A new tendency toward the design of artificial enzymes based on nanostructures (nanodots, nanofibers, mesoporous materials) has emerged. On one hand, nanotechnology bestows self-catalytic nanoparticles with a specific activity to achieve efficient reactions with low number of by-products. On other hand, the nanoparticles may behave as nanometric scaffolds for hosting enzymes, promoting their catalytic activity and stability. In this case, enzyme immobilization requires the preservation of the catalytic activity by preventing enzyme unfolding and avoiding its aggregation. These approaches render many other advantages like hosting/storing enzymes in nanotechnological solid, liquid, and gel-like media. This chapter focuses on the most up-to-date approaches to manipulate or mimic enzyme activity based on nanotechnology, and offers examples of their applications in the most promising fields. It also gives new insight into the creation of reusable nanotechnological tools for enzyme storage.
TopIntroduction
Enzymes are biocatalysts of specific substrates with a strong potential in multiple reactions. They are known to have capability of modifying their functions upon environmental changes, which is the key of the adaptation of organisms, like bacteria resistance to pesticides or even to drugs. Although the concept of enzymes is known from the 19th century (Payen & Persoz, 1833), it was not until the next century when researchers perceived their great potential. There is a diversity in the type and functions of enzymes which also depend on the type organism hosting. Additionally, enzymes may have different domain composition, being of single domain or multidomain which evolve new functionalities. The discipline of enzymology is very wide but entails notably benefits not only for other fields of application, such as in technology and medicine, but also for the industrial sector. One of its main interests is to explore the structural changes affecting enzyme functions and to rationally control new functions by re-designing existing enzyme frameworks. Many are the advances in creating artificial active enzymes to mimic Nature; thus, essential objectives are to stabilize enzymes and to control their catalytic performance in multistep processes of such complexity as those occurring in biological systems (e.g. metabolism, biosynthesis, signaling, channeling). However, the interest of the biotechnology is also to achieve a more efficient process control in large scale at low costs, very demanding in the industrial sector.
Some progress in the field has been made by supporting enzymes in different solid structures in order to maximize their catalytic activity, selectivity and recovery/reusability by incrementing their environmental tolerance. From the last two decades to now, the emerging advanced technology supported in nanoscale materials has attracted the attention of enzymologists. Nanotechnology offers a new perspective and opportunity not only to mimic but also to boost the enzymatic activity. Nanotechnology is considered as the manipulation of the nanomatter which brings unique physicochemical properties not observed for other materials, i.e., nanoparticles do not follow the fundamental rules known until now for micro and macroparticles; they can be explained thanks to the Nanoscience. Two main contributions of nanotechnology are highlighted. The first contribution is the use of those nanoparticles exhibiting catalytic activity by themselves; they are known as nanozymes. In the second approach, the nanostructures bring the possibility of hosting enzymes to improve both enzyme long-term stability against pH, heat and other external variables and reusability; it is possible thanks to their large surface area and unique physicochemical properties, which allow easy enzyme immobilization, in some cases with an improvement in catalytic activity, and in others, with high enzyme loading with minimum diffusion limitations, leading to great efficiencies in biocatalysts (Pundir, 2015).
It is important to note that enzyme immobilization requirements for achieving the best catalytic conditions are the preservation of the active conformational form of the enzyme and the prevention of any aggregation or unfolding. Thus, the main variables to consider are the type of enzyme, the nanoparticle surface, the mode of attachment and the matrix. With this aim in mind, many researchers have tried to understand the structural and conformational changes of enzymes in adequate micro- or nano- environments to create organized devices (Ariga, 2013). Thus, the convergence of nanotechnology and biotechnology lead to the nanobiotechnology, that has emerged for finding the foundations between the relationship of the enzyme structure and function in biological environments and evolution to a global industrial sector (Maine, 2014).
There exist a wide variety of analytical and biotechnological applications involving artificial enzymes coming from nanotechnology: nanozymes, enzyme-immobilized nanostructures and gels containing such enzyme-nanoparticles designated to the conversion of substrates to products, degradation of toxic substances, for sensing, delivering and/or imaging in biological systems, as well as creating more sophisticated self-assemblies.