Effect of valine 106 on structure-function relation of cytosolic human thymidine kinase

New Variants of Tomato Thymidine Kinase 1 Selected for Increased Sensitivity of E. coli KY895 towards Azidothymidine

Nucleoside analogues (NA) are prodrugs that are phosphorylated by deoxyribonucleoside kinases (dNKs) as the first step towards a compound toxic to the cell. During the last 20 years, research around dNKs has gone into new organisms other than mammals and viruses. Newly discovered dNKs have been tested as enzymes for suicide gene therapy. The tomato thymidine kinase 1 (ToTK1) is a dNK that has been selected for its in vitro kinetic properties and then successfully been tested in vivo for the treatment of malignant glioma. We present the selection of two improved variants of ToTK1 generated by random protein engineering for suicide gene therapy with the NA azidothymidine (AZT).We describe their selection, recombinant production and a subsequent kinetic and biochemical characterization. Their improved performance in killing of E. coli KY895 is accompanied by an increase in specificity for the NA AZT over the natural substrate thymidine as well as a decrease in inhibition by dTTP, the end product of the nucleoside salvage pathway for thymidine. The understanding of the enzymatic properties improving the variants efficacy is instrumental to further develop dNKs for use in suicide gene therapy.

Thymidine kinase 1 regulatory fine-tuning through tetramer formation

Thymidine kinase 1 (TK1) provides a crucial precursor, deoxythymidine monophosphate, for nucleic acid synthesis, and the activity of TK1 increases by up to 200-fold during the S-phase of cell division in humans. An important part of the regulatory checkpoints is the ATP and enzyme concentration-dependent transition of TK1 from a dimer with low catalytic efficiency to a tetramer with high catalytic efficiency. This regulatory fine-tuning serves as an additional control to provide a balanced pool of nucleic acid precursors in the cell. We subcloned and over-expressed 10 different TK1s, originating from widely different organisms, and characterized their kinetic and oligomerization properties. Whilst bacteria, plants and Dictyostelium only exhibited dimeric TK1, we found that all animals had a tetrameric TK1. However, a clear ATP-dependent switch between dimer and tetramer was found only in higher vertebrates and was especially pronounced in mammalian and bird TK1s. We suggest that the dimer form is the original form and that the tetramer originated in the animal lineage after the split of Dictyostelium and the lineages leading to invertebrates and vertebrates. The efficient switching mechanism was probably first established in warm-blooded animals when they separated from the rest of the vertebrates.

Characterization of oligomeric and kinetic properties of tomato thymidine kinase 1

The gene encoding thymidine kinase 1 from tomato (toTK1) has in combination with azidothymidine (AZT) recently been proposed as a powerful suicide gene for anticancer gene therapy. The toTK1/AZT combination has been demonstrated to have several advantages for the treatment of glioblastomas because AZT can easily penetrate the blood-brain barrier and toTK1 can efficiently phosphorylate AZT and also AZT-monophosphate. In a pursuit to further understand the properties of toTK1, we examined the oligomerization properties of recombinant toTK1 and its effect on enzyme kinetics. Previously, it has been shown that human TK1 is a dimer in the absence of ATP and a tetramer if preincubated with ATP. However, we show here that ATP preincubation did not result in a structural shift from dimer to tetramer in toTK1. For human TK1 pretreated with ATP, the K(m) value decreased 20-fold, but toTK1's K(m) value did not show a dependence on the presence or absence of ATP. Furthermore, toTK1 was always found in a highly active form.

Mass action models versus the Hill model: an analysis of tetrameric human thymidine kinase 1 positive cooperativity

Background

The Hill coefficient characterizes the extent to which an enzyme exhibits positive or negative cooperativity, but it provides no information regarding the mechanism of cooperativity. In contrast, models based on the equilibrium concept of mass action can suggest mechanisms of cooperativity, but there are often many such models and often many with too many parameters.

Results

Mass action models of tetrameric human thymidine kinase 1 (TK1) activity data were formed as pairs of plausible hypotheses that per site activities and binary dissociation constants are equal within contiguous stretches of the number of substrates bound. Of these, six 3-parameter models were fitted to 5 different datasets. Akaike's Information Criterion was then used to form model probability weighted averages. The literature average of the 5 model averages was K = (0.85, 0.69, 0.65, 0.51) μM and k = (3.3, 3.9, 4.1, 4.1) sec^-1 where K and k are per-site binary dissociation constants and activities indexed by the number of substrates bound to the tetrameric enzyme.

Conclusion

The TK1 model presented supports both K and k positive cooperativity. Three-parameter mass action models can and should replace the 3-parameter Hill model.

Reviewers

This article was reviewed by Philip Hahnfeldt, Fangping Mu (nominated by William Hlavacek) and Rainer Sachs.

Effective program against mother-to-child transmission of HIV at Saint Camille Medical Centre in Burkina Faso

