Powerful cell wall biomass degradation enzymatic system from saprotrophic Aspergillus fumigatus

Cell wall biomass, Earth’s most abundant natural resource, holds significant potential for sustainable biofuel production. Composed of cellulose, hemicellulose, lignin, pectin, and other polymers, the plant cell wall provides essential structural support to diverse organisms in nature. In contrast, non-plant species like insects, crustaceans, and fungi rely on chitin as their primary structural polysaccharide. The saprophytic fungus Aspergillus fumigatus has been widely recognized for its adaptability to various environmental conditions. It achieves this by secreting different cell wall biomass degradation enzymes to obtain essential nutrients. This review compiles a comprehensive collection of cell wall degradation enzymes derived from A. fumigatus, including cellulases, hemicellulases, various chitin degradation enzymes, and other polymer degradation enzymes. Notably, these enzymes exhibit biochemical characteristics such as temperature tolerance or acid adaptability, indicating their potential applications across a spectrum of industries.


Introduction
Aspergillus fumigatus, a common saprophytic fungus, plays a vital role in carbon and nitrogen recycling in the soil, serving as both a potent biological decomposer and an opportunistic pathogen causing invasive aspergillosis (Toyotome, 2019).Despite its pathogenic potential, studies highlight the remarkable ability of A. fumigatus to extract nutrients and energy from diverse sources, suggesting potential applications in various industries (Rhodes, 2006).While A. fumigatus cannot directly assimilate polysaccharides, it adopts a saprophytic lifestyle by secreting an array of cell wall-degrading enzymes, including cellulase, chitinase, phytase, chitosan enzyme, and more, with applications in enzyme engineering (Brugger et al., 2004;Saroj et al., 2022;Wang et al., 2023;Xia et al., 2001).These enzymes effectively break down cell wall biomass and polysaccharides into monomeric carbohydrates, facilitating absorption and metabolism (Nierman et al., 2005).Consequently, A. fumigatus, like other filamentous fungi, is utilized for the production of extracellular proteins (Adav et al., 2015).
A. fumigatus possesses a wealth of functional genes, exemplified by chitin synthase (Ries et al., 2017) and phytase (Mullaney et al., 2000), along with specific enzymes in its repertoire demonstrating remarkable thermal stability.This enzymatic diversity is advantageous for designing enzyme mixtures tailored for biomass utilization, including biofuel production, thereby establishing A. fumigatus as an exceptional source of industrial enzymes.However, unlike other industrially used Aspergillus species like Aspergillus niger and Aspergillus oryzae, certain strains of A. fumigatus are pathogenic, making them unsuitable for direct industrial enzyme production (Askew, 2008).Despite this limitation, recent research and reviews underscore the potential for studying biomassdegrading enzymes in A. fumigatus (Gonçalves et al., 2023).This review article aims to summarize the recent advancements in understanding these enzymes and their significant role in biomass degradation by A. fumigatus.

