Inhibition Effectiveness of Au Compounds

Inhibition Effectiveness of Au Compounds Function of the â€Å"Guiding Bar† on Inhibition Effectiveness of Au Compounds on Thioredoxin Reductase 3 Qi Liu Introduction Mammalian thioredoxin reductase (TR) is an Nicotinamide adenine dinucleotide phosphate (NADPH) dependent flavoprotein oxidoreductase, which is involved in reducing the disulfide of thioredoxin (Trx) using NADPH (Figure 1). Trx can reduce many important proteins, such as ribonucleotide reductase (RNR), which produces deoxyribonucleotides for DNA synthesis1, 2. Thus, TR and Trx play an important role in maintaining proteins in their reduced state, which involves the regulation of cellular redox reactions, proliferation, and differentiation3. In addition, Trx and TR is overexpressed in a lot of aggressive tumors, and the tumor progression and metastasis appear to be dependent on the Trx system, because tumor cells need constant DNA synthesis. However, tumor progression and metastasis were dramatically reduced when TR knockdown cells were injected into mice4. Thus, the TR was proposed to be a new anticancer drug target5. In mammals, there are three different TRs which have been characterized: the cytosolic form TR1, the mitochondrial form TR3, and thioredoxin glutathione reductase (TGR, also known as TR2)6. All of these TRs are selenoproteins with a selenocysteine (Sec) at their C-terminal active site. In the reaction mechanism of TR, the selenolate of Sec acts as an electron donor to Trx, also selenium can accept electrons from the N-terminal redox center of TR6. This redox center is located on a flexible arm, which is solvent-exposed and reactive towards electrophilic inhibitors, thus representing a target for antitumor drug development1. A number of recent reports suggest that gold-based drugs have strong inhibition ability on TRs, which could be from the direct coordination between Au and the active site Sec on TRs, so that the selenolate group is blocked7. Interestingly, the gold compounds,Bis[1,2-bis(dipyridylphosphino)ethane]gold(I) chloride ([Au(d2pype)2]Cl) and Bis[1,3-bis(di-2-pyridylphosphino)propane]gold(I) chloride ([Au(d2pypp)2]Cl), effect the inhibition effectiveness on human TR1(hTR1) and human TR3 (hTR3) differently (Figure2). The precise molecular mechanism of TR inhibition by gold compounds has not been elucidated. Thus, studies on inhibition of TRs by gold-based drugs are necessary for designing new cancer inhibitors. Available crystal structures of TRs include hTR1, rat TR1 (rTR1), mouse TR3(mTR3), DmTR (thioredoxin reductase from D. melanogaster), and PfTR (thioredoxin reductase from P. falciparum) enzymes9. A recent study showed the crystal structure of hTR1 with its substrate thioredoxin, in which the C-terminal tail of hTR1 was stabilized by a â€Å"guiding bar†, so that the flexible C-terminal tail can be observed (Figure 3). The guiding bar was first proposed in one of the structure studies of hTR1, and is composed of three amino acids: Trp407, Asn418, and Asn419. It functions to suppress random motions and positions the C-terminal tail in catalytically competent position through hydrogen bonding interactions9. However, the crystal structure of mTR3 does not show the C-terminus because of the random motion of the C-terminal without control from the guiding bar. Furthermore, the sequence comparison between hTR1 and hTR3 shows the absence of the guiding bar in TR3 (Figure 4). Research question Though inhibition of gold compounds on TRs has been studied, the function of the guiding bar on the gold compounds inhibition effectiveness on TRs has never been brought up to the studies of inhibition effectiveness on TRs. The goal of this proposal is to investigate the role of the guiding bar on the inhibition ability of gold based compounds on hTR1 and hTR3. The guiding bar will be constructed on hTR3 by mutations of K432W, A443N and S444N. Biophysical characterization will be applied to observe the presence of the C-terminus on TR3 after mutagenesis. My hypothesis is the C-terminal tail of the crystal structure of mutated TR3 will be observed, because the random motion of the C-terminus is reduced by restriction from the guiding bar. Then, two gold compounds, [Au(d2pype)2]Cl and [Au(d2pypp)2]Cl , will be applied to test the inhibition difference on normal TR3 and mutated TR3. I expect to see that the inhibition on mutated TR3 could be stronger than that on normal TR3. Significance of Proposed Research The thioredoxin system plays an important role in the intracellular redox enviornment and is composed of Trx and TR10. Proposed studies will give an insight into the function of the guiding bar on different TR enzymes. The guiding bar is expected to suppress the random motion of the C-terminal tail so that the redox center Sec on the C-terminus can have better coordination with the gold compounds. The coordination between metal and Sec will tightly block the Sec redox center, so the intracellular redox balance will be disturbed and result in potent TR inhibition4. Thus, the presence of the guiding bar can help TR to be a better drug target. Proposed studies Enzyme Expression and Purification Recombinant human TR1 and TR3 will be cloned into E. coli BL21(DE3) cells, and the growth and purification will be done as described previously.11 Mutation of Human TR3 In order to study the impact of the guiding bar residues on TR catalysis, the proposed mutations will be constructed on hTR3 with K432W, A443N, and S444N. These three mutation sites are chosen based on the sequence alignment of hTR1 in the guiding bar region of Pro376-Tyr422 (Figure 3), in which the critical amino acids, Trp407, Asn418 and Asn419, have the function of limiting random motion of the C-terminus. Thus, the same function of the guiding bar on hTR3 is expected to be observed via specific site mutation. Mutations will be introduced by the Quick Change mutagenesis method and confirmed by DNA sequencing. Structural Studies of Mutated Human TR3 The C-terminal tail in human and rat TR1 could be observed by X-ray crystallography, because the guiding bar limits its random motion. However, because