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A hemin/G-quadruplex acts as an NADH oxidase and NADH peroxidase mimicking DNAzyme.

Appropriately ordered guanosine bases in DNA chains form G-quartets stabilized by interbase hydrogen bonds. These Gquartets are further stacked, in the presence of ions, into a Gquadruplex structure. Different organic molecules are known to interact with G-quadruplex structures. Specifically, hemin was found to bind to a G-quadruplex, and the resulting complex revealed horseradish peroxidise mimicking activities. For example, the hemin/G-quadruplex catalyzes the H2O2-mediated oxidation of the 2,2’-azino-bis(3-ethylbenzthiazoline-6-sulfonate dianion) (ABTS ) to the colored product ABTSC [3] or leads to the generation of chemiluminescence in the presence of H2O2/luminol. Also, the hemin/ G-quadruplex structure linked to electrodes was reported to act as an electrocatalyst for the reduction of H2O2, [5] and the hemin/G-quadruplex associated with semiconductor quantum dots was found to quench the luminescence of the quantum dots. These properties enabled the use of the hemin/Gquadruplex as a versatile label for numerous sensors including enzyme-, DNA-, and aptamer-based sensors. In the present study, we demonstrate that the hemin/G-quadruplex acts not only as a horseradish peroxidase mimicking DNAzyme, but, also, as an NADH oxidase and NADH peroxidase mimicking DNAzyme. The regeneration of the nicotinamide adenine dinucleotide, NAD, cofactor attracted substantial interest for biotechnological applications, and NAD-dependent enzymes have been widely used for chemical transformations. Our results pave the way to use the hemin/Gquadruplex as a biocatalyst for the regeneration of the NAD cofactor and to apply the DNAzyme as a catalyst for enzymedriven transformations. NADH oxidase catalyzes the oxidation of NADH by O2 with the concomitant formation of H2O2. [11] The present system consists of reduced nicotinamide adenine dinucleotide (NADH), hemin/G-quadruplex (1), and Amplex Red (2), as a fluorescent reporter dye (Scheme 1A). Under aerobic conditions, NADH is oxidized to NAD (see below), while 2 is oxidized to resorufin (3), which generates fluorescence at lmax= 581 nm. Figure 1A, curve (a), shows the time-dependent increase in the fluorescence generated by the system. Control experiments reveal minor background fluorescence generated under an inert argon atmosphere (curve (e)), or upon exclusion of hemin from the system (curve (f)). Also, only a low fluorescence intensity is generated in the presence of only hemin and in the absence of 1 (curve (d)), or in the presence of hemin and a foreign nucleic acid (4) that cannot form the respective G-quadruplex (curve (c)). These results may suggest that the hemin/G-quadruplex-catalyzed oxidation of NADH by O2 yields H2O2, and the generated fluorescence occurs by the well-established hemin/G-quadruplex DNAzyme-catalyzed oxidation of 2 by H2O2. [12] Indeed, the intermediate formation of H2O2 in the system was confirmed by the addition of catalase. Figure 1A, curve (g), confirms that in the presence of catalase the fluorescence is blocked, consistent with the decomposition of the generated H2O2. Thus, in the first step, the hemin/Gquadruplex DNAzyme acts as an NADH oxidase, where NADH is oxidized by O2 to form NAD . It should be noted that the fluorescent resorufin (3) is formed also in the absence of NADH (Figure 1A, curve (b)). Presumably, Amplex Red by itself has a residual donor activity that participates in the activation of O2 (substituting NADH). The low fluorescence intensity changes should be considered as the background signal of the system. Scheme 1. A) Frame I: Hemin/G-quadruplex-catalyzed oxidation of NADH by O2 to NAD + and H2O2, respectively, and the concomitant oxidation of Amplex Red (2) to resorufin (3) by H2O2. Frame II: Coupled DNAzyme–alcohol dehydrogenase biocatalytic system for the formation of NADH and the concomitant regeneration of the NAD cofactor by the hemin/G-quadruplex. B) Suggested mechanism for the DNAzyme-catalyzed oxidation of NADH.