# Metal Nanozymes: New Horizons in Cellular Homeostasis Regulation
## 4 Strategies in the Quest for New Nanozymes
Specific steric features of the matrix of the nanozymes with active metallic centers can bring other effects allowing for targeted selectivity of the obtained nanoparticles. For example, upon being entrapped on the amine-terminated PAMAM dendrimer (AuNCs-NH\({}_{2}\)), gold nanoclusters unexpectedly lost their peroxidase-like activity while still retaining their CAT-like activity in physiological conditions [105]. The possible mechanism was proposed that the enrichment of polymeric 3\({}^{\circ}\)-amine on the surface of AuNCs-NH\({}_{2}\) (or AuNCs-OH) provided sufficient suppression of the critical mediator \({}^{\bullet}\)OH, which is responsible for the peroxidase-like activity. Because their CAT-like activity was superior to that of AuNCs-OH and their hidden peroxidase-like activity diminished cytotoxic effects, AuNCs-NH\({}_{2}\) were further chosen for H\({}_{2}\)O\({}_{2}\)-mediated cytotoxicity tests in primary neuronal cells. Testing indicated a potential for the use of AuNCs-NH\({}_{2}\) in protecting primary neuronal cells against H\({}_{2}\)O\({}_{2}\)-induced toxicity. Many metallic NPs can display multienzymatic activity, as it is in the case of Pt nanoparticles, which serve as both CAT and SOD mimics [59, 106, 107, 108]. Simultaneous expression of multiple antioxidant enzymes is more effective than single or double expression in combating oxidative stress [109]. It has been reported that CAT-GPx cooperativity is important for the proper control of H\({}_{2}\)O\({}_{2}\) levels under pathophysiological conditions. While CAT is responsible for the removal of excess H\({}_{2}\)O\({}_{2}\) during oxidative stress, GPx is known to fine-tune the concentration of H\({}_{2}\)O\({}_{2}\) for cell signaling. Unlike CAT activity that is common for many metal and metal oxide materials, GPX is a selenium-based enzyme. GPx is a GSH-dependent antioxidant enzyme that can catalyze peroxides such as H\({}_{2}\)O\({}_{2}\) to form nontoxic products in the presence of glutathione (GSH) [110, 111]. Thus, it plays an important role in the maintenance of intracellular redox homeostasis. Until recently, mimicking GPx has mainly focused on designing selenium-based organic molecules [112]. However, these molecules can have disadvantages such as complicated preparation process, high toxicity, low cycle efficiency, etc. Therefore, the development of new and biocompatible GPx mimics is of great importance [113, 114, 115, 116]. [PERSON], [PERSON], and colleagues also discovered that V\({}_{2}\)O\({}_{5}\) nanowires could as well mimic GPx activity by using cellular GSH [115]. Although bulk V\({}_{2}\)O\({}_{5}\) is known to be toxic to the cells, the property is altered when converted into a nanomaterial form. The vanadium-based nanozymes were readily internalized into mammalian cells and fully restored the redox balance without perturbing the cellular antioxidant defense. The potential of the biocompatible V\({}_{2}\)O\({}_{5}\) nanowires in the treatment of aging, heart disease, and many neurological diseases was thus shown. This work was a breakthrough in mimicking GPXs by the non-selenium-based materials. [PERSON], [PERSON], et al. [116] reported that Mn\({}_{3}\)O\({}_{4}\) nanoparticles with flower-like morphology (Mnf) could potentially exhibit the activities of all three primary antioxidant enzymes namely SOD, CAT, and GPX. They studied the GPx-like activity of Mnf using the classical coupled assay system involving glutathione reductase (GR) and NADPH to confirm the formation of GSSG. The catalytic properties of Mnf, investigated using steady-state kinetics by independently varying the concentrations of H\({}_{2}\)O\({}_{2}\) (0-1.8 mm) and GSH (0-5.0 mm), followed the Michaelis-Menten kinetics. A proportional dependence of the initial rate corresponding to the first-order kinetics was observed as the concentration of Mnf was varied by keeping the concentration of other reactants constant.