Lignocellulose hydrolases
The growing cell wall in plants is characterized by its thinness, strength, and flexibility, with the extracellular layer playing a fundamental role in maintaining plant integrity.This structure comprises cellulose microfibrils ensconced in a hydrated matrix of intricate polysaccharides (cellulose-binding glycans) and a small proportion of structural proteins (Cosgrove, 2005).Lignocellulosic biomass, a primary constituent of plant cell walls, consists of cellulose, hemicellulose, and lignin (Hendriks and Zeeman, 2009;Singh et al., 2014;Van Dyk and Pletschke, 2012).Globally, projections indicate an annual production of over 20 billion tonnes of lignocellulosic biomass, establishing it as one of the most abundant and sustainable carbon resources on Earth (Liu et al., 2020).The predominant method for converting plant lignin biomass involves enzymatic hydrolysis technology.Effective hydrolysis of lignocellulosic biomass depends on the synergistic action of endoglucanases, exoglucanases and β-glucosidases (Kumar et al., 2008).While Trichoderma reesei is renowned for its production of endoglucanases, cellobiohydrolase, and β-glucosidases; however, its utility is hampered by low β-glucosidase activity, limiting further application.On the other hand, A. niger has the potential to produce cellulases with high β-glucosidase activity, yet its efficacy in cellulose hydrolysis is impeded by low endoglucanase expression (Liu et al., 2013).
Given the intricate composition and challenging breakdown of lignocellulosic biomass, employing multiple enzymes becomes imperative for effective deconstruction and utilization.A. fumigatus emerges as an important producer of lignocellulolytic enzymes, yielding substantial quantities of hydrolytic enzymes.The cellulase system, exemplified by the A. fumigatus Z5 cellulase system, boasts remarkable thermal stability and a broad pH range, offering distinct advantages.Notably, its enzymatic activity surpasses that of the highly cellulolytic strain T. reesei RUT30-C (Gouvêa et al., 2018).
Despite the stability and efficiency of fungal-secreted endoglucanases at elevated temperatures, research on those secreted by A. fumigatus remains limited.However, A. fumigatus strain LMB-35Aa, isolated from the Peruvian Amazon rainforest, exhibits significant cellulase activity, including the presence of essential cellulose-degrading genes such as endoglucanase A, endoglucan protease B, and endoglucanase-like D (Paul et al., 2017).While the virulence and pathogenicity of A. fumigatus have traditionally hindered its application, the LMB-35Aa strain is recognized for its potent production of neutral/ basic endoglucanase.The putrefactive status of this strain and its genetic differences from clinical isolates was further explored by Rebaza et al.Optimal conditions for endoglucanase production and activity of A. fumigatus LMB-35Aa were found to be 37 • C and pH 7.6.Notably, this temperature inhibited virulence factors such as gliZ, atrF and medA, with L. Tong et al.only minor expression of virulence-related genes observed.
Endoglucanases from A. fumigatus typically exhibit high optimal reaction temperatures ranging from 45 to 60 • C. Das et al. purified a halotolerant endoglucanase with a calculated molecular weight of approximately 56.3 kDa from a culture extract of A. fumigatus ABK9.This enzyme showed maximum activity at 50 • C and pH 5.0, demonstrating high stability within a pH range of 4.0-7.0 and up to a NaCl concentration of 3.0 M (Das et al., 2013).In another study, Aline Vianna Bernardi et al. heterologously expressed the gene encoding a GH7 family endoglucanase (Afu6g01800) from A. fumigatus Af293 in Escherichia coli.The enzyme exhibited a broad optimal temperature range spanning from 40 to 60 • C (Bernardi et al., 2018).Additionally, when the endoglucanase gene from the heat-resistant A. fumigatus DBiNU-1 strain was expressed in the lactic acid-producing Kluyveromyces lactis, the purified recombinant endoglucanase displayed peak activity at 60 • C (Rungrattanakasin et al., 2018).
Cellulase enzymes originating from A. fumigatus exhibit remarkable temperature tolerance.Notably, an endoglucanase belonging to the AA9 family (formerly GH61), isolated from alkali-resistant A. fumigatus MKU1 (derived from pulp and paper industry waste), demonstrating resilience at temperatures up to 70 • C (Meera et al., 2011).Another A. fumigatus strain A-16, produces enzymes with an optimum reaction temperature of 70 • C, retaining over 80% activity even after heat treatment at 80 • C for 90 min (ZHANG Kai-ping et al., 2022).The endoglucanase from this strain displays optimal activity between 55 and 80 • C.This property of high-temperature tolerance is advantageous for constructing cost-effective and stable strains suitable for industrial conditions, making them promising candidates for various industrial applications (Bhiri et al., 2010;Maheshwari et al., 2000;Rungrattanakasin et al., 2018).
Cellobiose hydrolase, a cellulose disaccharide hydrolase, targets the crystalline region of cellulose, playing a pivotal role in the hydrolysis of natural cellulose into cellulose disaccharides.The industry has perennially grappled with challenges related to pH stability, thermal stability, and low conversion efficiency.The crystal structure of A. fumigatus CBHI Cel7A has undergone analysis, revealing a classic β sandwich structure in its three-dimensional configuration.AfCel7A demonstrates commendable stability, retaining high activity even after incubation at 65 • C (Moroz et al., 2015).In addition, Dodda et al. successfully achieved a comprehensive enhancement in the catalytic activity and stability of the mutant N449V through protein engineering (Dodda et al., 2016).These advancements not only contribute to overcome industry challenges but also establish a robust foundation for ongoing research and the practical application of cellobiose hydrolase.

