13.7C: Nucleotide and Nonnucleotide Reverse Transcriptase Inhibitors
- Summarize the mechanism of action for reverse transcriptase inhibitors
Reverse transcriptase is an enzyme that has the ability to transcribe single-stranded DNA from a single-stranded RNA chain. This is the reverse of the usual flow of information when RNA is synthesized from DNA. Viruses that use reverse transcriptase to convert their genetic material (RNA) into DNA are called retroviruses. One of the most prominent representative of a retrovirus is HIV. Due to the high prevalence of HIV/AIDS in the world, it is important to have drugs that will prevent or cure the infection. This enzyme is also found in tumors and cancer cells.
Drugs that inhibit the function of this enzyme are divided into three groups:
- nucleoside analog reverse transcriptase inhibitors
- nucleotide analog reverse transcriptase inhibitors
- non-nucleoside reverse transcriptase inhibitors
The first two inhibitors act on the same principle. They mimic, respectively, nucleosides or nucleotides but lack a free hydroxyl group at the 3′ end. The major difference between them is that the nucleosides need to be phosphorylated by cellular kinases. The enzyme reverse transcriptase recognizes them as regular nucleotides and inserts them into the newly synthesized DNA chain. But once inserted the elongation stops at them because no more nucleotides can be added due to the lack of the 3′ hydroxyl group and the inability of the formation of 5′-3′ phosphodiester bond. This process is called chain termination. Nucleoside and nucleotide inhibitors are also called competitive substrate inhibitors. Examples of such drugs are Zidovudine (AZT) and Lamivudine. AZT was the first FDA approved drug for the treatment of HIV. Lamivudine is used for the treatment of both HIV and hepatitis B. Since some viruses, such as hepatitis B, carry RNA-dependent DNA polymerases reverse transcriptase inhibitors can be used to treat these infections as well.
Non-nucleotide reverse transcriptase inhibitors bind to a different site, not the active one, of the reverse transcriptase enzyme. That leads to conformational changes that distort the position of the DNA binding sites in the enzyme and lead to halt in DNA polymerization. Non-nucleotide inhibitors are non-competitive inhibitorsof reverse transcriptase. Such drugs are Efavirenz and Nevirapine.
Resistance occurs to all drug groups. The mechanisms for resistance against the nucleoside (nucleotide) inhibitors are two. The first one is due to mutations in the N-terminal polymerase domain of the reverse transcriptase that makes it less likely to incorporate the analogues. The second mechanism is caused by mutations in the transcriptase that allow the removal of the incorporated inhibitor and hence restart of DNA replication.
Resistance to the non-nucleotide inhibitors is caused by mutations in the inhibitor binding site of the enzyme. Such mutations prevent the binding of the inhibitor to the enzyme.
Key Points
- Nucleoside and nucleotide inhibitors are competitive substrate inhibitors that mimic the structure of a normal nucleotide but lack the 3′ hydroxyl group needed for the addition of the next nucleotide for DNA elongation.
- Non-nucleotide inhibitors bind to a site different than the active one and cause rearrangements of the protein domains needed for DNA polymerization.
- Mutations in the reverse transcriptase gene can cause resistance to both types of drugs.
Key Terms
- competitive substrate inhibitors : Molecules that bind to the active site of an enzyme and prevent the real substrate from binding to it.
- non-competitive inhibitors : Molecules that bind to sites other than the active site of an enzyme while still being able to indirectly inhibit its function.
- nucleotide : the monomer comprising DNA or RNA biopolymer molecules, consisting of a nitrogenous heterocyclic base; a five-carbon pentose sugar; and a phosphate group