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Antimetabolites are substances that interfere with the normal metabolism of an organism, thereby causing its death. They are widely used in the medical sciences because they have the ability to 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 antimetabolite works by inhibiting the action of an enzyme that is crucial to the process of metabolism. When the enzyme is immobilized, the series of reactions by which metabolism occurs is interrupted and the microorganism dies.

One of the classic examples of antimetabolite action is that of the sulfa drugs, discovered in the 1930s. Some examples of the sulfa drugs are 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. Folic acid is then used in the manufacture of nucleic acids in the bacterium.

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

Antimetabolites generally work in one of three ways to interrupt the metabolism of an organism. First, as in the example above, they may prevent the formation of nucleic acids, essential for the production of DNA in the organism. Second, they may interfere with the synthesis of proteins in the cell of a microorganism. Third, they may interfere with the synthesis of a cell wall, causing the cell to break apart and die.

Today, a wide variety of antimetabolite drugs are available to physicians. In addition to the sulfa drugs described above, other examples of such drugs include members of the penicillin family, the tetracyclines, chloramphenicol, streptomycin, and the anti-cancer drug known as 5-fluorouracil.

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