β-glucosidases
In the early 1970s and 1975, Parry et al. successfully characterized two sources of β-glucosidase from A. fumigatus (Rudick andElbein, 1975, 1973).The β-glucosidase derived from A. fumigatus employs an acidbase catalytic mechanism, facilitating the hydrolysis of β-glucosidic bonds and releasing glucose from the substrate.Structurally, it typically consists of an N-terminal signaling peptide, a catalytic domain, and a Cterminal domain, with a molecular weight falling within the range of 60-80 kDa (Agirre et al., 2016).
A. fumigatus Z5 emerges as an excellent candidate for cellulase production (Liu et al., 2013(Liu et al., , 2012)).Both nBgl3 and rBgl3, its heat-resistant β-glucosidases, retain more than 40% and 50% of their activity, respectively, even when treated at 70 • C for one hour.The degradation of cellulose to produce glucose typically necessitates the coordinated action of at least three distinct and complementary cellulases: endoglucanase, exoglucanase and β-glucosidase.Among these, β-glucosidase plays a crucial role in preventing the accumulation of cellobiose and alleviating its inhibitory effect on cellulase activity.This enzyme ultimately determines the efficiency of the cellulose saccharification process, and serves as a key catalyst for promoting cellulose degradation.Therefore, the stability of β-glucosidases is essential for ensuring the robustness of industrial bioenergy utilization process.These β-glucosidases (nBgl3 and rBgl3) hold promise for diverse applications in bioenergy and food processing, consequently.
Xylanases exhibit significant diversity, with various microorganisms capable of simultaneously producing different types of xylanases.Fungi, including A. fumigatus, are known to synthesize and secrete xylanases into the external environment (Damasio et al., 2017;Damis et al., 2019;Miao et al., 2015b).In a study by Miao et al., 10 different hemicellulases from A. fumigatus Z5 were identified and quantified.Among them, four endo-xylanases, two xyloglucosidases, one acetyl-xylan esterase, and one α-L-arabinofuranosidase were identified as crucial participants in xylan degradation (Miao et al., 2015b).Xylanases from A. fumigatus, like other enzymes, exhibit high reaction stability and a broad pH range.
In industrial applications, finding fungal xylanases that are both alkaline and heat-stable proves challenging.Typically, most bacterial and fungal GH11 xylanases are alkaline, while GH10 xylanases are acidic.Through a computational analysis of the A. fumigatus genome (KEY83365), Dodda et al. identified alkaline xylanases belonging to the GH10 family.The study highlighted that, GH11 xylanase showed higher binding energy than GH10 xylanase.However, with a few exceptions, GH10 xylanases demonstrate a higher number of hydrogen bonds, salt bridges, and helices compared to GH11 counterparts, contributing to their superior heat resistance (Dodda et al., 2021).Furthermore, GH10 xylanases typically possess a higher molecular weight than GH11 xylanases.A thermotolerant albino strain of Aspergillus (A. fumigatus var.niveus), isolated from Brazilian rainforest composted floors, yielded a GH10 xylanase (AFUMN-GH10) that was cloned and heterologously expressed.AFUMN-GH10 exhibits an optimal temperature of 60 • C and retains over 60% of its maximum activity within the temperature range of 30 to 75 • C. Xylanases belonging to the GH10 family from other filamentous fungi also showed optimal reaction temperatures ranging from 60 to 75 • C (Velasco et al., 2019).However, this strain has the same pathogenicity as A. fumigatus Af293 (Couger et al., 2018).Therefore, careful selection of a heterologous host for protein expression is imperative.
fumigatus not only displays xylanase activity but also features other hemicellulases, including galactosidase.Galactosidase, a member of the glycoside hydrolase (GH) family, plays a role in catalyzing the hydrolysis of galactosides into monosaccharides.An α-galactosidase purified from the thermotolerant A. fumigatus strain IMI 385708 was identified to catalyze the glycosylation of internal sugar residues in oligosaccharides (Puchart and Biely, 2005).Utilizing recombinant technology, the genes encoding two α-L-arabinofuranosidases (AbfI and AbfII) from GH family 62, as identified in the A. fumigatus genome, were expressed in Pichia pastoris.Following purification and characterization, both ABFI and ABFII displayed optimum temperatures of 37 • C and 42 • C, respectively, with an optimal pH range of 4.5 to 5.0.Notably, ABFII demonstrated higher thermostability (Pérez and Eyzaguirre, 2016).These findings underscore the diverse enzymatic capabilities of A. fumigatus in the context of hemicellulose degradation (Table 2) and offer valuable insights for industrial applications.

