Introduction

Biotechnology today is present everywhere and has a large influence on several diverse industries, including the production of feed and food, biofuel as well as the efficient synthesis of valuable chemical compounds (Elleuche et al. 2014). With the increasing demand for bioethanol, the need for possible substitutes of fossil-based biofuel has increased substantially too. One such alternative source that have been used extensively in the recent times is lignocellulosic biomass. Lignocellulose is a polysaccharide combination of cellulose, hemicellulose and lignin and is the most prevalent source of organic material in the world (Plácido and Capareda 2015). Lignocellulolytic enzymes are biocatalysts involved in the bioprocessing of lignin and cellulosic materials into their constituents for further breakdown into significant products such as biofuel (Chukwuma et al. 2020). With the advancement of biotechnology to its present state, the processing of biomass to biofuel using extremophilic microorganisms has gained significant importance. Extremophilic microbial process converts the biomass into biofuels using microorganisms that can survive in harsh environments like high temperature, high salt concentrations, high pressure, low pH, or intense radiation. The ability to produce biofuels from a larger variety of biomass sources (e.g., lignocellulosic biomass) can be made feasible by utilizing the unique abilities of extremophilic microbes (Jönsson et al. 2013; Zhu et al. 2020). Extremophiles can produce stable enzymes called extremozymes that are frequently able to withstand variations in external factors, such as pH and temperature. Such organisms e.g., Clostridium thermocellum and their associated enzymes e.g., extremozyme cellulase have a wide range of potential uses in bioengineering, with the manufacture of biofuel like ethanol and hydrogen (Ahmad et al. 2021).

Extremozymes go through a number of alterations to adjust to the harsh environments they live in. These enzymes differ from related mesophilic proteins as they have modified shape, structure, or functionality. Despite their near similarities, the terms, shape and structure each refer to a separate feature of protein conformation. The structure of a protein refers to the particular configuration of atoms inside the protein molecule, whereas the shape of a protein refers to its overall three-dimensional geometry. The form refers to the overall look of the protein, whereas structure refers to the specific arrangement of the protein’s secondary, tertiary, and quaternary structures. Alteration in the tertiary and quaternary structure of the protein is necessary for extremozymes to adapt their form, structure, or functioning in accordance with their surroundings (Rehman et al. 2022). For example, the thermophilic lignocellulolytic enzyme CelA, isolated from the bacterium Caldicellulosiruptor bescii, has a modular structure and contains multiple carbohydrate-binding domains, which allow it to efficiently degrade lignocellulosic biomass even under higher temperature as compared to the enzymes, which work under mesophilic conditions (Brunecky et al. 2013). The conformation of proteins from psychrophiles is different to those of mesophiles and thermophiles to adapt flexibility in their structures and boost the effectiveness of their catalytic activity at low temperature e.g., cold-active cellulase enzyme produced by Nocardiopsis dassonvillei (Sivasankar et al. 2022). The enzymatic activity of the proteins in thermophiles is further known to be increased by amino acid substitution (Akanuma et al. 2019) and the biochemical role of n-glycosylation in thermostable endoglucanases from Chaetomium thermophilum has also been explained (Han et al. 2020).

In the future, efforts to increase the performance of biofuel synthesis and discover the energy capacity of agricultural and agro-industrial waste water would be streamlined by the understanding of the structure–function of extremoCAZymes (a class of extremophilic enzymes that belongs to the carbohydrate-active enzymes (CAZymes) family), the advancement of editing the genomes of microorganisms and expression systems using CRISPR-CAS system, and developments in extremophilic enzyme immobilization techniques (Souii et al. 2022). Extremophiles and extremozymes excel in the pre-treatment of biomass e.g., cellulase from obligate halophilic Aspergillus flavus (Bano et al. 2019), bioremediation e.g., extremophilic red alga G. sulphuraria (Singh et al. 2023), lignin depolymerisation e.g., a halotolerant and alkalophilic bacteria Bacillus ligniniphilus, which significantly breakdown lignin at the ideal pH 9 and create aromatic chemicals like vanillic acid and vanillin (Zhu et al. 2017). Because of their enhanced performance under a wide range of external environmental circumstances, they have a lot of significant applications in industrial processes.

