Growing concerns over climate change, depletion in fossil fuels, and a rapid increase in population have driven efforts to find alternative resources for the industrial production of energy and chemicals [1]. According to the U.S. Energy Information Administration, the total renewable energy production has increased from ∼6000 to ∼12,000 trillion Btu over the last two decades [2]. In 2022, 13% of the total energy consumed originated from renewable resources [3]. Among others, biomass feedstocks, including agricultural and forestry residues, are promising raw materials, currently responsible for 37% of renewable energy production [4]. Biomass feedstocks can also be converted into high-value chemicals (e.g., proteins and triglycerides) in biorefinery [4]. Collectively, biorefinery is a term referring to such a utilization or conversion process [5].

Lignocellulosic biomass is one of the most abundant bioresources available on earth [6]. It is typically composed of cellulose, hemicellulose, and lignin [7]. While the relative compositions of these biopolymers vary depending on sources, nearly 50% of lignocellulosic biomass consists of cellulose, which is present as the main component of cell walls [8] and is composed of repeating β-D-glucopyranose units [9]. In biorefinery, cellulosic materials are degraded either chemically or enzymatically. An enzymatic degradation of cellulose is more preferred due to its lower environmental impact and milder reaction conditions [10]. This enzymatic process involves the synergistic action of three different cellulases: endo-1,4-β-glucanase (EC 3.2.1.4), exo-1,4-β-glucanase (EC 3.2.1.91), and β-glucosidase (EC 3.2.1.21). First, endo-1,4-β-glucanase cleaves internal bonds randomly within the cellulose chain. This random cleavage produces reducing and non-reducing ends, from which further degradation into cellobioses and small oligosaccharides can occur by exo-1,4-β-glucanase. Lastly, the released cellobioses are broken into glucoses, via catalysis by β-glucosidase [11].

As one of the three categories of cellulases, β-glucosidases are commercially produced and provided in a hydrolytic cocktail, for example, Cellic® Ctec3 (Novozymes) [12], [13] and Accellerase® Trio (Genencor) [13]. Among other origins, Trichoderma Reesei is the most common source of such cellulase cocktails, though β-glucosidase activity is low relative to other cellulase activities in the cellulase mixture directly obtained from T. Reesei. Moreover, β-glucosidase from T. Reesei is highly susceptible to inhibition by its product, glucose [10], [14]. Thus, β-glucosidase from a different source, such as Aspergillus niger, is usually supplemented in these cellulase cocktails. In addition to biofuel production, β-glucosidase is frequently used in other industrial applications. In the food industry, β-glucosidase enhances flavors by releasing aromatic compounds from precursors [15] and produces antioxidants (e.g., aglycones) from agricultural crops (e.g., soybean) [16]. β-glucosidase is also heavily used in the cosmetics and tobacco industries [17].

In nature, β-glucosidases serve varying roles in bacteria, fungi, plants, insects, and mammals. In microorganisms, β-glucosidase is involved in breaking plant cell walls and producing glucose from plant biomass [18]. In plants, β-glucosidase plays a key role in defense, signaling, and secondary metabolism. For example, the hydrolysis of secoiridoid glycosides by β-glucosidase releases aucubin, which triggers a defense mechanism to denature proteins in plant pathogens and herbivores during tissue disruption in Plantago lanceolata [19]. In maize, β-glucosidase activates hormone conjugates to release cytokinin, which mediates cell division [20]. In insects and mammals, β-glucosidase is utilized for the uptake of dietary glucosides and their subsequent metabolisms [18]. Thus, β-glucosidase is an important enzyme in both nature and industries.

This review highlights up-to-date findings on the diversity of β-glucosidases, in terms of enzyme classifications, catalytic mechanisms, enzymatic properties, and kinetic models to depict the catalytic behaviors. The information collected in this paper is anticipated to not only provide physicochemical fundamentals of β-glucosidases but also benefit the selection of a proper enzyme source as well as the subsequent enzyme engineering endeavors for desired biotechnological applications.