Chitin hydrolases
Chitin, constituting 10-30% of fungal cell walls, stands as the principal component in these structures (van Leeuwe et al., 2020).This linear polymer consists of β-(1,4)-linked N-acetylglucosamine (Fig. 1), and chitin molecules are classified into three types based on the arrangement of adjacent sugar chains: α-Chitin, β-Chitin, and γ-Chitin (Chen et al., 2023).Although chitin is insoluble in water and challenging to decompose, its deacetylated product, chitosan, features numerous free amino groups with positively charged molecules, rendering it soluble in acidic and neutral aqueous solutions (Elango et al., 2023).Despite minimal difference in molecular structure, both chitin and chitosan play crucial roles in providing structural stability for fungal cell walls (Brown et al., 2020).Within the realm of chitin metabolism, chitinase, chitin deacetylase, and chitosanase are essential members for chitin hydrolases.Currently, there is limited research on chitin hydrolases derived from A. fumigatus compared to other filamentous fungi (Table 3).This observation underscores the potential for further exploration and understanding of A. fumigatus chitinolytic enzymes and their implications in fungal cell wall dynamics.

Chitinase
Chitinases (EC 3.3.1.14)are widespread in viruses, fungi, and bacteria, with each source exhibiting distinct functions.Those produced by filamentous fungi are commonly associated with processes such as cell wall cleavage, spore formation and germination, mycelial growth, and fungal parasitism (Flach et al., 1992).Given their pivotal physiological roles and broad application potential, chitinases have become a highly researched topic in recent years (Di Giambattista et al., 2001).As the major structural component within the cell walls of all human pathogenic fungi, chitin polymers are absent in mammals.This absence positions chitin-metabolizing enzymes as promising targets for the development of novel antifungal agents.Chitinases are classified into two categories based on different hydrolysis modes: endogenous chitinases (EC 3.2.1.14)and exogenous chitinases (EC 3.2.1.29)(He et al., 2022).Further differentiation is observed in protein sequence, structure, and catalytic mechanism, placing chitinases into two families, GH18 and GH19 (Henrissat, 1991), with fungal chitinases predominantly belonging to GH18 (Khan et al., 2015).Among the GH18 chitinases identified in microorganisms, varying degrees of glycosylation activity have been reported (Aguilera et al., 2003).The versatile functions of chitinases and their potential as targets for antifungal therapies underscore the importance of understanding their mechanisms and activities, especially within the context of fungal pathogenesis.
The genome of the fungus A. fumigatus is known to harbor a rich array of chitinase genes, numbering at least 17 (He et al., 2022).A comprehensive study identified a complex chitinase catabolism in A. fumigatus, revealing 14 distinct chitinases through phylogenetic analysis (Taib et al., 2005).The successful cloning and overexpression of the 45 kDa chitinase gene ChiB1 from A. fumigatus in Saccharomyces cerevisiae, revealed a retaining mechanism typical of GH18 family chitinases (Jaques et al., 2003).Additionally, a heat-resistant chitinase isolated from A. fumigatus YJ-407 displayed stability within a pH range of 4.0 to 8.0, with peak activity at pH 5.0 and an optimal temperature of 60 • C.This enzyme exhibited endo-chitinase, exo-chitinase, and transglycosylation activities (Xia et al., 2001).Unlike most bacteria, fungal chitinases typically exhibit optimal activity under acidic conditions.
While limited studies on chitinases in marine fungi have hindered the understanding of their potential, recent investigations successfully identified and characterized chitinase genes in two marine A. fumigatus strains, df673 (He et al., 2022) and df347 (Wu et al., 2022).AfChiJ and AfChi28, derived from these marine strains, stand out as acid-, salt-, and temperature-tolerant bifunctional enzymes with endo-and exo-endonucleation capabilities.AfChiJ, with an optimum temperature of 45 • C in colloidal chitin and a pH of 4.0, maintained high activity (≥97.96%) in 1-7% NaCl.These findings suggest that marine fungal chitinases hold significant potential for the high-value utilization of chitin biomass and may emerge as promising candidates for applications in green industries.The exploration of marine fungal chitinases expands our understanding of their diverse characteristics and applications, potentially unlocking valuable opportunities in sustainable and environmentally friendly industrial processes.

