Two new β-glucosidases from ethanol-fermenting fungus Mucor circinelloides NBRC 4572: enzyme purification, functional characterization, and molecular cloning of the gene

Novel β-Glucosidase Mibgl3 from Microbacterium sp. XT11 with Oligoxanthan-Hydrolyzing Activity

The enzymatic pathway of xanthan depolymerization has been predicted previously; however, the β-glucosidase and unsaturated glucuronyl hydrolase in this system have not been cloned and characterized. This lack of knowledge hinders rational modification of xanthan and exploration of new applications. In this work, we report on the properties of Mibgl3, a xanthan-degrading enzyme isolated from Microbacterium sp. XT11. Mibgl3 exhibits typical structural features of the GH3 family but shares low sequence identity with reported GH3 enzymes. The activity of Mibgl3 can be inhibited by Cu²⁺, Fe²⁺, Zn²⁺, and glucose. Unlike most β-glucosidases, Mibgl3 can tolerate a wide pH range and is activated by high concentrations of NaCl. This improves the commercial value of Mibgl3. In particular, Mibgl3 exhibits higher substrate specificity toward oligoxanthan than other β-glucosidases. Ion chromatography, ultrahigh-performance liquid chromatography-mass spectrometry (UPLC-MS), and GC-MS results showed that Mibgl3 could effectively hydrolyze oligoxanthan to release glucose and glucuronate. Therefore, Mibgl3 might play an important role in xanthan depolymerization by functioning as hydrolase of both the xanthan backbone and sidechains. This knowledge of the enzymatic properties and hydrolysis mechanism of a β-glucosidase will be beneficial for future applications.

Mucoromycota fungi as powerful cell factories for modern biorefinery

Biorefinery employing fungi can be a strategy for valorizing low-cost rest materials, by-products and wastes into several valuable bioproducts through the fungal fermentation. Mucoromycota fungi are soil fungi with a highly versatile metabolic system that positions them as powerful microbial cell factories for biorefinery applications. Lipids, pigments, chitin/chitosan, polyphosphates, ethanol, organic acids and enzymes are main Mucoromycota products that can be refined from the fermentation process and applied in nutrition, chemical or biofuel industries. In addition, Mucoromycota biomass can be used as it is for specific purposes, such as feed. Mucoromycota fungi can be employed in developing co-production processes, whereby several intra- and extracellular products are simultaneously formed in a single fermentation process, and, thus, economic viability of the process can be improved. This mini review provides a comprehensive overview over the recent advances in the production of valuable metabolites by Mucoromycota fungi and fermentation strategies which could be potentially applied in the industrial biorefinery settings. KEY POINTS: • Biorefineries utilizing Mucoromycota fungi as production cell factories can provide a wide range of bioproducts. • Mucoromycota fungi are able to perform co-production of various metabolites in a single fermentation process. • Versatile metabolism of Mucoromycota allows valorization of a various low-cost substrates such as wastes and rest materials.

Diversity of Cellulase-Producing Filamentous Fungi From Tibet and Transcriptomic Analysis of a Superior Cellulase Producer Trichoderma harzianum LZ117

Filamentous fungi are widely used for producing cellulolytic enzymes to degrade lignocellulosic biomass. Microbial resources from Tibet have received great attention due to the unique geographic and climatic conditions in the Qinghai-Tibet Plateau. However, studies on cellulase producing fungal strains originated from Tibet remain very limited, and so far no studies have been focused on regulation of cellulase production of the specific strains thereof. Here, filamentous fungal strains were isolated from soil, plant, and other environments in Tibet, and cellulase-producing strains were further investigated. A total of 88 filamentous fungal strains were identified, and screening of cellulase-producing fungi revealed that 16 strains affiliated with the genera Penicillium, Trichoderma, Aspergillus, and Talaromyces exhibited varying cellulolytic activities. Among these strains, T. harzianum isolate LZ117 is the most potent producer. Comparative transcriptome analysis using T. harzianum LZ117 and the control strain T. harzianum K223452 cultured on cellulose indicated an intensive modulation of gene transcription related to protein synthesis and quality control. Furthermore, transcription of xyr1 which encodes the global transcriptional activator for cellulase expression was significantly up-regulated. Transcription of cre1 and other predicted repressors controlling cellulase gene expression was decreased in T. harzianum LZ117, which may contribute to enhancing formation of primary cellulases. To our knowledge, this is the first report that the transcription landscape at the early enzyme production stage of T. harzianum was comprehensively described, and detailed analysis on modulation of transporters, regulatory proteins as well as protein synthesis and processing was presented. Our study contributes to increasing the catalog of publicly available transcriptome data from T. harzianum, and provides useful clues for unraveling the biotechnological potential of this species for lignocellulosic biorefinery.