Naturally the extremophilic organisms have a diverse habitat from mountains to volcano sites, dessert to ocean and glaciers to rocks. Deep sea is known to be one of the biggest biomes on earth and is home to a number of extremophiles (Jin et al. 2019). Extremophiles are divided into groups based on the environments in which they thrive (Table 1).

Table 1 Primary groups and division of extremophiles
Full size table

All of these organisms possess several advanced level machineries to survive and thrive in the harsh conditions they are exposed to. These microorganisms safeguard their genetic systems in such environments and retain the mobility and integrity of their membranes under harsh conditions. Thermophiles have heat stable proteins and their cellular membranes resist protein degradation by not getting dissociated at high temperature (Arora and Panosyan 2019). In comparison to their mesophilic counterparts, thermophilic enzymes and thermostable proteins have evolved to use a variety of techniques to tolerate harsh conditions. Some universal signatures of thermal adaptation have been reported based on primary, secondary and tertiary structure of protein (Fig. 1). In Primary structure properties, it was observed that small amino acids are disfavoured as they are associated with large backbone entropy, while charged amino acids are highly preferred on protein surface, which result in enhanced solvent interaction and abundant salt bridges. To avoid thermal damage caused by de-amidation mechanism of protein damage in high temperatures, the amide amino acids are highly disfavoured. On the other hand, among tertiary structure properties, large hydrophobic core, exposed charged groups, Electrostatic interactions mediated by salt bridges and higher number of cation–π interactions were observed (Hait et al. 2020).

Fig. 1
figure 1

Some universal signatures of thermal adaptation of protein. Large octagonals connected with solid lines represents more significant, while small octagonals connected with dotted lines represent less significant contribution in thermostability

Full size image

Beside bacteria and archaea, fungi are among the eukaryotes that can survive in extremely hostile conditions. The presence of thermophilic fungus is typically found in soil or environments where plant matter, such as grains, manure, waste materials, and other organic components, decomposes in an environment that is humid and aerobic. In these settings, thermophiles exist both as dormant propagules or as functional mycelia based on the nutrients and the environmental situation around it. The occurrence of thermophilic fungus takes place due to the spread of propagules through mass of organic material. The genera Zygomycetes, Ascomycetes, and Deuteromycetes all contain thermophilic fungi (Chaturvedi and Sarethy 2022). Fungi and aerobic bacteria breakdown of lignocellulosic biomass through the free enzyme system, while anaerobic bacteria use a different lignocellulolytic system that utilizes intricate protein complexes like cellulosomes and xylanosomes during the hydrolysis of lignocellulose (Malgas et al. 2017).

Lignocellulolytic extremozymes

The lignocellulolytic extremozymes are a large group of extracellular proteins consisting of ligninolytic and cellulolytic enzymes. In the coming years, biofuels that can be substituted for fuel derived from fossils, will be made possible due to the enzyme's assistance in degrading lignin. Thermostable lignin peroxidase is employed in the elimination of dyes, lignin by-products, and environmental contaminants. The textile sector can benefit from the utilization of thermostable laccases for blanching (Liu et al. 2020). The biological processing of biomass to biofuel has proven to be more cost effective and environmental friendly as compare to chemical or physical techniques, and hence has acquired increasing attention in the past few decades, despite the fact that chemical and thermal techniques are the existing technology for producing bioenergy (Sharma et al. 2023).

Cellulolytic enzymes

Cellulolytic enzymes are proteins that allow hemicellulases and cellulases, respectively, to break down cellulose and hemicellulose into glucose and xylose. The finished goods can be applied to the manufacturing of bioethanol. Saccharification is the primary step for biofuel production and the operation conditions include high temperature and salt concentration. Under such conditions, thermo/halophilic enzymes may be a good choice e.g., Halophilic Virgibacillus salaries produced cellulase enzyme, which exhibited stability at 12% NaCl and 60 ℃. This enzyme could utilize non-pretreated lignocellulosic substrates. Among different substrates, utilization of wheat bran demonstrated maximum saccharification yield upto 90% (Yousef and Mawad 2023).

Some cellulolytic extremozymes produced by different microorganisms were reported in last few years (Table 2). These extremozymes demonstrated their optimum enzyme activity from 60 ℃ to 80 ℃, while most of the enzymes were able to work at a lower PH and few showed their optimum pH as low as 3. Thermostability/Half-life was also given in the table for ready reference.

