Bio-inspired Materials : Antioxidant and Phosphotriesterase Nanozymes
Vernekar, Amit A
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Bio-inspired or biomimetic chemistry deals with the replication of the nature’s fundamental processes, which can help in understanding the functioning of biological systems and develop novel applications. Although a large number of researchers worked towards the replication of natural synthetic pathways through biogenetic syntheses, enzyme mimicry by the small organic molecules and inorganic complexes emerged in leaps and bounds over the years. The development of biomimetic chemistry then continued in designing the molecules that can function like enzymes. And now, with the advent of nanotechnology, nanostructured materials have been shown to exhibit enzyme-like activities (nanozymes). Interestingly, the two distinct fields, biology and materials science, have been integrated to form an entirely new area of research that has captured a great attention. Along with the pronounced application of nanomaterials as drug delivery vehicles, anticancer agents, antimicrobials, etc., research is also focused on designing nanomaterials for the biomimetic applications. The thesis consists of five chapters. The first chapter provides a general overview of the recently discovered nanozymes that mimic heme-peroxidase, oxidase, superoxide dismutase, catalase, haloperoxidase and phosphatase. This chapter also deals with the nanozymes’ application in sensing and immunoassay, and as antioxidants, neuroprotective agents. The factors affecting the nanozymes’ activity and the challenges associated with them is also covered in this chapter. Chapter 2 is divided into two parts and it deals with the biomimetic properties of graphene-based materials. In part A, the remarkable peroxynitrite (PN) reductase and isomerase activities of hemin-functionalized reduced graphene oxide (rGO) is discussed. In part B, the activity of graphene oxide (GO) as peroxide substrate for the glutathione peroxidase (GPx) enzyme is discussed. In chapter 3, the oxidant material, V2O5, is shown to exhibit significant GPx-like antioxidant activity in its nano-form. Chapter 4 deals with the oxidase-like activity of MnFe2O4 nanooctahedrons for the antibody-free detection of major oxidative stress biomarker, carbonylated proteins. In chapter 5, the phosphotriesterase mimetic role of vacancy engineered nanoceria is discussed. instead of H2O2 for glutathione peroxidase (GPx) enzyme. As partial reduction of GO was observed when treated with GPx enzyme due to the fact that large sheet-like structures cannot be accessible to the active site, we studied the reaction with some GPx mimetics (Fig. 2). Varying the concentration of cofactor glutathione (GSH) required for the reaction, GPx mimic, ditelluride, could accomplish the reduction of GO following Michaelis-Menten kinetics. As the structure of GO is elusive and under active investigation, our study highlights the presence of peroxide linkages as integral part of GO other than hydroxyl, epoxy and carboxylic groups. This study also highlights an important fact that the modification of GO by biologically relevant compounds such as redox proteins must be taken into account when using GO for biomedical applications because such modifications can alter the fundamental properties of GO. Figure 2. The GO reductase and decarboxylase activities of GPx mimetic ditelluride compound, suggesting the presence of peroxide linkages on GO. In chapter 3, we have discussed about the novel antioxidant nanozyme that combats oxidative stress. During our attempts in the investigation of antioxidant nanozymes, we surprisingly noticed that the oxidant material, V2O5, shows significant GPx-like antioxidant activity in its nano-form. The Vn readily internalize in the cells and exhibit remarkable protective effects when challenged against reactive oxygen species (ROS). Although Vn has been shown to protect cells from ROS-induced damage, cells treated with bulk V2O5 and few vanadium complexes resulted in generation of ROS and severe toxicity. Detailed investigation on the mechanism of this interesting phenomenon Chapter 4 deals with the development of novel methodology for detection of biomarkers. Inspired by the use of antibodies and enzymes for detection of a specific antigen, we have shown for the first time that the nanozymes can entirely replace antibodies and enzymes in Enzyme-linked Immunosorbent Assays (ELISA). As a specific example, we focused on the antibody-free detection of chief oxidative stress biomarker, carbonylated proteins, as our target. To achieve this, we designed MnFe2O4 nanooctahedrons that can function as oxidase enzyme and form signaling point of detection. We functionalized MnFe2O4 nanooctahedrons with hydrazide terminating groups so that carbonylated proteins can be linked to nanozymes by hydrazone linkage (Fig. 4a). Treatment of various carbonylated proteins (hemoglobin (Hb), Myoglobin (Mb), Cytochrome c (Cyt c), RNase and BSA) coated in well plate with hydrazide-terminated MnFe2O4 nanooctahedrons and then with 3,3’,5,5’-tetramethylbenzidine substrate, resulted in instantaneous detection by well plate reader (Fig. 4b). Considering the challenges and difficulties associated with the conventional methods used to detect such modified proteins, this methodology opens up a new avenue for the simple, cost-effective, instantaneous and entirely antibody-free ELISA-type detection of carbonylated proteins. Our results provide a cumulative application of nanozymes’ technology in oxidative stress associated areas and pave a new way for direct early detection of post translational modification (PTM) related diseases. Figure 4. a) Nanozyme linked to the carbonylated protein coated on a plate through hydrazone linkage. b) General bar diagram showing detection of oxidized (carbonylated) proteins by nanozymes. Synopsis Figure 5. a) A cartoon view of surface of ceria showing vacancy. b) Zoomed portion of high resolution transmission electron microscopic image showing few vacancies on the surface of nanoceria. c) Catalytic mechanism of detoxification of paraoxon at the defect site. In the final chapter, chapter 5, we have discussed about the nanomaterial that can function as phosphotriesterase enzyme. Phosphotriesterase enzyme is a bacterial enzyme that is involved in the rapid hydrolysis of sarin gas-related deadly nerve agents such as paraoxon, parathion and malathion. When encountered with these orgnaophospatetriesters, living beings tend to undergo nerve shock to cause paralysis by inhibiting an extremely important enzyme called acetylcholine esterase. They are also known to cause severe oxidative stress problems and are associated with neurodegenerative disorders. Therefore, curbing the toxic effects and detoxification of these nerve agents is a world-wide concern and many research teams have focused their attention to address this important problem. Working on the development of nanozymes for important problems, we found that nanoceria, especially the vacancy engineered one (Fig. 5a,b), can serve as active mimic of phosphotriesterase enzyme in the presence of N-methylmorpholine (acting as a distal base histidine). Vacancy engineered nanoceria has been shown to catalyze the hydrolysis of high amounts of paraoxon quiet efficiently and within few minutes with very low activation energy and high kcat. Detailed mechanistic investigation revealed that the presence of both Ce(III) and Ce(IV) is very essential for detoxification activity (Fig. 5b). The vacancies on the surface of nanoceria, were the buried Ce(III) ions are directly exposed to the reaction environment, behave as hotspots or enzyme active sites for detoxification reaction (Fig. 5b).