## 2 Main Lacase Types and Their Structure  
Lacase (EC 1.10.3.2, p-diphenyl: dioxygen oxidoreductaseuse) is related to the superfamily of multicopper oxidases (MCOs), an enzyme group consisting of many proteins with different substrate specificities and various biological functions. It was one of the first enzymes to ever be identified, an enzyme from the Chinese lacquer tree (_Rhus vernicifera_) having been first isolated in 1883 [25]. Since then, nearly 140 years of research have disclosed the unique properties and the important roles of these copper-containing enzymes. The first bacterial lacase (_Azospirillum lipoferum_) was isolated in 1993 from the rhizosphere of rice [26]. To date, about 280 lacases have been characterized according to the BRENDA database ([[http://www.brenda-enzymes.org/](http://www.brenda-enzymes.org/)]([http://www.brenda-enzymes.org/](http://www.brenda-enzymes.org/)) accessed on 22 September 2023), found mainly in fungi (Basidiomycetous and Ascomycetous) [27] but also in bacteria, insects, and algae [6]. The first bacterial lacase (_Azospirillum lipoferum_) was isolated in 1993 from the rhizosphere of rice [26]. Nowadays, it is well known that the classification of lacases using only substrate spectra is not accurate because some of them are very similar to those of tyrosinase and bilirubin oxidase [28; 29]. A large class of substances (mono-, di-, poly-, and methoxyphenols, aromatic and aliphatic amines, hydroxyindoles, benzenethiods, carbohydrates, and inorganic/organic metal compounds) can be oxidized by lacases [6; 30; 31]. Because of this wide substrate range and the fact that lacases use only oxygen as the final electron receptor, they have widespread applications in various industries and in environmental remediation. Figure 1: The redox cycle for substrate oxidation is catalyzed by lacase. Many crystallographic studies on the structure of lacases are now available, especially regarding proteins of fungal or bacterial origin. Plant and animal lacases are difficult to obtain as crystals due to their low abundance and complex purification methods. However, despite their wide taxonomic distribution and substrate diversity, the molecular structure of lacase has proven to be common to all polyvalent copper oxidases [32; 33]. Lacases' domains and amino acid sequences share similarities with those of all multivalent copper oxidases. They have domains like those of copper oxidases and can reduce oxygen to water. The amino acid sequences of lacases, like all multivalent copper oxidases, contain small cupredoxin-like domains of 10-20 kDa and have a relatively simple 3D structure consisting mainly of beta sheets and turns. The crystalline three-dimensional structure of _T. versicolor_ was reported for the first time in 2002 [34]. The molecular mass of monomeric lacase has a molecular mass varying from 60 to 110 kDa with a 10-50% rate of glycosylation. Lacases are thermostable up to 70 \({}^{\circ}\)C due to the high carbohydrate content [35]. Fungal lacases are generally expressed in extracellular form, but some fungi can produce intracellular lacases with a molecular weight between 38 and 150 kDa [36]. Lacases typically include three cupredoxin-like domains, like those described in fungi, plants, insects, and some bacteria, mainly consisting of about 500 amino acids. Bacterial lacases are characterized by a molecular weight between 50 and 70 kDa and are mostly intracellular and belong to two genera, Bacillus and Streptomyces. Small bacterial lacases comprising about 200 amino acids were more recently discovered and considered as two-domain enzymes. Furthermore, similar lacases that are smaller than the typical ones and consist of only two domains can be produced from some fungal species, such as _Pleutrotus ostreatus_[37]. Notwithstanding the low similarity in amino acid sequence between fungal and bacterial lacases, their molecular structure is analogous, and the geometrical spatial arrangement of their active sites is considerably conserved [32; 38; 39].