetabolite drugs are available, including members of the penicillin family, the tetracyclines, chloramphenicol, streptomycin, and the anticancer drug known as 5-fluorouracil.