Table 2 Different cellulolytic extremozymes, microorganisms and their optimum conditions
Full size table

Cellulases

The most prevalent polymer on earth, cellulose also serves as the structural foundation for the cell walls of green plants. It consists of millions of glucose units connected by 1,4-glycosidic linkages. Being made up of both amorphous and extremely organized crystalline sections, it is a heterogeneous structure (Cabrera and Blamey 2018). It takes three different forms of cellulases—Endoglucanases, Cellobiohydrolases, and β-Glucosidase—to hydrolyze cellulose into glucose. High levels of polymerization oligomers are produced when endoglucanases haphazardly hydrolyse the 1,4-glycosidic connections in cellulose’s amorphous structure. Two glucose units (cellobiose) are removed from the ends of the cellulose chain by cellobiohydralases and finally, Cello-oligosaccharides and cellobiose are metabolized into glucose by β-glucosidase (Zuccaro et al. 2019).

Endoglucanase

An important stage in the bioconversion of lignocellulosic resources into biofuel is the earliest phase of cellulose breakdown, which is engaged with endoglucanases. (Yennamalli et al. 2011). To make the ends of the cellulose chain available to cellobiohydrolase, they function by hydrolysing the internal β-glycosidic linkages in the cellulose chain (Yennamalli et al. 2013). The composition of amino acids in thermophilic endoglucanases and their mesophilic counterparts differs greatly, which explains why they are more stable. Variations in interactions act as determining forces for thermostability, and even though thermostability in endoglucanases is typically imparted through varying amino acid composition, in some cases even a single-point mutation modification can induce thermostability (Fig. 2). Surprisingly, these structural variations are particular to enzyme families and protein folds, and they influence the intramolecular associations in a fold-dependent method (Yennamalli et al. 2011). Thermophiles have increased concentrations of particular amino acids like Arg and Glu. The energy of unwinding, the count of Van der Waals contacts per repeat, the number of hydrogen bonds per base, or the number of residues engaged in the secondary structure of protein are possible additional parameters that contribute to the enzyme’s thermostability (Yennamalli et al. 2013). Intramolecular interactions were computed to determine the sequence of amino acids that contribute to the enhanced stability of endoglucanase. The protein length was normalized, and a t-test was run to determine statistical significance (Yennamalli et al. 2011).

Fig. 2
figure 2

Yenamalli’s experimental studies evaluated the already known endoglucanase structures from the PDB and have demonstrated that protein folds instead of protein families are more crucial when trying to define rules for thermophilicity (Yennamalli et al. 2011)

Full size image

An archaeal endoglucanase Vul_Cel5A, was found to be highly active at a temperature of 115 ℃ and can efficiently break down various polysaccharides including barley β-glucan, lichenan, carboxymethyl cellulose, and locust bean gum. This enzyme demonstrated a long half-life time of 46 min at 100 ℃ and remained active even after 48 h of incubation at 75 ℃ (Suleiman et al. 2019). Current research findings imply that endoglucanases from a few particular bacteria may be useful as laundry detergent additives (Dutta et al. 2008). Endoglucanases are extremely useful in the manufacture of textiles, paper, and food for both humans and animals. Endoglucanases are also thought to increase industrial operations’ cost effectiveness by reducing the requirement for heating and, consequently, energy use (Khalili Ghadikolaei et al. 2018). Endoglucanases aid in the biodegradation process that leads to the production of biofuel by converting lignocellulosic substrates into soluble sugars (Mandeep et al. 2021). They are potentially valuable biocatalysts for various transformative biomass processing industrial applications. They find applications in food and fruit processing industries, textile and wood processing; they also act as exogenous nutraceutical enzyme supplements for human foods, and are extensively used in the food and feed industries (Fan et al. 2021). In addition to cellulose degradation, they may also be beneficial for altering fibrous substrates and making cellulose oligosaccharides (Wu and Wu 2020). Due to their stability in an alkaline environment and their ability to function at relatively cold temperature, psychrophilic endocellulase from Pseudoalteromonas sp. and cold-active endoglucanase from Paenibacillus sp. has been suggested to be used in the food business and in the chemical industry to produce laundry additives (Dong et al. 2016; Yuan et al. 2018).

Cellobiohydrolase

Cellobiohydrolases (CBH) degrade cellulose by hydrolysing the 1,4-D-glycosidic linkages in its chain. CBH is of two types, CBH I and CBH II. CBH I enzyme cleaves the cellulose chain from the reducing end, while CBH II, acts on the non-reducing end of the cellulose chain. They both act primarily as exoglucanases releasing free cellobiose molecules from the ends of the polymeric cellulose chain (Hildén and Mäkelä 2018).

