in the treatment of HIV:
inhibits the action of reverse transcriptase. It functions as an
analog for thymidine (one of the nucleotide building blocks of DNA).
the same shape as thymidine and can be incorporated into the growing
strand of DNA. It is first phosphorylated to the triphosphate form
by host cell enzymes. After thymidine has been incorporated into
the DNA chain, the 3' -OH group (i.e. the -OH group attached
to the 3' carbon of the sugar) can bind to the phosphate group of
the next nucleotide. However, zidovudine does not have such an -OH
group and instead has an azido (-N3) group. The azido
group cannot form a bond with a phosphate group, and so once zidovudine
has been included in the DNA chain, no more nucleotides can be added.
This means reverse transcription stops, and the chain is terminated
after zidovudine is incorporated.