HIV reverse transcriptase enzyme

Reverse Transcriptase - 3 -Azido-2 3 -dideoxythymidine (AZT), when converted to the corresonding 5 triphosphate in cells, is an inhibitor of the HIV reverse transcriptase enzyme, which is responsible for making viral DNA from viral RNA. Other nucleoside analogs, such as 2 3 -dideoxycytidine (ddC), 2 3 -dideoxyinosine (ddl), and 2 3 -didehydro-3 -deoxythymidine (d4T) (see here) are also converted to triphosphates and act by blocking DNA chain elongation after they are incorporated into DNA. 

The most successful older anti-AIDS drug, azidothymidine (AZT, also called zidovudine), is one of the drugs used in combination with a protease inhibitor. AZT is a derivative of deoxythymidine, a nucleoside. (Nucleosides are nucleotides without the phosphate group see Section 15.11.) Because AZT resembles a natural nucleoside, it is accepted by the HIV reverse transcriptase enzyme and placed into the viral DNA. Once there, its structure prevents additional nucleosides being added to the chain, and cell division is halted. AZT has been found to reduce the mortality of babies born to HIVpositive mothers to close to zero. 

Nucleoside reverse transcriptase inhibitor (NRTI)/nucleotide reverse transcriptase inhibitor (NtRI) A modified version of a naturally-occurring nucleoside or nucleotide that prevents human immunodeficiency virus (HIV) replication by interfering with the function of the viral reverse transcriptase enzyme. The nucleoside/nucleotide analog causes early termination of the proviral DNA chain. For activity, an NRTI requires three phosphorylation steps once inside the cell, whereas an NtRI has a phosphate group attached and needs only two phosphorylation steps inside the cell for activity. 

There are a few key enzymes for the proliferation of human immunodeficiency virus (HIV). Reverse transcriptase is one of them since HIV is a member of the DNA viruses. Efavirenz (1) is an orally active non-nucleoside reverse transcriptase inhibitor (NNRTI) and was discovered at Merck Research Laboratories [1] for treatment of HIV infections. Efavirenz was originally licensed to DuPont Merck Pharmaceuticals which was later acquired by Bristol-Myers Squibb.11 The typical adult dose is 600 mg once a day and 1 is one of three key ingredients of the once-a-day oral HIV drug, Atripla (Figure 1.1). 

Thiourea compounds have been observed to inhibit human immunodeficiency virus (HIV) reverse transcriptase, a viral enzyme that is responsible for the reverse transcription of the retroviral RNA to proviral DNA. Phenethylthiazoylthiourea (PETT) compounds were discovered as potent inhibitors of HIV type 1 and display certain structure-activity relationships among various substituents in their structure.199 207 Furthermore, thiourea derivatives have been found to be potent and selective viral inhibitors, antifungal and antibacterial compounds.208 215... 

The work that paved the way toward enzymatic inhibition was published in the early 1990s by Wudl and coworkers (Schinazietal., 1993 Friedmanetal., 1993 Sijbesma et al., 1993) and since then studies regarding antiviral activity, mainly HIV-protease inhibition, have been carried out to find active compounds. Up to now, the most effective fullerene derivatives are the trans-2, -dimethy 1-bis-fulleropyrrolidin-ium salt (Fig. 1.4) (Marchesan et al., 2005) and the dendrofullerene reported by Hirsch (Schuster et al., 2000) both of them present an ECJ0 of 0.2pM. Also HIV reverse transcriptase can be inhibited by, -dimcthyl-bis-fulleropyrrolidinium salts (Mashino et al., 2005). The same compounds are also active against acetylcholine esterase (AChE), an enzyme that hydrolyzes a very important neurotransmitter. 

The next stage is to ensure that the recombinant DNA molecule is copied by the enzymes which s)mthesize nucleic acids. These DNA and RNA polymerases synthesize an exact copy of either DNA or RNA from a pre-existing molecule. In this way the DNA polymerase duplicates the chromosome before each cell division such that each daughter cell will have a complete set of genetic instructions which are then passed to the newly formed RNA by RNA polymerase. While both DNA and RNA polymerase require a preformed DNA template, some viruses (such as HIV) have an RNA genome. To duplicate that genome, and incorporate it into a bacterial or mammalian cell, the viruses encode a reverse transcriptase enzyme which produces a DNA copy from an RNA template. 

Cross-resistance between indinavir and HIV reverse transcriptase inhibitors is unlikely because the enzyme targets involved are different. Cross-resistance was noted between indinavir and the protease inhibitor ritonavir. Varying degrees of cross-resistance have been observed between indinavir and other HIV-protease inhibitors. 

Pharmacology Zalcitabine, active against HIV, is a synthetic pyrimidine nucleoside analog of the naturally occurring nucleoside deoxycytidine in which the 3 -hydroxyl group is replaced by hydrogen. Within cells, zalcitabine is converted to the active metabolite, dideoxycytidine 5 -triphosphate (ddCTP), by cellular enzymes. ddCTP inhibits the activity of the HIV-reverse transcriptase both by competing for utilization of the natural substrate, deoxycytidine 5 -triphosphate (dCTP), and by its incorporation into viral DMA. 

