Au4 cluster inhibits human thioredoxin reductase activity via specifically binding of Au to Cys189

Studies have shown that Au triggers tumor cell apoptosis by directly inhibiting human thioredoxin reductase (hTrxR), thus making Au potential drugs for cancer treatment. However, the exact molecular mechanism of Au inhibiting hTrxR has never been demonstrated via experimental and theoretical studies. Here, we synthesized an Au4 cluster coated by an artificial RGD peptide. This gold cluster can bind with hTrxR (KD: ∼1.74 µM) in vitro, and significantly inhibit hTrxR activity in tumor cells. To analyze the underlying mechanism, we established hTrxR-overexpressing tumor cells and fished Au-hTrxR adducts from tumor cells previously treated by Au clusters. Using single-particle cryo-electron microscopy (Cryo-EM) and mass spectra (MS), we found unexpectedly that, Au (I) ion, metabolism of Au cluster, specifically binds to the Cys189 of hTrxR. Molecular dynamic simulation revealed such specific Au-Cys189 binding effectively broke the electron transfer from Sec498 of hTrxR to Cys32 of thioredoxin. This inhibition of hTrxR activity by Au4 cluster induced ROS-regulated tumor cell apoptosis and significantly suppressed tumor growth in an A549 lung cancer xenograft model. In summary, this study revealed the structural mechanism of Au inhibiting hTrxR activity and shed light to the design and application of novel hTrxR-inhibitory anti-tumor nanodrugs.