Recently, a hyper-thermostable cellobiohydrolase (HmCel6A) was isolated metagenomically from hot spring sediment and expressed in E. coli. This study revealed several unique features associated with the thermostability of this enzyme. These structural features include characteristic metal binding, disulphide bonds and shortened loops around the substrate tunnel. Besides, an additional tryptophan, was located at the enzyme’s tunnel entrance, and suggested to contribute in substrate recognition and thermostability of the enzyme. The optimum temperature for this enzyme was 75 ℃, while some variants demonstrated an increased optimum temperature i.e., 95 ℃ (Takeda et al. 2022). Similarly, extremophilic cellobiohydrolase (AnCel7B) from marine Aspergillus niger showed its thermostable property with the half-live of 90 min at 90 ℃. It was suggested that charged amino acid residues in the enzyme engendered sufficient salt bridges to make the structure thermostable (Cai et al. 2022).

Cellobiohydrolases, similar to β-glucosidases and endoglucanases, have high industrial importance due to their widespread use in paper, textiles, pharmaceutical, and animal food items (Ejaz et al. 2021). Additionally, they are frequently used in conjunction with other cellulases in the saccharification of lignocellulose to create fermentable sugars that could then be turned into bioethanol, the next-generation fuel.

β-Glucosidase

The final and most important stage in the hydrolysis of cellulose is catalyzed by β-glucosidases, which are a crucial element of the cellulase system (cellulose metabolising enzymes). These enzymes are found in all types of organisms and are crucial for the conversion of biomass in microbes. They are also known to operate as a pest deterrent, to activate phytohormones, and to start the breakdown of cell walls in plants. They are also involved in the breakdown of glycolipids and the lignification process (Singh and Vinod 2016). In an experiment, the structures deposited in PDB were compared to the crystal structure of a family GH3 β-glucosidase from the fungus Chaetomium thermophilum (CtBgL). It became clear that glycosylation and charged fragments involved in electrostatic interactions may be responsible for the enzyme’s stability at high temperatures (Mohsin et al. 2019).

β-glucosidases have characteristics like tolerance to sugar, substrate selectivity, and heat stability that can be helpful for their uses in the food business, pharmaceutical sector, and lignocellulose breakdown (Godse et al. 2021). β-glucosidase is a possible substitute for the generation of cellulosic ethanol (Huang et al. 2021). They are potential tools for producing bioethanol, and in the coming years, they may be able to meet the demands of alternative sources of clean energy (Tiwari et al. 2013). They are also known to reduce the bitterness of citrus fruits and can also be used as additions to hydrolyze non-starch polysaccharides. They enhance the texture, flavour, and scent of vegetables and fruits (Kuhad et al. 2011). They also serve as flavouring agents for various juices and wine (Singh et al. 2016). Additionally, β-glucosidase is used in the detoxification of cassava, the formation of oligosaccharides, and the degradation of isoflavones (Ahmed et al. 2017). Mammalian β-glucosidases are believed to be involved in the breakdown of dietary glucosides and glycolipids as well as signalling activities. This enzyme is used biotechnologically in a variety of sectors, including the food, surfactant, biofuel, and agricultural ones (Aryan et al. 1987).

β-Glycosidases play an important role in winery by enhancing wine aroma. An extremophilic β-glucosidase A (BglA) produced by Halothermothix orenii released volatile aglycones from non-volatile glycosides. BglA was active at 3.5 pH, and could tolerate upto 10–14% ethanol concentration, while it released sufficient volatiles in sweeter wines and in grape juices. The extremozyme β-glucosidase A exhibits a potential to be used in the initial phases of wine production and in the juice processing (Delgado et al. 2020).

Hemicellulases

Hemicellulose is the second most prevalent biopolymer in plant biomass. The primary component of the cell wall, a highly homogeneous 1 4–6-associated polyglucan, sets it apart from cellulose. Hemicellulose’s structure is amorphous, irregular, and weak. Hemicellulose's hydrophilic properties help to make the plant cell wall stiffer and more flexible when combined with cellulose and lignin. Additionally, it helps the plant cell wall retain water (Machmudah et al. 2017). The primary constituent of hemicellulose is xylan, which is a heterogeneous molecule with a main chain made of repeats of xylose which are connected by β-1,4-glycosidic linkages (Cabrera and Blamey 2018). Xylan is hydrolyzed by the action of xylanases.