The present research was aimed to prevent mother-to-child transmission of HIV; to use RT-PCR in order to detect, 6 months after birth, infected children; and to test the antiretroviral resistance of both children and mothers in order to offer them a suitable therapy. At the Saint Camille Medical Centre, 3,127 pregnant women (aged 15-44 years) accepted to be enrolled in the mother-to-child transmission prevention protocol that envisages: (i) Voluntary Counselling and Testing for all the pregnant women; (ii) Antiretroviral therapy for HIV positive pregnant women and for their newborns; (iii) either powdered milk feeding or short breast-feeding and RT-PCR test for their children; (iv) finally, pol gene sequencing and antiretroviral resistance identifications among HIV positive mothers and children. Among the patients, 227/3,127 HIV seropositive women were found: 221/227 HIV-1, 4/227 HIV-2, and 2/227 mixed HIV infections. The RT-PCR test allowed the detection of 3/213 (1.4%) HIV infected children: 0/109 (0%) from mothers under ARV therapy and 3/104 (2.8%) from mothers treated with Nevirapine. All children had recombinant HIV-1 strain (CRF06_CPX) with: minor PR mutations (M36I, K20I) and RT mutations (R211K). Among them, two twins had Non-Nucleoside Reverse Transcriptase Inhibitor mutation (Y18CY). Both mothers acquired a major PR mutation (V8IV), investigated 6 months after a single-dose of Nevirapine. Prevention by single-dose of Nevirapine reduced significantly mother-to-child transmission of HIV, but caused many mutations and resistance to antiretroviral drugs. Based on present study the antiretroviral therapy protocol, together with the artificial-feeding, might represent the ideal strategy to avoid transmission of HIV from mother-to-child.

Purification and characterization of wild-type and mutant TK1 type kinases from Caenorhabditis elegans

Caenorhabditis elegans has a single deoxynucleoside kinase-like gene. The sequence is similar to that of human TK1, but besides accepting thymidine as a substrate, the C. elegans TK1 (CeTK1) also phosphorylates deoxyguanosine. In contrast to human TK1, the CeTK1 exclusively exists as a dimer with a molecular mass of approximately 60 kDa, even if incubated with ATP. Incubation with ATP induces a transition into a more active enzyme with a higher kcat but unchanged Km. This activation only occurs at an enzyme concentration in the incubation buffer of 0.5 micro g/ml (8.42 nM) or higher. C-terminal deletion of the enzyme results in lower catalytic efficiency and stability.

Effect of C-terminal of human cytosolic thymidine kinase (TK1) on in vitro stability and enzymatic properties

Thymidine kinase (TK1) is a key enzyme in the salvage pathway of nucleotide metabolism and catalyzes the first rate-limiting step in the synthesis of dTTP, transfer of a gamma-phosphate group from a nucleoside triphosphate to the 5'-hydroxyl group of thymidine, thus forming dTMP. TK1 is cytosolic and its activity fluctuates during cell cycle coinciding with the DNA synthesis rate and disappears during mitosis. This fluctuation is important for providing a balanced supply of dTTP for DNA replication.The cell cycle specific activity of TK1 is regulated at the transcriptional level, but posttranslational mechanisms seem to play an important role for the level of functional TK1 protein as well. Thus, the C-terminal of TK1 is known to be essential for the specific degradation of the enzyme at the G2/M phase. In this work, we have studied the effect of deletion of the C-terminal 20, 40, and 44 amino acids of TK1 on in vitro stability, oligomerization, and enzyme kinetics. We found that deletion of the C-terminal fold markedly increased the stability as well as the catalytic activity.

High-level expression and purification of human thymidine kinase 1: Quaternary structure, stability, and kinetics

Human cytosolic thymidine kinase (hTK1) is the key enzyme of the pyrimidine salvage pathway and phosphorylates thymidine to thymidine monophosphate, a precursor building block of the DNA. Wild-type hTK1 (hTK1W) as well as a truncated form of the enzyme (hTK1M) carrying deletions at the N- and C-terminal regions were cloned as His(6)-tagged fusion proteins. Expression, isolation, and purification protocols have been established, leading to high yields of soluble and active wild type (approximately 35 mg) and truncated hTK1 (approximately 23 mg) per liter of culture. The protein was purified to near homogeneity. The chaperone DnaK was identified to be the major contaminant that could be removed by applying an additional ATP-MgCl(2) incubation and washing step. hTK1W was a permanent tetramer in solution, whereas the truncated construct hTK1M appears to be a dimer in absence and presence of substrates. Both hTK1W and hTK1M exhibit pronounced thermal stability with transition temperatures (T(m)) of 71.7 and 73.4 degrees C, respectively, when measured without adding substrates. The presence of substrates stabilized both hTK1W (DeltaT(m) ranging from 5.6 to 12.5 degrees C) and hTK1M (DeltaT(m) ranging from 0.8 to 5.3 degrees C). Both enzymes show high activity over a broad range of pH, temperature, and ionic strength. Kinetic studies determined a K(M) of 0.51 microM and a k(cat) of 0.28 s(-1) for wild-type hTK1. The truncated hTK1M has a K(M) of 0.87 microM and k(cat) of 1.65 s(-1), thus exhibiting increased catalytic efficiency. The availability of recombinant human TK1 will facilitate further biochemical and crystallographic studies.

Structures of thymidine kinase 1 of human and mycoplasmic origin

Cytosolic thymidine kinase 1, TK1, is a well known cell-cycle-regulated enzyme of importance in nucleotide metabolism as well as an activator of antiviral and anticancer drugs such as 3'-azido-3'-deoxythymidine (AZT). We have now determined the structures of the TK1 family, the human and Ureaplasma urealyticum enzymes, in complex with the feedback inhibitor dTTP. The TK1s have a tetrameric structure in which each subunit contains an alpha/beta-domain that is similar to ATPase domains of members of the RecA structural family and a domain containing a structural zinc. The zinc ion connects beta-structures at the root of a beta-ribbon that forms a stem that widens to a lasso-type loop. The thymidine of dTTP is hydrogen-bonded to main-chain atoms predominantly coming from the lasso loop. This binding is in contrast to other deoxyribonucleoside kinases where specific interactions occur with side chains. The TK1 structure differs fundamentally from the structures of the other deoxyribonucleoside kinases, indicating a different evolutionary origin.