The reverse transcriptase enzyme (RT) is the primary enzyme responsible for the conversion of the viral single-strand RNA to the double-strand DNA. The reverse transcriptase enzyme is a component of the virion and is encoded by the pol gene. The RT is manufactured in the HIV-infected cells as a gag-pol fusion polyprotein. The RT is not the only enzyme necessary for the translation of RNA to DNA. The other enzymes for this conversion include RNA-dependent DNA polymerase, DNA-dependent DNA polymerase, and RNase H (Gilboa and Mitra, 1978 Prasad and Gogg, 1990). The reverse transcriptase enzyme has a high error rate (1 in 2000 bases), which produces higher incidents of mutation. Some of these mutations make the virus resistant to NNRTI treatment. 

Lamivudine is a synthetic cytidine analogue used in the treatment of HIV (see Chapter 51) and HBV. Its activation requires phosphorylation by cellular enzymes. Lamivudine triphosphate competitively inhibits HBV DNA polymerase and HIV reverse transcriptase and causes chain termination. It inhibits the activity of mammalian DNA polymerases with a much lower potency. 

Mechanism of Action An antiviral that inhibits HIV reverse transcriptase by viral DNA chain termination. Also inhibits RNA- and DNA-dependent DNA polymerase, an enzyme necessary for HIV replication. Therapeutic Effect Interrupts HIV replication, slowing the progression of HIV infection. 

Other DNA polymerases. Reverse transcriptases synthesize DNA using an RNA template strand. They are best known for their function in retroviruses (Chapter 28). The HIV reverse transcriptase is a heterodimer of 51- and 66-kDa subunits. The larger subunit contains a ribonuclease H domain.288-2893 The enzyme is a prime target for drugs such as AZT and others.290 291 A different reverse transcriptase is found in all eukaryotic cells in telom-erase, an enzyme essential for replication of chromosome ends. Reverse transcriptases have also been found in rare LI sequences that are functioning ret-rotransposons (Section D).292... 

Figure 6. Amplification of RNA molecules by assays that are sequence- insensitive. The first assay (upper part) combines the polymerase chain reaction (PCR) of DNA templates with reverse transcription and transcription. Commonly used enzymes are TAQ-polym-erase, HIV reverse transcriptase and bacteriophage T7 RNA polymerase. The assay requires a temperature program applying higher temperatures for double strand dissociation. The second assay (lower part) shows the self-sustained sequence replication reaction (3SR) which can be carried out isothermally because double strand dissociation is replaced by enzymatic digestion of the RNA strand in the RNA-DNA duplex. The enzymes used are HIV reverse transcriptase, RNase H and T7 RNA polymerase.

Mechanism of Action. RTIs impair HIV replication by inhibiting the reverse transcriptase enzyme that is needed to convert viral RNA to viral DNA (Fig. 34-3). With regard to zidovudine and the other NRTIs, these agents enter viral-infected cells, where they are progressively phosphorylated (activated) by... 

TIs also inhibit the reverse transcriptase enzyme s ability to perform one of the initial steps in HIV replication. The NNRTIs, however, directly inhibit the active (catalytic) site on this enzyme, whereas zidovudine and other NRTIs serve as false substrates that take the place of the substance (thymidine) normally acted on by this enzyme (see Reverse Transcriptase Inhibitors Mechanism of Action ). Hence, NNRTIs provide another way to impair one of the key steps in HIV replication, and these drugs can be used along with other agents (NRTIs, protease inhibitors) to provide optimal benefits in preventing HIV replication and proliferation (see the next section). 

Reverse transcription (which occurs in both prokaryotes and eukaryotes) is the synthesis of DNA from an RNA template. A class of RNA viruses, called retroviruses, are characterized by the presence of an RNA-dependent DNA polymerase (reverse transcriptase). The vims that causes AIDS, Human Immunodeficiency Virus (HIV), is a retro-vims. Because nuclear cell division doesn t use reverse transcriptase, the most effective anti-HIV drugs target reverse transcriptase, either its synthesis or its activity. Telomerase, discussed in the previous section, is a specialized reverse transcriptase enzyme. See Figure 12-5. 

Retroviruses like HIV, the pathogen responsible for AIDS, incorporate an RNA template that is copied into DNA during infection. The reverse transcriptase enzyme that copies RNA into DNA is relatively nonselective and error-prone, leading to a high mutation rate. Its lack of selectivity is exploited by the anti-HIV drug AZT (3 -azido-2, 3 -dideoxythymidine), which becomes phosphorylated and is incorporated by reverse transcriptase into DNA, where it acts as a chain terminator. Mammalian DNA polymerases are more selective, having a low affinity for AZT, so its toxicity is relatively low. 

A major breakthrough that has facilitated the identification of active mutant DNA polymerases has been the use of E. coli recA718polA12, a bacteria that encodes a temperature-sensitive mutant DNA polymerase I. At elevated temperatures the mutant E. coli fails to form colonies unless complemented by a DNA polymerase that can effectively substitute for DNA polymerase I [34], The E. coli recA71S polA12 strain was first utilized in identification of active mutants of rat DNA Pol-/ and then in analyzing mutations in HIV reverse transcriptase [35], In both situations, complementation required very active mutant enzymes and it was only feasible to screen libraries that contained thousands of mutant genes. 

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