Xylanases

A vital class of depolymerizing enzymes known as xylanases is employed to hydrolyze xylan, which is potentially a key component of hemicellulose. Thermostable xylanases are employed in numerous industrial processes and some of their thermostability may be linked to a larger proportion of aromatic residues for improved compactness and a greater percentage of charged residues upon the surface of the protein for enhanced water-mediated hydrogen-bonding (van Bueren et al. 2012). To find out the difference between xylanase derived from extremophiles thriving in harsh conditions and xylanase derived from mesophiles, a mesophilic GH11 xylanase and a GH11 family xylanase (Xyn11A) with good heat stability and enzymatic activity were compared. It was discovered that Xyn11A had more H-bonds and was more compact. The mutagenesis results demonstrated that the high thermostability and activity of Xyn11A may be due to the electrostatic interactions in the thumb and palm region. The ideal reaction temperature would rise to 90 ℃ with improved activity if a disulfide link were added further at the n-terminus (Yi et al. 2021).

In comparison to the closest described xylanolytic enzyme, a novel heat stable GH10 xylanase known as XynDZ5 only shared 26% of its amino acid sequence. The thermostability at different temperatures may not be that much high in the extremozymes, they may work upto certain high temperatures but not beyond that. As observed, XynDZ5 tolerated elevated temperatures as up to 65 ℃, the enzyme retained over 80% of its activity even after 20 h of exposure, while it rapidly lost its catalytic activity when exposed to high temperatures at 75 ℃ (Fig. 3). XynDZ5 was considered as a natural thermophilic enzyme. A newly created bioinformatic analytical tool was used to find this novel protein from a culture of a Thermoanaerobacterium strain in an Icelandic hot spring. The protein was able to preserve its high catalytic activity even after being incubated at extreme temperatures for many hours, in the influence of substantial quantities of a variety of metal ions, and in the vicinity of inactivating agents. Computer modelling of its 3D structure predicted a (β/α)8 TIM barrel fold, which is extremely among family GH10 enzymes. This stimulated structure offered hints regarding structural features that could explain the enhanced activity of the enzyme (Zarafeta et al. 2020).

Fig. 3
figure 3

Thermostability of enzyme XynDZ5: assessed by determining the residual activity of the enzyme after incubation at 65 ℃ (■), 70 ℃ (▲), and 75 ℃ (•), for a duration of 20 h (Zarafeta et al. 2020)

Full size image

Some organisms may produce both cellulase and xylanase, which are active at high temperature. For instance, fungus Thielavia terrestris produced a bifunctional cellulase/xylanase GH7 enzyme called TtCel7A. This enzyme demonstrated its optimal activity at 60 ℃ and 50 ℃ for cellulolytic and xylanolytic activities, respectively at pH 5.5 (López-López et al. 2023). On the other hand, two halophilic fungi, Aspergillus flavus and Aspergillus penicillioides, were reported to produce CMCase, FPase, xylanase, and β-xylosidase under solid-state fermentation conditions. These enzymes demonstrated their optimum activity at pH levels (9.0–10.0), high temperatures (50–60 ℃), and in the presence of 15–20% salt. These enzymes were also stable in the presence of metal salts and ionic liquids. When used together to treat bamboo, the cellulase-hemicellulase co-treatment produced higher yields of reducing sugars than separate cellulase and hemicellulase treatments (Zhao et al. 2021).

In the paper and pulp business, xylanases are used to bleach cellulose fibres instead of chlorine. Xylanases are employed as nutritional supplements or to treat indigestion in the pharmaceutical business and in human health. As low-calorie sweeteners, however, xylan hydrolytic products are employed (Cabrera and Blamey 2018). The use of xylanases in food and feed, paper and fibre, clothing, medicine, and lignocellulosic biorefinery has led to an increase in demand for them on a global scale (Dar and Dar 2021). The enzyme has also been used for pretreatment of biomass and bioethanol production, e.g., a highly thermostable crude xylanase from Geobacillus sp. DUSELR13 demonstrated its optimum activity at 75 ℃. This xylanase was highly thermostable, with a half-life of 48, 38, and 13 days at 50 ℃, 60 ℃, and 70 ℃, respectively. The crude xylanase was found to perform better than its commercial counterparts e.g., Novozymes Cellic HTec at high temperatures. This enzyme also showed potential in the hydrolysis of pretreated and untreated lignocellulosic biomass, i.e., prairie cord grass and corn stover, and was able to produce ethanol when grown in co-culture with Geobacillus thermoglucosidasius (Bibra et al. 2018).

