Synergistic effect of the enzyme complexes comprising agarase, carrageenase and neoagarobiose hydrolase on degradation of the red algae

Systematic review on carrageenolytic enzymes: From metabolic pathways to applications in biotechnology

Carrageenan, the major carbohydrate component of some red algae, is an important renewable bioresource with very large annual outputs. Different types of carrageenolytic enzymes in the carrageenan metabolic pathway are potentially valuable for the production of carrageenan oligosaccharides, biofuel, and other chemicals obtained from carrageenan. However, these enzymes are not well-developed for oligosaccharide or biofuel production. For further application, comprehensive knowledge of carrageenolytic enzymes is essential. Therefore, in this review, we first summarize various carrageenolytic enzymes, including the recently discovered β-carrageenase, carrageenan-specific sulfatase, exo-α-3,6-anhydro-D-galactosidase (D-ADAGase), and exo-β-galactosidase (BGase), and describe their enzymatic characteristics. Subsequently, the carrageenan metabolic pathways are systematically presented and applications of carrageenases and carrageenan oligosaccharides are illustrated with examples. Finally, this paper discusses critical aspects that can aid researchers in constructing cascade catalytic systems and engineered microorganisms to efficiently produce carrageenan oligosaccharides or other value-added chemicals through the degradation of carrageenan. Overall, this paper offers a comprehensive overview of carrageenolytic enzymes, providing valuable insights for further exploration and application of these enzymes.

Preparation and Characterization of Carrageenase Immobilized onto Polyethyleneimine-Modified Pomelo Peel

In this study, carrageenase immobilization was evaluated with a concise and efficient strategy. Pomelo peel cellulose (PPC) modified by polyethyleneimine (PEI) using the physical absorption method was used as a carrier to immobilize carrageenase and achieved repeated batch catalysis. In addition, various immobilization and reaction parameters were scrutinized to enhance the immobilization efficiency. Under the optimized conditions, the enzyme activity recovery rate was more than 50% and 4.1 times higher than immobilization with non-modified pomelo peels. The optimum temperature and pH of carrageenase after immobilization by PEI-modified pomelo peel, at 60°C and 7.5 respectively, were in line with the free enzyme. The temperature resistance was reduced, inconsistent with free enzyme, and pH resistance was increased. A significant loss of activity (46.8%) was observed after reusing it thrice under optimal reaction conditions. In terms of stability, the immobilized enzyme conserved 76.0% of the initial enzyme activity after 98 days of storage. Furthermore, a modest decrease in the kinetic constant (Km) value was observed, indicating the improved substrate affinity of the immobilized enzyme. Therefore, modified pomelo peel is a verified and promising enzyme immobilization system for the synthesis of inorganic solvents.

Extracellular expression of agarolytic enzymes in Clostridium sp. strain and its application for butanol production from Gelidium amansii

In this study, Clostridium sp. strain WK-AN1 carrying both genes of agarase (Aga0283) and neoagarobiose hydrolase (NH2780) were successfully constructed to convert agar polysaccharide directly into butanol, contributing to overcome the lack of algal hydrolases in solventogenic clostridia. Through the optimization by the Plackett-Burman design (PBD) and response surface methodology (RSM), a maximal butanol production of 6.42 g/L was achieved from 17.86 g/L agar. Further application of utilizing the butyric acid pretreated Gelidium amansii hydrolysate demonstrated the modified strain obtained the butanol production of 7.83 g/L by 1.63-fold improvement over the wild-type one. This work for the first time establishes a novel route to utilize red algal polysaccharides for butanol fermentation by constructing a solventogenic clostridia-specific secretory expression system for heterologous agarases, which will provide insights for future development of the sustainable third-generation biomass energy.

Enhanced biodegradation of waste poly(ethylene terephthalate) using a reinforced plastic degrading enzyme complex

Poly(ethylene terephthalate) (PET) is synthesized via a rich ester bond between terephthalate (TPA) and ethylene glycol (EG). Because of this, PET degradation takes a long time and PET accumulates in the environment. Many studies have been conducted to improve PET degrading enzyme to increase the efficiency of PET depolymerization. However, enzymatic PET decomposition is still restricted, making upcycling and recycling difficult. Here, we report a novel PET degrading complex composed of Ideonella sakaiensis PETase and Candida antarctica lipase B (CALB) that improves degradability, binding ability and enzyme stability. The reaction mechanism of chimeric PETase (cPETase) and chimeric CALB (cCALB) was confirmed by PET and bis (2-hydroxyethyl terephthalate) (BHET). cPETase generated BHET and mono (2-hydroxyethyl terephthalate (MHET) and cCALB produced terephthalate (TPA). Carbohydrate binding module 3 (CBM3) in the scaffolding protein greatly improved PET film binding affinity. Finally, the final enzyme complex demonstrated a 6.5-fold and 8.0-fold increase in the efficiency of hydrolysis from PET with either high crystalline or waste to TPA than single enzymes, respectively. This complex could effectively break down waste PET while maintaining enzyme stability and would be applied for biological upcycling of TPA.