A Novel Thermostable GH3 β-Glucosidase from Talaromyce leycettanus with Broad Substrate Specificity and Significant Soybean Isoflavone Glycosides-Hydrolyzing Capability

A novel β-glucosidase gene (Bgl3B) of glycoside hydrolase (GH) family 3 was cloned from the thermophilic fungus Talaromyce leycettanus JM12802 and successfully expressed in Pichia pastoris. The deduced Bgl3B contains 860 amino acid residues with a calculated molecular mass of 91.2 kDa. The purified recombinant Bgl3B exhibited maximum activities at pH 4.5 and 65°C and remained stable at temperatures up to 60°C and pH 3.0-9.0, respectively. The enzyme exhibited broad substrate specificities, showing β-glucosidase, glucanase, cellobiase, xylanase, and isoflavone glycoside hydrolase activities, and its activities were stimulated by short-chain alcohols. The catalytic efficiencies of Bgl3B were 693 and 104/mM/s towards pNPG and cellobiose, respectively. Moreover, Bgl3B was highly effective in converting isoflavone glycosides to aglycones at 37°C within 10 min, with the hydrolysis rates of 95.1%, 76.0%, and 75.3% for daidzin, genistin, and glycitin, respectively. These superior properties make Bgl3B potential for applications in the food, animal feed, and biofuel industries.

Cellulases from Thermophiles Found by Metagenomics

Cellulases are a heterogeneous group of enzymes that synergistically catalyze the hydrolysis of cellulose, the major component of plant biomass. Such reaction has biotechnological applications in a broad spectrum of industries, where they can provide a more sustainable model of production. As a prerequisite for their implementation, these enzymes need to be able to operate in the conditions the industrial process requires. Thus, cellulases retrieved from extremophiles, and more specifically those of thermophiles, are likely to be more appropriate for industrial needs in which high temperatures are involved. Metagenomics, the study of genes and gene products from the whole community genomic DNA present in an environmental sample, is a powerful tool for bioprospecting in search of novel enzymes. In this review, we describe the cellulolytic systems, we summarize their biotechnological applications, and we discuss the strategies adopted in the field of metagenomics for the discovery of new cellulases, focusing on those of thermophilic microorganisms.

Expression and characterization of a novel β-glucosidase, with transglycosylation and exo-β-1,3-glucanase activities, from Rhizomucor miehei

A novel β-glucosidase gene, designated RmBglu3B, was cloned from the thermophilic fungus, Rhizomucor miehei CAU432. Its 2196-bp open reading frame encoded 731 amino acids. Its deduced amino-acid sequence showed highest identity (66%) with a glycoside hydrolase family 3 β-glucosidase from R. miehei NRRL5382. RmBglu3B was successfully expressed in Escherichia coli. The recombinant enzyme was purified to homogeneity with 18.2-fold purification and 59% recovery yield. Molecular masses of 76.5 kDa, by SDS-PAGE, and 66.4 kDa, by gel filtration, suggested that it is a monomer. Optimal pH and temperature of the purified enzyme were 5.0 and 50°C, respectively. RmBglu3B exhibited a broad range of substrate specificity, catalyzing the cleavage of β-1,2, β-1,3, β-1,4 and β-1,6 linkages, in various oligosaccharides, to liberate glucose. RmBglu3B also showed relatively high activity (19.1 U/mg) toward laminaran and transglycosylation activity, enabling gentiobiose production. This enzyme is a potential candidate for several industrial applications.

Identification and characterization of D-xylulokinase from the D-xylose-fermenting fungus, Mucor circinelloides

D-Xylulokinase catalyzes the phosphorylation of D-xylulose in the final step of the pentose catabolic pathway to form d-xylulose-5-phosphate. The D-xylulokinase activity was found to be induced by both D-xylose and L-arabinose, as well as some of the other enzymes involved in the pentose catabolism, in the D-xylose-fermenting zygomycetous fungus, Mucor circinelloides NBRC 4572. The putative gene, xyl3, which may encode D-xylulokinase, was detected in the genome sequence of this strain. The amino acid sequence deduced from the gene was more similar to D-xylulokinases from an animal origin than from other fungi. The recombinant enzyme was purified from the E. coli transformant expressing xyl3 and then characterized. The ATP-dependent phosphorylative activity of the enzyme was the highest toward D-xylulose. Its kinetic parameters were determined as Km (D-xylulose) = 0.29 mM and Km (ATP) = 0.51 mM, indicating that the xyl3 gene encoded D-xylulokinase (McXK). Western blot analysis revealed that McXK was induced by L-arabinose as well as D-xylose and the induction was repressed in the presence of D-glucose, suggesting that the enzyme may be involved in the catabolism of D-xylose and L-arabinose and is subject to carbon catabolite repression in this fungus. This is the first study on D-xylulokinase from zygomycetous fungi.