Ligninolytic enzymes

A highly complex 3D amorphous polymer called lignin makes up 15% of the biomass on the planet. By bridging the gaps between various polysaccharides in the cell wall, it gives the plant mechanical strength. The following oxidative enzymes are needed to break down lignin: laccase, manganese peroxidase, and lignin peroxidase (Niladevi 2009). Recently, a number of microorganisms have been isolated, which showed the production of ligninolytic extremozymes (Table 3).

Table 3 Different ligninolytic extremozymes produced by microorganisms along with their optimum conditions
Full size table

Laccase

High-stability glycoproteins known as laccase can exist as monomers, dimers, or tetramers. By employing molecular oxygen and electron-donating groups, they are known to oxidize both aromatic and non-aromatic molecules. They are extra-cellular and have an inducible nature, which results in limited substrate specificity for breakdown. Because of their very compact hydrophobic core, hydrogen bonds, and increased polar surface area, laccases are known to be very thermostable (Chandra et al. 2017). Through the method of site-directed mutagenesis, it has been possible to enhance specific laccase qualities like thermal stability, activations, and compactness. During an experiment, negative glu 188 was replaced by the three AA amino acids Arg, Ala, and Lys which increased the enzyme's activity by at least nine times (Fig. 4) (Mollania et al. 2011).

Fig. 4
figure 4

Local protein structure of laccase. The location of Glu188 and surrounding residues in the wild-type and the Glu188Arg variant are indicated in (A) and (B), respectively. Stick format displays are used for residues. Black lines denote the salt bridges (Mollania et al. 2011)

Full size image

There are numerous industries where extremophilic ligninolytic enzymes are used. Laccases are useful in the detoxification of pulp, paper, and petrochemical industry industrial effluents. Extremophilic laccase (LacT) produced by Brevibacillus agri, has been exploited for its in denim bleaching. LacT demonstrated thermostable and acidophilic properties along with tolerance to high salt, different organic solvent, and few divalent metal ions. Optimum denim bleaching efficiency this enzyme was observed at pH 4.0 (Panwar et al. 2020). Similarly, in bioremediation, a recombinant spore-coat laccase produced by Bacillus sp. FNT, a thermoalkaliphilic bacterium, was tested for its ability to oxidise different polycyclic aromatic hydrocarbons, and also found to efficiently oxidise anthracene. This laccase worked optimally at temperatures from 70 to 80 ℃ (Bueno-Nieto et al. 2023). Thermotolerant laccase has been used for functionalization of chitosan film. A thermophilic bacterial strain Bacillus sp. PC-3 T produced laccase enzyme, which demonstrated its maximum activity at 60 ℃ and pH 7. It showed a half-life of 3.75 h at 60 ℃ and retained 95% activity at pH 7 for 240 min. Additionally, the enzyme was used to modify a chitosan film, which resulted in improved antioxidant and antimicrobial activity (Sharma et al. 2019).

Thermophilic laccase has also been used in the field of bioelectrocatalysis. A native laccase enzyme from thermophilic bacteria Bacillus sp. FNT showed maximum activity at a high temperature of 70 ℃ and a neutral to slightly alkaline pH range of 7 to 8. The amino acid sequence of the enzyme indicated a higher proline content than that of mesophilic T. versicolor laccase. In addition, a recombinant form of the laccase enzyme was used to create an enzyme-multi-walled carbon nanotube composite catalyst, which was found to be more stable and effective for the oxygen reduction reaction compared to similar conjugates made using commercially available T. versicolor laccase (Atalah et al. 2018).

