• About This Group

    Matt Shachat is a junior majoring in Chemistry and Chinese. He is an undergraduate researcher in the Scheck group at Tufts University. Ariq Rahman is a junior majoring in Biochemistry and Chinese.
  • Recent Comments

    NRTIs

    NRTIs are nucleoside reverse transcriptase inhibitors. This means that they are mimics of the nucleoside bases that reverse transcriptase uses to make DNA. These molecules are designed to be extremely similar to RTase substrates while acting as antagonists. [1] This is opposite NNRTIs (non-nucleoside reverse transcriptase inhibitors) that function by allosterically binding to HIV reverse transcriptase and altering the shape of the active site. For NRTIs, that means attaching a nucleobase to a molecule that will fit in the reverse transcriptase polymerase site. Often this is a ribose sugar with a substitution at the C3 position, however there are also other molecules used. [2] Through competitive inhibition, they lower the success of HIV reverse transcriptase because they ruin any transcript they are put in by causing chain termination. [1]

    AZT

    For example, the first NRTI clinically cleared for use in HIV+ patients was AZT (Zidovudine); AZT took a record 2 years from creation to approval by the FDA – the shortest amount of time for any drug ever. [3] AZT is a thymidine analog and gets triphosphorylated and integrated into the group viral DNA transcript by RTase. However, in AZT a 3′ azide group replaces the 3′ hydroxyl group. As a result, RTase can no longer add any bases on to the end of that transcript and the integration of AZT results in a chain termination. At the maximum allowed dosage, AZT and other NRTIs will clog up RTase and viral DNA formation without hampering DNA polymerase. [4]

    To the left is an image of normal HIV-1 reverse transcriptase. To the right is an AZT-resistant mutant of HIV-1 reverse transcriptase. Domains are colored as follows: magenta - fingers, blue - palm, green - thumb, red - RNAse domain, grays - connecting domains

    To the left is an image of normal HIV-1 reverse transcriptase. To the right is an AZT-resistant mutant of HIV-1 reverse transcriptase. Domains are colored as follows: magenta – fingers, blue – palm, green – thumb, red – RNAse domain, grays – connecting domains. Image created by the authors.

    However, NRTIs are very susceptible to HIV’s greatest strength – mutation. Since the strength of an NRTI comes from competitive inhibtion, they still have to fit into the binding pocket for RTase, or else they won’t be integrated into the DNA transcript and they will be rendered useless. As a result, HIV RTase mutations can cause an NRTI-based treatment to become completely ineffective when a mutation that allows the base but not the NRTI analog into the binding pocket as shown above. [5] Sterically, it is clear how these mutations might arise – for example a mutation that allows a 3′ OH group to fit in but not a 3′ azide could change the binding pocket to a smaller version or move a coordination amino acid group in the protein around.

    Structures of HIV-1 NRTIs. Each lacks a 3' OH, and as a result acts as a chain terminator when introduced into a DNA transcript.

    Structures of HIV-1 NRTIs. Each lacks a 3′ OH, and as a result acts as a chain terminator when introduced into a DNA transcript. Image created by the authors.

     

    Although HIV’s mutation rate is staggering, HIV isn’t the only thing that can adapt to its environment. Modern science has found new NRTIs like Tenofovir that have the nucleobase attached to something other than a deoxyribose and has made these molecules fit into the binding pocket of HIV RTase. [2] Tenofovir is a phosphonic acid ether, but due to the high degree of freedom of rotation the molecule has, it can fit inside of many different binding pockets HIV RTase can create. As a result, Tenofovir has become one of the three drugs that are the core of anti-HIV treatment plans today. Tenofovir can also be taken with Emtricitabine in a daily medication called Pre-Exposure Prophylaxis (PrEP) which is 92% effective at preventing HIV infection in HIV-negative individuals. [6] PrEP equips the cells with NRTIs, so in the case of an incoming HIV infection HIV RTase is immediately inhibited and HIV cannot integrate into a single cell – thus stopping HIV before it gets a hold in the body.

    NRTIs have also found use in other diseases, specifically AMD (Age-related Macular Degeneration.) This study found a link between NRTIs and anti-inflammatory activity in RPE cells in the eye.

    Citations:

    1. Cheng Y. C., Dutschman G. E., Bastow K. F., et. al. Human immunodeficiency virus reverse transcriptase. General properties and its interactions with nucleoside triphosphate analogs. J Biol Chem. 262. 2187-2189. (1987)

    2. Singh K., Marchand B., Kirby K. A., et al. Structural Aspects of Drug Resistance and Inhibition of HIV-1 Reverse Transcriptase. Viruses. (2010)

    3. Cimons, Marlene (21 March 1987). “U.S. Approves Sale of AZT to AIDS Patients”. Los Angeles Times. p. 1.

    4. Marty R., Ouameur A. A., Neault J. F., et al. AZT-DNA interaction. DNA Cell Biol. 23(3). 135-140. (2004)

    5. Tu X., Das K., Han Q., et al. Crystal structure of unliganded AZT-resistant HIV-1 reverse transcriptase.

    6. Grand RM et al. Preexposure chemoprophylaxis for HIV prevention in men who have sex with men. New Engl Jour Med. 363(27). 2587-2599. (2010)

    Leave a Reply

    Your email address will not be published.

    • Categories

      • No categories
    • Archives

    • Meta