Galactomannan degradation by thermophilic enzymes: a hot topic for biotechnological applications

Abstract

Extremophilic microorganisms are valuable sources of enzymes for various industrial applications. In fact, given their optimal catalytic activity and operational stability under harsh physical and chemical conditions, they represent a suitable alternative to their mesophilic counterparts. For instance, extremophilic enzymes are important to foster the switch from fossil-based to lignocellulose-based industrial processes. Indeed, more stable enzymes are needed, because the conversion of the lignocellulosic biomass to a wide palette of value-added products requires extreme chemo-physical pre-treatments. Galactomannans are part of the hemicellulose fraction in lignocellulosic biomass. They are heteropolymers constituted by a β-1,4-linked mannan backbone substituted with side chains of α-1,6-linked galactose residues. Therefore, the joint action of different hydrolytic enzymes (i.e. β-mannanase, β-mannosidase and α-galactosidase) is needed to accomplish their complete hydrolysis. So far, numerous galactomannan-degrading enzymes have been isolated and characterized from extremophilic microorganisms. Besides applications in biorefinery, these biocatalysts are also useful to improve the quality (i.e. digestibility and prebiotic properties) of food and feed as well as in paper industries to aid the pulp bleaching process. In this review, an overview about the structure, function and applications of galactomannans is provided. Moreover, a survey of (hyper)-thermophilic galactomannans-degrading enzymes, mainly characterized in the last decade, has been carried out. These extremozymes are described in the light of their biotechnological application in industrial processes requiring harsh conditions.

Recombinant production and characterization of six novel GH27 and GH36 α-galactosidases from Penicillium subrubescens and their synergism with a commercial mannanase during the hydrolysis of lignocellulosic biomass

“…These properties are within range of other fungal α-galactosidases with reported activity optima at pH 4.0-5.0 and 40-50°C (Kurakake et al, 2011; Nakai et al, 2010; Sinitsyna et al, 2008;Zeilinger et al, 1993). In general, the enzymes showed a broad thermostability, being a favorable property for the increased reaction rates at high temperatures and lower risk of contamination in industrial applications (Aulitto et al, 2019). AglB, AglD, and AglE retained more than 80% activity after 1 h incubation up to 40°C, whereas AglA and AglC maintained 80% activity up to 50°C, and AglF up to 60 °C (Fig.…”

“…Among these polysaccharides, hemicelluloses, including galacto(gluco)-mannan, xylan and xyloglucan, constitute the second most abundant biopolymer present in nature, after cellulose (Zeilinger et al, 1993). However, the distribution of these hemicellulosic polysaccharides in hardwoods (angiosperms), softwoods respectively, which contain distinctive galactose and mannose ratios (Aulitto et al, 2019).…”

“…Besides applications in plant biomass conversion, α-galactosidases are also used in other important biotechnological and medical applications. Examples of these are enhancing the kraft pulp bleaching for the paper industry, the synthesis of galacto-oligosaccharides via transglycosylation, hydrolysis of indigestible oligosaccharides to improve their nutritional utilization and digestibility, as crystallization aids in the conversion of raffinose to sucrose in the sugar industry, medical treatments such as the modification of blood group glycomarkers on erythrocytes, and enzyme replacement therapy for the Fabry's disease (Aulitto et al, 2019;Katrolia et al, 2014). Despite these existing applications, the identification of novel α-galactosidases with different substrate specificities, high catalytic efficiencies, great synergistic capacity, and high production levels, remains a challenge.…”

Characterization of a highly stable α-galactosidase from thermophilic Rasamsonia emersonii heterologously expressed in a modified Pichia pastoris expression system

“…In addition to glycoproteins and glycolipids, α-linked galactosyl residues are found in nature in two major forms: (1) as branched residues in galactomannans and galactoglucomannans that are classified into hemicellulose [1, 2], the second most abundant biopolymer on earth after cellulose, both of which are mainly present in plant cell walls, and (2) as moieties of oligosaccharides including melibiose, raffinose, and stachyose that are present in sugar beets and soys [3].…”

“…α-Galactosidases are of great interest in a variety of biotechnological applications [1, 3]. In pulp and paper manufacturing, α-galactosidases are used with other kinds of hemicellulases to enhance pulp bleaching [1, 5].…”