Chitosanase
Chitosan, a derivative of chitin through deacetylation, is characterized by its composition of β-(1→4)-linked D-glucosamine and N-acetyl-D-glucosamine units (Zhang et al., 2019) (Fig. 1).This biopolymer plays a crucial structural role in fungal cell walls, potentially influencing the virulence of pathogenic fungi (Beck et al., 2014).Chitosanase (EC 3.2.1.132)targets the β-1,4-glycosidic bond in chitosan, releasing oligosaccharides with heightened biological activity (Cheng et al., 2006).Unlike bacterial chitosanases, the physiological function of fungal chitosanases remains unclear, primarily because fungi expressing chitosanase are unable to grow on chitosan (Sugita et al., 2012).However, Chitosan serves as a pivotal mediator and target in fungal pathogenesis, making it an attractive therapeutic target for fungicide development.In the CAZy database, chitosanases are categorized into GH3, GH5, GH7, GH8, GH46, GH75, and GH80 families based on amino acid sequence homologies (Xu et al., 2023).The GH75 family predominantly includes A. fumigatus-derived chitosanases, with the endonuclease CSN being the first chitosanase studied in this family (Cheng et al., 2006).However, CSN is predominantly expressed as inclusion bodies in E. coli BL21 (DE3), limiting downstream purification and industrial applications.To overcome this challenge, Chen et al. explored Pichia pastoris as a host for CSN secretion expression.This approach yielded high activity, with 3 g of the enzyme converting 60 kg of chitosan into oligosaccharides within 24 h at 200 • C. Remarkably, the enzyme exhibited a half-life of 32 min at 100 • C.This research highlights the potential of fungal chitosanases for industrial applications and provides insights into their production and stability under extreme conditions.
Chitosanases GH75 from Aspergillus sp.exhibit a notable ability to hydrolyze chitosan into DP 2-7 chitooligosaccharides, showcasing potential applications in the field of medicine (Zhang et al., 2015).Notably, the promotion of chitosanase production was observed by analyzing the phenotype of an A. fumigatus strain with a deleted CsnB gene.Recombinant protein experiments further unveiled the presence of an IgG response protein, suggesting that A. fumigatus chitosanase CsnB may have utility in serological diagnosis (Beck et al., 2014).This information underscores the diverse applications and potential medical relevance of chitosanases produced by Aspergillus species, particularly A. fumigatus.

Chitin deacetylase
Chitin and chitosan differ primarily in their degree of acetylation, with polymers having a degree at or above 50% commonly known as chitin, and those with a degree below 50% referred to as chitosan (Kasaai, 2009).Chitin deacetylase (CDA), a key enzyme responsible for converting chitin to chitosan, belongs to the carbohydrate esterase family 4 (CE4) (EC 3.5.1.41)(Caufrier et al., 2003).Within fungal genomes containing chitin, the CE4 family is prevalent, and CDA plays a crucial role in cell wall morphogenesis and spore formation (Baker et al., 2007).Phytopathogenic fungi are able to evade host immune defenses during infection by secreting CDA, thus CDA is essential for fungal virulence (Liu et al., 2023).