Manganese peroxidase

A vital enzyme in the breakdown of lignin is manganese peroxidase (MnP), an oxi-reductase enzyme that contains heme. MnP enzyme is secreted by ligninolytic bacteria in both solid and liquid states. Histidine (His), aspartic acid (Asp), and arginine (Arg) residues are found in the enzyme's active site. Oxidizing Mn+2 into Mn+3 ions, which are reactive and steadied by chelators such as oxalic acids, is the primary activity of the enzyme. Free radicals are created when MnP, a diffusible redox intermediate, breaks down phenolic lignin (Sundaramoorthy et al. 2005). One minor helix, 10 major helixes, and 5 disulphide bridges make up the monomeric protein MnP and contribute to its stability. Its ideal pH spans from 3.5 to 9.0, and its ideal temperature ranges from 25 to 70℃. The enzyme has a MW range of 25 to 68 kDa (Kumar and Arora 2022).

A highly useful biocatalyst for the breakdown of dangerous environmental pollutants, dye wash water decolorization in particular, is manganese peroxidase (Chang et al. 2021). Manganese peroxidase produced by alkaliphilic bacterial strain Pseudomonas mendocina breaks down the reactive azo dyes, and thus, are used in environmental remediation (Gomathy et al. 2021). MnP’s capacity to oxidize and purify a variety of resistant toxic compounds, including synthetic colours, has also attracted substantial attention for detoxifying textile waste (Kalsoom et al. 2022).

Lignin peroxidase

A heme-containing enzyme called lignin peroxidase, or oxireductase, helps lignin and its derivatives degrade when H2O2 is available (Kumar and Chandra 2020). The fungus Aspergillus sydowii MS 19, a psychrophile collected from the antarctic region, was shown in a study to synthesize LiP and MnP at temperatures as cold as 3 ℃. However, the optimum pH for the enzyme’s synthesis and function is 4.2, and the optimum temperature ranges from 30 to 34 ℃ (Cong et al. 2017). Another investigation using cold-adapted enzyme synthesis demonstrated Cladophora's ability to produce LiP at 15 ℃. Due to its strong redox potential for the oxidation of phenolic and non-phenolic chemicals in lignin breakdown, lignin peroxidase, with a mw ranging from 38 to 46 kDa, exhibits a broad spectrum of uses (Liu et al. 2021). Lignin peroxidase serves as second-generation biofuels (Biko et al. 2020). Significant use is made in the cosmetics, dermatological preparations, and bio pulping and bio bleaching sectors (Chowdhary et al. 2020).

Conclusion and future prospects

Extremozymes have been employed widely in the last few years because of their robustness and capacity to survive in harsh environments. Due to their strong resilience to extremes in temperature, chemicals, organic solvents, and pH, thermophilic enzymes in particular have a wide range of uses in various fields. The importance of extremophilic enzymes is found in both industrial and medical operations because they can endure high temperatures, prevent unfolding, and function at their peak under harsh conditions. Enzyme structure along with their amino acid composition play an important role to make them tolerant of extreme conditions.

The limited availability of extremophilic enzymes, which can be challenging to isolate and produce in large amounts, is one of the major bottlenecks in its applications. In addition, the high cost as well as the technological obstacles that are linked with their production and purification must be addressed. Another obstacle is the optimization of enzyme performance, which may require changes to the enzymes themselves or the catalytic mechanisms they employ. This could include the use of genetic engineering, directed evolution, or the usage of cofactors or co-substrates to increase the activity and specificity of enzymes. Enzyme engineering has, however, received little research in attempts to increase the stability and catalytic effectiveness of lignocellulosic enzymes. Therefore, research should concentrate on improving the stability and catalytic performance of proteins by mutations, domain substitutions, computational modelling, and hybrid enzyme technologies in the coming future. To create new enzymes having exceptional properties and stability, genomic information must also be exploited more thoroughly.

Extremophilic enzymes that break down lignocellulose have numerous possible uses in the bioenergy, bioremediation, and biorefining sectors. However, there may be compatibility concerns with the reaction conditions that are necessary for the application of these enzymes in the bioprocessing industry. For instance, certain reactions require extremely high pH or temperature, both of which are incompatible with many other aspects of the process. To solve this problem, one solution would be to use immobilised enzymes, but there are also other ways to ensure that the enzymes are kept in conditions that allow them to function at their highest potential without affecting the process as a whole (Fig. 5). Future prospects for the use of synthetic and chimeric extremozymes for lignocellulosic biomass valorization should be encouraged. Beside enzymes, some extremophilic microorganisms may also be utilized in a variety of biotechnological fields to further the development of a bio-based society and for further research.

Fig. 5
figure 5

Some key industrial conditions, which require specific extremozymes

Full size image