Insight into How Telomeric G-Quadruplexes Enhance the Peroxidase Activity of Cellular Hemin

The toxic oxidative damage of G-quadruplexes (G4), linked to neurodegenerative diseases, may arise from their ability to bind and oxidatively activate cellular hemin. However, there have been no precise studies on how telomeric G4 enhances the low intrinsic peroxidase activity of hemin. Herein, a label-free and nanopore-based strategy was developed to explore the enhancement mechanism of peroxidase activity of hemin induced by telomeric G4 (d(TTAGGG)_n ). The nanopore-based strategy demonstrated that there were simultaneously two different binding modes of telomere G4 to hemin. At the single-molecule level, it was found that the hybrid structural telomeric G4 directly binds to hemin (the affinity constant (K_a )≈10⁶  m^-1 ) to form a tight complex, and some of them underwent a topological change to a parallel structure with an enhancement of K_a to approximately 10⁷  m^-1 . Through detailed analysis of the topology and peroxidase activity and molecular modeling investigations, the parallel telomere G4/hemin DNAzyme structure was proven to be preferable for high peroxidase activity. Upon strong π-π stacking, the parallel structural telomere G4 supplied a key axial ligand to the hemin iron, which accelerated the intermediate compound formation with H₂ O₂ in the catalytic cycle. Our studies developed a label-free and single-molecule strategy to fundamentally understand the catalytic activity and mechanism of telomeric DNAzyme, which provides some support for utilizing the toxic, oxidative-damage property in cellular oxidative disease and anticancer therapeutics.