β-glucosidases (EC. 3.2.1.21) are enzymes that hydrolyze glucosidic bonds of oligosaccharides, in special disaccharides, such as cellobiose, realizing glucose at the end of the process. They are highly used in second-generation biofuel production.
In second-generation biofuel production, they act in synergy with other classes of enzymes, such as endo-glucanases and exo-glucanases [2][3], to obtain fermentable sugars from biomass. However, the literature has described that most β-glucosidase enzymes are inhibited by glucose, which has been considered one of the most impacting bottlenecks for high-efficient industrial production [4].
Recently, a more efficient class of β-glucosidases have been described in the literature. They are called glucose-tolerant, due to their high resistance to inhibition even in high glucose concentrations [5]. This class of enzymes has excellent potential for use in industrial applications such as biofuel production. Hence, glucose-tolerant β-glucosidase enzymes have been an essential target of several studies to detect mutations that transform non-tolerant enzymes into glucose-tolerant enzymes [6].
Most glucose-tolerant β-glucosidases are obtained from glycoside-hydrolase family 1 (GH1). Proteins from this family have a conserved folding structure called TIM-barrel [7]. Figure 1 illustrates the three-dimensional structure of GH1 β-glucosidase from the fungus Humicola insolens complexed with a glucose molecule [8].
Figure 1. GH1 β-glucosidase from the fungus Humicola insolens. Obtained from PDB ID: 4MDP. Figure generated using ChimeraX [9].
In conclusion, β-glucosidase enzymes have an excellent potential for second-generation biofuel production. However, genetic engineering applications are still necessary to improve the activity of non-tolerant enzymes. Additionally, bioinformatics applications have been successfully used to bring new insights to detect sites for mutations that could improve their activity.