Other exocrine proteins
The structural foundation of fungal walls is layered, featuring a robust core scaffold comprised of fibrous and gelatinous carbohydrate polymers.This scaffold serves as the base to which various proteins and additional surface components are added, forming branched chains that attach to proteins and other polysaccharides.The specific composition of these components varies among fungal species (Gow et al., 2017).In the case of C. albicans and S. cerevisiae, β-(1,6) glucan serves as a linker molecule, binding cell wall proteins (CWPs) to the β-(1,3) glucan-chitin backbone through glycosylphosphatidylinositol residues.In yeast cells, CWPs constitute 30--50% of the cell wall's dry matter (Klis et al., 2001).

Protease
Proteases, enzymes essential for peptide bond hydrolysis, are highly valued in industry for their remarkable activity, specificity, stability, and tolerance to diverse conditions such as temperature, metal ions, surfactants, and organic solvents.These proteases are categorized into various groups, including cysteine, serine proteases, metallo-, and aspartic acid proteases, among others (Morya et al., 2012).The filamentous fungus A. fumigatus is renowned for secreting numerous allergens with protease activity, and this secretion is associated with various allergic conditions (Farnell et al., 2012).Recent research highlights that the quantity, type, and activity of the main allergen proteases secreted by A. fumigatus are influenced by physiological substrates or specific protein substrate reactions.A. fumigatus demonstrates the ability to sense different protein substrates in its environment, regulating the secretion of proteases accordingly.For example, in a culture medium where collagen serves as the sole nitrogen and carbon source, A. fumigatus predominantly secretes serine alkaline protease (ALP) and metalloproteinase (MEP) (Jaton-Ogay et al., 1994).Additionally, beyond complex protein substrates, the fungus adjusts its protease secretion based on the pH of the growth medium (Sriranganadane et al., 2010).At neutral pH values, A. fumigatus secretes a set of proteases, including neutral and alkaline endonucleases, while acidic pH values promote the secretion of aspartate protease.This dynamic regulation of protease secretion in response to environmental cues highlights the adaptability of A. fumigatus and its ability to tailor its enzymatic arsenal to the prevailing conditions.
A. fumigatus, a highly pathogenic fungus, is well-known for causing invasive aspergillosis (IA), aspergilloma, and allergic bronchopulmonary aspergillosis (ABPA).There exists a demonstrated correlation in mice between the elastase produced by A. fumigatus strains and their capacity to cause IA.This correlation strongly suggests that the toxicity of A. fumigatus is closely linked to the production of extracellular proteases (Monod et al., 1991).Notably, the secretion of these proteases is considered essential for overcoming the host protein barrier (Gifford et al., 2002).Compared to other fungi within the Aspergillus genus, such as A. oryzae, A. niger, A. flavus, and A. tereus, A. fumigatus stands out with an increasing number of identified proteases in its secretome.These identified proteases exhibit the capability to hydrolyze various protein substrates, including the degradation of smaller peptide segments.This characteristic is believed to be associated with A. fumigatus's efficient acquisition of nitrogen sources (Reeves et al., 2004).In summary, investigating the pathways related to A. fumigatus proteases or protease expression holds promise as a potential target for future drug therapy in fungal infections.The understanding of A. fumigatus's proteolytic mechanisms may offer valuable insights into developing therapeutic strategies to combat fungal infections.

