Heme in its ferrous and ferric states [heme(Fe) and heme(Fe), respectively] binds selectively to the 3'-terminal G-quartet of all parallel-stranded monomeric G-quadruplex DNAs formed from inosine(I)-containing sequences, i.e., d(TAGGGTGGGTTGGGTGIG) DNA(18mer) and d(TAGGGTGGGTTGGGTGIGA) DNA(18mer/A), through a π-π stacking interaction between the porphyrin moiety of the heme and the G-quartet, to form 1:1 complexes [heme-DNA(18mer) and heme-DNA(18mer/A) complexes, respectively]. These complexes exhibited enhanced peroxidase activities, compared with that of heme(Fe) alone, and the activity of the heme(Fe)-DNA(18mer/A) complex was greater than that of the heme(Fe)-DNA(18mer) one, indicating that the 3'-terminal A of the DNA sequence acts as an acid-base catalyst that promotes the catalytic reaction. In the complexes, a water molecule (HO) at the interface between the heme and G-quartet is coordinated to the heme Fe atom as an axial ligand and possibly acts as an electron-donating ligand that promotes heterolytic peroxide bond cleavage of hydrogen peroxide bound to the heme Fe atom, trans to the HO, for the generation of an active species. The intermolecular nuclear Overhauser effects observed among heme, DNA, and Fe-bound HO indicated that the HO rotates about the HO-Fe coordination bond with respect to both the heme and DNA in the complex. Thus, the HO in the complex is unique in terms of not only its electronic properties but also its dynamic ones. These findings provide novel insights into the design of heme-deoxyribozymes and -ribozymes.