Mechanisms of substrate selectivity for Bacillus anthracis thymidylate kinase

Bacillus anthracis is well known in connection with biological warfare. The search for new drug targets and antibiotics is highly motivated because of upcoming multiresistant strains. Thymidylate kinase is an ideal target since this enzyme is at the junction of the de novo and salvage synthesis of dTTP, an essential precursor for DNA synthesis. Here the expression and characterization of thymidylate kinase from B. anthracis (Ba-TMPK) is presented. The enzyme phosphorylated deoxythymidine-5'-monophosphate (dTMP) efficiently with K (m) and V (max) values of 33 microM and 48 micromol mg(-1) min(-1), respectively. The efficiency of deoxyuridine-5'-monophosphate phosphorylation was approximately 10% of that of dTMP. Several dTMP analogs were tested, and D-FMAUMP (2'-fluoroarabinosyl-5-methyldeoxyuridine-5'-monophosphate) was selectively phosphorylated with an efficiency of 172% of that of D-dTMP, but L-FMAUMP was a poor substrate as were 5-fluorodeoxyuridine-5'-monophosphate (5FdUMP) and 2',3'-dideoxy-2',3'-didehydrothymidine-5'-monophosphate (d4TMP). No activity could be detected with 3'-azidothymidine-5'-monophosphate (AZTMP). The corresponding nucleosides known as efficient anticancer and antiviral compounds were also tested, and d-FMAU was a strong inhibitor with an IC(50) value of 10 microM, while other nucleosides--L-FMAU, dThd, 5-FdUrd, d4T, and AZT, and 2'-arabinosylthymidine--were poor inhibitors. A structure model was built for Ba-TMPK based on the Staphylococcus aureus TMPK structure. Docking with various substrates suggested mechanisms explaining the differences in substrate selectivity of the human and the bacterial TMPKs. These results may serve as a start point for development of new antibacterial agents.     

3'-Azido-3'-deoxythymidine-(5')-tetraphospho-(5')-adenosine, the product of ATP-mediated excision of chain-terminating AZTMP, is a potent chain-terminating substrate for HIV-1 reverse transcriptase

The resistance of HIV-1 to 3'-azido-3'-deoxythymidine (AZT) involves phosphorolytic excision of chain-terminating AZT-5'-monophosphate (AZTMP). Both pyrophosphate (PPi) and ATP act as excision substrates in vitro, but the intracellular substrate used during replication of AZT-resistant HIV is still unknown. PPi-mediated excision produces AZT-5'-triphosphate (AZTTP), which could be immediately re-used as a substrate for viral DNA chain termination. In contrast, ATP-mediated excision produces the novel compound AZT-(5')-tetraphospho-(5')-adenosine (AZTp4A). Since little is known of the interaction of AZTp4A with HIV-1 RT, we carried out kinetic and molecular modeling studies to probe this. AZTp4A was found to be a potent inhibitor of HIV-1 RT-catalyzed DNA synthesis and of both ATP- and PPi-mediated AZTMP excision. AZTp4A is in fact an excellent chain-terminating substrate for AZT-resistant RT-catalyzed DNA synthesis, better than AZTTP (k(pol)/Kd = 6.2 and 11.9 for AZTTP and AZTp4A, respectively). The affinity of AZT-resistant HIV-1 RT for AZTp4A is at least 30,000-fold greater than that for the excision substrate ATP and approximately 10-fold greater than that for AZTTP. Dissociation of newly formed AZTp4A from RT may therefore provide a significant rate-limiting step for continued HIV-1 DNA synthesis. Our studies show that the products of PPi- and ATP-mediated excision of chain-terminating AZTMP (AZTTP and AZTp4A, respectively) are both potent chain-terminating substrates for HIV-1 RT, suggesting that there is no obvious benefit to HIV using ATP instead of PPi as the excision substrate.