# Metal Nanozymes: New Horizons in Cellular Homeostasis Regulation
## 5 Metal Nanozymes as Enzyme Inhibitors  
In addition to biocatalytic functions, nanozymes can be used to selectively bind various enzymes and inhibit their activity [130]. This is related to their abovementioned features: a developed surface that is easy to modify and functionalize, and their architecture, which can be highly ordered and often has features important from the point of view of biocatalysis, such as chirality [131]. Protein surface recognition is an important issue in biomedical and biomaterial research. In addition to its use in immunoimaging methods, it is necessary for the specific inhibition of enzymes [132]. Until recently, small molecules, such as polypeptides or metal complexes, have been used as structures recognizing the active sites of enzymes [133], but the complicated geometry of enzyme active sites often requires structurally complex inhibitors difficult to precisely design. The required large receptor-protein contact surfaces and the inherent complexity of protein surfaces, the need to consider the hydrophobic nature of the active electrostatic center, and hydrogen bonds are still a challenge for modern biochemistry [131]. To obtain functional nanomaterials compatible with protein surfaces, [PERSON] et al. [132] constructed carboxylic acid-functionalized gold nanoparticles with various L-amino acids and investigated their effect on the rate of R-chymotrypsin (ChT) catalyzed N-succinyl-L-alanine-p-nitroanilide (SPNA) hydrolysis. The authors observed an explicit charge-dependent inhibition of the enzyme. In the presence of negatively charged amino acid-modified gold nanoparticles, the rate of ChT-catalyzed SPNA hydrolysis was reduced, while the positively charged amino-acid-modified gold nanoparticles had no obvious effect on ChT activity. This is well following the fact that the ChT active site is surrounded by cationic residues. Circular dichroism (CD) analysis showed conformation changes dependent on the side chains of negatively charged amino acids with which the nanoparticles were decorated. At the same time, in the presence of gold particles with a hydrophobic side chain of the NP-L-Leu amino acid, almost no changes in the CD ChT spectrum were observed. In addition, the hydrophobicity was postulated decisively for the observed inhibition effect: After 24 h incubation with nanoparticles, hydrophilic NP_L-Asp induced approximately 90% denaturation of ChT, while hydrophobic NP_L-Leu induced only 20% denaturation. Moving on, [PERSON] et al. showed that the interaction between nanoparticles and protein influences both the catalytic constants and Michaelis constant values, resulting in the modulation of the association/dissociation processes. The recognition of the ChT surface by charged nanoparticles opens up new horizons for research into the affinity of protein-active centers [132, 134]. As mentioned, apart from electrostatic interactions, the surface geometry is also of great importance for the interaction with the active center of the enzyme. This situation has been observed by [PERSON], [PERSON] et al. [135], who showed that small zinc oxide nanoparticles (ZnO NP), pyramids, plates, and spheres, can inhibit the activity of \(\beta\)-galactosidase (GAL), a proteolytic enzyme, the upregulation of which is usually associated with primary ovarian cancer and cell aging. The inhibition of enzymes by ZnO nanoparticles is reversible and strongly depends on their geometry [135]. [PERSON] et al. [136] synthesized several functionalized two-dimensional molybdenum disulfide (2D-MoS\({}_{2}\)) nanomaterials capable of inhibiting penicillin, also known as \(\beta\)-lactamase, which is responsible for penicillin-resistance bacterial strains [137]. The authors showed that carboxylate-functionalized negatively charged MoS\({}_{2}\) effectively blocked the \(\beta\)-lactamase active site and exhibited a competitive inhibitory effect.