Enzymes are catalytic proteins that facilitate chemical reactions within a living organism. They are widely used in a variety of industries due to their high specificity, catalytic power, and environmental safety (Singhania et al., 2015). Nonetheless, their activity can be hindered by operational conditions such as heat, pH, and solvents; thus, enzymatic immobilization techniques are employed to protect enzymes against these harsh conditions and to allow them to be recycled and stored for longer periods (Huang et al., 2022). Laccases (E.C. 1.10.3.2) are natural oxidases that are involved in lignin biosynthesis and degradation. They are monomeric glycoproteins with molecular weights between 50 and 85 kDa depending on their origin, and their carbohydrate content can represent up to 25% of the protein's weight. Laccases can be easily obtained from a variety of sources, and they are widely used in biotechnological processes due to their ability to catalyze oxidation reactions using only atmospheric oxygen as an electron acceptor, unlike other cofactor-dependent oxidases, making them attractive for application in bioremediation processes, biosensing and many other applications that will be further discussed (Arregui et al., 2019; Barrios-Estrada et al., 2018b; Orlikowska et al., 2018).

Several materials and nanomaterials can be used to immobilize enzymes, including natural polymers such as collagen, alginate, chitosan, and cellulose and inorganic materials such as zeolites, ceramics, silica, charcoal, and glass (Datta et al., 2013). Enzymes are immobilized into these carriers by adsorption, covalent linkage, pore entrapment, encapsulation, crosslinking, or ligand affinity. Many techniques can be utilized to synthesize and/or prepare such materials in which to anchor enzymes including electrospinning, matrix-assisted pulsed laser evaporation (MAPLE), soft plasma polymerization, 3D and laser printing, and the use of crosslinked enzyme aggregates (CLEAs), nanoflowers and, of course, MOFs (Alvarado-Ramírez et al., 2021a).

MOFs constitute a class of nanoporous and usually crystalline materials composed of metal ions or clusters and organic linkers. These structures are widely used as carriers for enzymes due to their biocompatibility, large surface area, tunability, and well-defined pores, giving them a tremendous advantage over other carriers (Du et al., 2022). However, perhaps the most important factor is the chemical affinity between enzymes and MOFs, which prevents leaching (Chen et al., 2014; Gascón et al., 2017; Xu et al., 2021a). This set of properties enables MOFs to provide a suitable (micro) environment for enzymes to achieve efficient catalytic activities (Gkaniatsou et al., 2017). Immobilization “in” a MOF (or any other support) suggests that the enzyme is bound to the internal surface of pores; therefore, the enzymes should be described as being “immobilized in/into the support”. However, some supports do not offer internal surfaces for enzyme binding, and immobilization can occur only on the external surface of the support. In these cases, the enzymes should be described as being “immobilized on/onto the support”.

There are countless strategies/approaches by which enzymes are immobilized using MOFs as supports, leading to differences in the processes and results of immobilization. In this review, we use the abbreviation Enz@MOF (Lac@MOF for the particular case of the enzyme laccase) to designate any solid biocatalyst resulting from the immobilization of enzymes in/on MOFs, regardless of the strategy used, which is consistent with the widespread nomenclature of the type enzyme@MOF, Enzyme@MOF or Enz@MOF. Enz@MOF composites can be prepared by two principal methods: postsynthesis and in situ or de novo immobilization. The postsynthesis approach requires a preexisting MOF for the enzyme to be immobilized within. In this case the enzyme can be embedded into the MOF through adsorption or covalent bonding. Immobilization by this strategy can occur within inter- or intracrystalline spaces wide enough to accommodate the enzyme, but this involves difficult diffusion of the enzyme through the microporous network of MOFs. Therefore, immobilization often occurs on the external surface of the particle. Alternatively, in situ synthesis is a method by which Enz@MOF is synthesized via the nucleation and growth of the MOF around the enzyme (Molina et al., 2021; Xia et al., 2020). The conditions for the synthesis of most MOFs (extreme pH values, high temperatures or in the presence of organic solvents) are not compatible with enzymatic activity, and this issue is a high-priority topic in research within the scientific community. Sánchez-Sánchez et al. (2015) proposed a synthesis approach in aqueous media by deprotonating the organic linker in an alkaline medium to make it soluble in an aqueous solution at room temperature. This discovery is a milestone that permitted the generation of MOFs in the presence of enzymes and thus significantly improved the enzyme loading in the final biocatalysts and prevented leaching.

Nanozymes are among the most emerging research topics in heterogeneous catalysis. They are nonbiological nanomaterials that exhibit enzyme-like catalytic performance under certain conditions. In other words, they can be considered artificial enzymes. Compared to enzymes, these materials have certain key catalytic advantages: much lower cost, much higher stability, easier storage, and broader application conditions. Many of the most studied nanozymes are based on MOFs (Wang et al., 2020c). Although MOFs and other immobilization supports provide an option to overcome the aforementioned disadvantages of natural enzymes, nanozymes have been paving a new path since the last decade, when biotechnology and nanotechnology were combined to develop new efficient technologies. This development also created a synergy between natural catalysts and synthetic materials. Additionally, MOFs have become a popular material for designing artificial enzyme materials. However, nanozymes can mimic only a limited type of enzyme, and their catalytic power is no better than that of natural enzymes (Liang and Yan, 2019). In fact, only a few laccases mimicking MOF-based nanozymes have been synthesized to date, but research on biomimetic compounds seems to be increasing exponentially, especially in the area of biosensing. Herein, we present a review to discuss the advantages and disadvantages of postsynthetic and in situ or de novo immobilization into MOFs, focusing on laccases and their potential applications, as well as the possibilities of laccase-like MOF-based nanozymes to compete with their homolog Enz@MOF composites. We discuss how the synthesis conditions and the physicochemical properties of the final MOF or Lac@MOF composite and the nature of the laccase-like MOF-based nanozyme can affect their (bio)catalytic performance in different applications.