the guiding bar is not present in mouse TR3, the C-terminal tail is too mobile to generate sufficient electron density without restriction from the guiding bar. This leads to the absence of the C-terminal tail in the crystal structure of mouse TR39. The mutated hTR3 after insertion of the guiding bar will be studied by X-ray crystallography. The presence of an ordered C-terminal tail in the crystal structure and the interaction between the guiding bar and the C-terminal tail will suggest restricted motion of the C-terminal tail. I expect to see that the interaction of the guiding bar on the C-terminal tail will improve the inhibition effectiveness of Au compounds on hTR3. Electrospray ionization mass spectroscopy (ESIMS) Characterization ESI is an ionization technique which is used to detect high molecular weight molecules, such as proteins, peptides and other macromolecules, so ESI MS is an important technique for studying a complex biological sample9. Instead of fragmenting the macromolecules into smaller charged particles, this process turns the macromolecules into small droplets by ionization, and these droplets are further desolvated into even smaller droplets, which creates molecules with attached protons12. As a putative target for anticancer metallodrugs, the selenoenzyme TR is the drug target for gold compounds, such as [Au(d2pype)2]Cl and [Au(d2pypp)2]Cl. The TR inhibition is thought to occur through direct binding of the gold to the active site Sec following ligand substitution. So, TR3 and mutated TR3 will be separately incubated with the two gold compounds mentioned above for 30 mins at room temperature, and then the products will be analyzed by EIS-MS. The results will be compared and are expected to explain the binding of the gold onto the hTRs. I hope to see the gold compounds have better binding on mutated hTR3 than normal hTR3. This can be explained as the guiding bar can reduce the motion of flexible C-terminal tail, so that the gold compounds can bind onto the Sec of mutated hTR3 easier than the normal one and cause more inhibition on the mutated hTR313. TR Inhibition Assay The solvent-accessible selenolate group, arising from enzyme reduction, very likely constitutes a high affinity binding site for gold compounds. Tightly blocking the active site Sec through metal coordination should result in potent TR inhibition9. Thus, after ESI-MS characterization, the TR inhibition study will be performed with the two different gold compounds, [Au(d2pype)2]Cl and [Au(d2pypp)2]Cl, on hTR1, hTR3 and mutated hTR3 respectively. In addition to Trx, some low molecular weight disulfide-containing substrates, including 5,5†²-dithiobis-(2-nitrobenzoic acid) (DTNB), lipoic acid, and lipoamidem, can also be reduced by TRs9. Here, DTNB would be chosen as the substrate for TR inhibition assay to keep consistent with the previous studies from other groups9. The NADPH dependent TR catalyzed reduction of DTNB will be monitored and determined by the increase in absorbance at 412nm. Inhibition of hTR1, hTR3 and mutated hTR3 will be done by using different concentrations of go ld compounds ([Au(d2pype)2]Cl and [Au(d2pypp)2]Cl). The inhibition results from different TRs and gold compounds will be compared. Based on the assumption that the guiding bar can suppress the motion of the C-terminal tail, greater inhibition effectiveness on mutated hTR3 than that on normal hTR3 is expected to be seen. This will show that the C-terminus is limited by the guiding bar on mutated hTR3 helps the gold compounds inhibit the active site Sec more. Binding Studies by Isothermal Titration Calorimetry (ITC) ITC is a technique used to determine the thermodynamic parameters of interaction in solution. It can be applied to study the interactions between protein-protein, enzyme-inhibitor, protein-small molecules, protein-DNA, and so on. Thus, the binding between gold compounds and the three TRs (hTR1, hTR3 and mutated hTR3) can be investigated by ITC. Applying this method will tell us how well the inhibitors, the gold compounds, bind to TRs. The binding affinity (Ka) could show how strongly the gold compounds and TRs will bind, and the higher Ka, the stronger the binding. In addition, enthalpy changes (ΔH) can tell the amount of energy released or obtained. Then, the entropy change (ΔS) and Gibbs energy changes (ΔG) can be determined by the following equation (1): ΔG = -RTlnKa = ΔH-TΔS (1) Comparison of the binding parameters between gold compounds ([Au(d2pype)2]Cl and [Au(d2pypp)2]Cl) and TRs (hTR1, hTR3 and the mutated hTR3) will give more information about the function of the guiding bar on the inhibition effectiveness of TRs. The higher Ka of hTR1 compared to normal hTR3 is expected to be seen, because the absence of guiding bar in hTR3 will lead to a flexible C-terminal tail, which will give a lower inhibition. In addition, Ka of the mutated hTR3 is expected to show higher binding affinity than the normal hTR3, because the guiding bar mutated on hTR3 will help reduce the random motion of the C-terminal tail, so that the redox center Sec can be easily inhibited by the gold compounds. Thus, the guiding bar can help improve the inhibition effectiveness of gold compounds on mutated hTR3. Conclusion Through mutation and structural studies, the presence of the C-terminal tail which is restricted by the inserted guiding bar is expected to be seen in the mutated hTR3. The ESI-MS characterization and binding study can give information about if the gold can successfully bind to the specific active site on the mutated hTR3. Finally, a better understanding of the inhibition effectiveness will be tested and compared by the inhibition assay with hTR1, hTR3 and mutated hTR3. Thus, a better understanding of the function of the guiding bar in the TR system will give an insight into the effect of the guiding bar on the drug inhibition effect on TR3 so that it can become a better drug target. References O. Rackham., A. M. Shearwood., R. Thyer., E. McNamara., S. M. Davies., B. A. Callus., A. Miranda-Vizuete., S. J. Berners-Price., Q. Cheng., E. S. J. Arner and A. 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