Phytase
Phytic acid, a phosphorylated derivative of myo-inositol, plays a crucial role as the primary storage form of phosphorus in plant seeds.Myo-inositol phosphates, which include phytic acid, exhibit a diverse array of functions in plants, ranging from acting as signaling molecules and osmoprotectants to being integral components of cell walls (Hegeman et al., 2001).In most plants, the primary storage form of phosphorus is phytate, specifically myo-inositol hexakisphosphate.However, monogastric animals, such as pigs and birds, encounter challenges in directly utilizing phytate due to insufficient levels of digestive enzymes in the intestine (Tahir et al., 2012).To address the phosphorus requirements for the growth of these animals, additional inorganic phosphorus may be added, but this can lead to significant ecological problems.Phytase (EC 3.1.3.8),belonging to the histidine phosphatase family, is primarily found in microorganisms and plants.These enzymes catalyze the release of phosphate from phytic acid, the main form of phosphorus storage in plants.Consequently, the supplementation of feed with phytase can enhance the utilization rate of phytate in monogastric animals while simultaneously reducing phytate phosphorus pollution (Han et al., 2018).
In the realm of phytase, possessing robust acid stability and thermal stability is imperative (Tomschy et al., 2002).While A. niger phyA phytase (AnP) demonstrates superior enzyme activity (102.5 U/mg) compared to A. fumigatus ATCC 13073A wild-type phytase (Afp), Afp stands out for its remarkable thermal stability and a broad range of optimal pH values (Gu et al., 2007).Even when exposure to 100 • C for 20 min, Afp retains 90% of its initial enzyme activity (Pasamontes et al., 1997).Notably, Afp demonstrates the ability to hydrolyze various substrates such as phytate, fructose, glucose 6-phosphate, etc. (Tomschy et al., 2002).In a comprehensive structural analysis of Afp, it consists of a small α-structure domain and a large α/β-structure domain composition, sharing a similar folding pattern to AnP. Liu et al. identified a larger catalytic pocket of Afp compared to E. coli phytase, potentially explaining its broader substrate specificity (Liu et al., 2004).Additionally, an acid phytase has been cloned from A. fumigatus ATCC34625 (Rodriguez et al., 2000), A. fumigatus NF191 (Gangoliya et al., 2015) and A. fumigatus WY-2 (Wang et al., 2007).This discovery holds significant implications for the application of phytase in the feed industry, showcasing the potential for enhanced enzyme performance and versatility in various feed formulations.

Conclusion and outlook
As a widely distributed saprophytic fungus in nature, A. fumigatus faces the challenge of limited directly utilizable monosaccharides in its environment.In response to this, it employs a strategy of secreting substantial amounts of proteins to break down organisms in its habitat, including cell wall biomass, acquiring nutrients for growth.While other filamentous fungi like T. reesei, Penicillium oxalate, and A. niger also secrete hydrolytic proteins, A. fumigatus has sparked industrial interest due to its remarkable enzyme production performance and the heat and acid resistance properties of its proteins.Enzymes produced by A. fumigatus exhibit thermal stability across diverse environments, including high-temperature compost piles, laboratory settings, and preclinical conditions (Bernardi et al., 2018;Kadam and Drew, 1986;Miao et al., 2015b).This characteristic may be attributed to the fungus's robust environmental resilience and adaptability.While pathogenicity poses a barrier to direct utilization in production, the versatile and efficient enzyme system of A. fumigatus can be tailored to develop optimized enzyme systems for industrial carbohydrate degradation.Moreover, the use of safe fungal heterologous expression systems, such as Pichia pastoris, offers a solution to this challenge.
Beyond producing large-molecule enzymes, A. fumigatus can generate peptides with antifungal activity, prompting further research and development.Efforts to overcome A. fumigatus's pathogenicity focuse on developing a safe degradation system for industrial production.This involves screening non-pathogenic high-yield hydrolytic enzyme strains and mitigating their toxicity through genetic engineering or efficient heterologous expression.By implementing thorough detection and pre-treatment protection measures, the untapped potential of A. fumigatus in enzyme engineering can be harnessed across research, development, production, and application of its enzymes.This approach aims to leverage the beneficial aspects of A. fumigatus's enzyme system while addressing safety concerns for industrial applications, particularly in cell wall and cell surface modification.

Fig. 1 .
Fig. 1.A schematic representation of the composition of various cell wall biomasses in nature.

Table 1
The cellulose hydrolases from the reported A. fumigatus strains.NA, not available.

Table 2
Reported hemicellulose hydrolases from A. fumigatus strains.NA, not available.

Table 3
Reported chitin hydrolases from A. fumigatus strains.NA, not available.