Which of the following is an inhibitor of dihydrofolate reductase –
**Core Concept**
Dihydrofolate reductase (DHFR) is an enzyme that plays a crucial role in the synthesis of tetrahydrofolate, an essential cofactor for DNA synthesis and repair. Inhibition of DHFR can lead to impaired DNA synthesis, making it an attractive target for cancer chemotherapy and antimicrobial therapy.
**Why the Correct Answer is Right**
Methotrexate is a potent inhibitor of dihydrofolate reductase. It binds to the enzyme, preventing the conversion of dihydrofolate to tetrahydrofolate, which is necessary for DNA synthesis. This mechanism of action is particularly effective against rapidly dividing cells, such as cancer cells and certain microorganisms. Methotrexate works by exploiting the high demand for thymidine synthesis in these cells, leading to an accumulation of dihydrofolate and a subsequent inhibition of DNA replication.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately represent a dihydrofolate reductase inhibitor. While some antimicrobials may target folate metabolism, they do not specifically inhibit DHFR.
**Option B:** This option is incorrect because it is not a well-known inhibitor of dihydrofolate reductase. Although it may have some effects on folate metabolism, it is not a primary target of DHFR inhibition.
**Option C:** This option is incorrect because it is actually a substrate for dihydrofolate reductase, rather than an inhibitor. Dihydrofolate is converted to tetrahydrofolate by the action of DHFR.
**Clinical Pearl / High-Yield Fact**
Methotrexate is a classic example of a chemotherapeutic agent that targets a specific enzyme in the folate metabolism pathway. Its mechanism of action is closely related to that of other antimicrobials, such as sulfonamides, which also exploit the high demand for thymidine synthesis in rapidly dividing cells.
**Correct Answer:** A. Methotrexate