Effect of carbohydrate binding modules alterations on catalytic activity of glycoside hydrolase family 6 exoglucanase from Chaetomium thermophilum to cellulose

Exoglucanase (CBH) is the rate limiting enzyme in the process of cellulose degradation. The carbohydrate binding module (CBM) can improve the accessibility of cellulase to substrate, thereby promoting the enzymatic hydrolysis of cellulase. In this study, the influence of CBM on the properties of GH6 exoglucanase from Chaetomium thermophilum (CtCBH) is systematically explored from three perspectives: the fusion of exogenous CBM, the exogenous CBM replacement of its own CBM, and the deletion of its own CBM. The parental and reconstructed CtCBH presented the same optimum pH (6.0) and temperature (60 °C) for maximum activity. Fusion of exogenous CBM increased the binding capacity of CtCBH to Avicel by 8% and 9%, respectively, but it had no significant effect on its catalytic activity. The exogenous CBM replacement of its own CBM resulted in a 12% reduction in the binding ability of CtCBH to Avicel, and a 26% reduction in the catalytic activity of Avicel. The deletion of its own CBM significantly reduced the binding ability of CtCBH to Avicel by approximately 53%, but its catalytic activity was not obviously reduced. These observations suggest that binding ability of CBM is not necessary for the catalysis of CtCBH.

Review on hydrothermal liquefaction aqueous phase as a valuable resource for biofuels, bio-hydrogen and valuable bio-chemicals recovery

Hydrothermal liquefaction (HTL) of biomass results in the formation of bio-oil, aqueous phase (HTL-AP), bio-char, and gaseous products. Safer disposal of HTL-AP is difficult on an industrial scale since it comprises low molecular acid compounds. This review provides a comprehensive note on the recent articles published on the effective usage of HTL-AP for the recovery of valuable compounds. Thermo-chemical and biological processes are the preferred techniques for the recovery of biofuel, platform chemicals from HTL-AP. From this review, it was evident that the composition of HTL-AP and product recovery are the integrated pathways, which depend on each other. Substitute as reaction medium in HTL process, growth medium for algae and microbes are the most common mode of reuse and recycle of HTL-AP. Future research is needed to depict the mechanism of HTL process when HTL-AP is used as a reaction medium on an industrial scale. Need to find a solution for the hindrance in commercializing HTL process and recovery of value-added compounds from HTL-AP from lab scale to industry level. Integrated pathways on reuse and HTL-AP recycle helps in reduced environmental concerns and sustainable production of bio-products.

Enhanced CO2 fixation and lipid production of Chlorella vulgaris through the carbonic anhydrase complex

Photosynthesis of C. vulgaris shows slow growth and low lipid production due to the low solubility of CO₂, and it is thus necessary to increase the dissolved inorganic carbon source to solve this problem. In this study, carbonic anhydrase (CA) was fused with dockerin to form a CA complex by cohesion-dockerin interaction. The CA complex was displayed on the surface of C. vulgaris by a cellulose binding module. The CA complex increased activity and stability compared to those of a single enzyme. Additionally, C. vulgaris showed an average of 1.6-fold rapid growth during log phase through the influence of the CA complex. The bicarbonate produced by the CA complex increased the lipid production about 1.7-fold (23.3%), compared to 13.6% for the control group. The present results suggest that the CA complex successfully enhances the CO₂ fixation, which should be an essential study for 4th generation biofuels.

Trienzymatic Complex System for Isomerization of Agar-Derived d-Galactose into d-Tagatose as a Low-Calorie Sweetener

d-Tagatose is a rare monosaccharide that is used in products in the food industry as a low-calorie sweetener. To facilitate biological conversion of d-tagatose, the agarolytic enzyme complexes based on the principle of the cellulosome structure were constructed through dockerin-cohesin interaction with the scaffoldin. The construction of agarolytic complexes composed of l-arabinose isomerase caused efficient isomerization activity on the agar-derived sugars. In a trienzymatic complex, the chimeric β-agarase (cAgaB) and anhydro-galactosidase (cAhgA) from Zobellia galactanivorans could synergistically hydrolyze natural agar substrates and l-arabinose isomerase (LsAraA Doc) from Lactobacillus sakei 23K could convert d-galactose into d-tagatose. The trienzymatic complex increased the concentration of d-tagatose from the agar substrate to 4.2 g/L. Compared with the monomeric enzyme, the multimeric enzyme showed a 1.4-fold increase in tagatose production, good thermostability, and reusability. A residual activity of 75% remained, and 52% of conversion was noted after five recycles. These results indicated that the dockerin-fused chimeric enzymes on the scaffoldin successfully isomerized d-galactose into d-tagatose with synergistic activity. Thus, the results demonstrated the possibility of advancing efficient strategies for utilizing red algae as a biomass source to produce d-tagatose in the industrial food field that uses marine biomass as the feedstock.

Marine oligosaccharides originated from seaweeds: Source, preparation, structure, physiological activity and applications

Marine polysaccharides originated from seaweeds, including agar, alginate, carrageenan, and fucoidan, possess various kinds of physiological activities and have been widely used in food, agricultural and medical areas. However, the application has been greatly limited by their poor solubility and low bioavailability. Thus marine oligosaccharides, as the degradation products of those polysaccharides, have drawn increasing attentions due to their obvious biological activities, good solubility and excellent bioavailability. This review will summarize the recent advances on the source, molecular structure and physiological activity of marine oligosaccharides, emphasizing their application as functional food additives. Furthermore, the relationship between the structure and the physiological activity of marine oligosaccharides is also elucidated and highlighted. The review concludes with an outlook toward potential applications for preparing the functional oligosaccharides in food biotechnology and agriculture fields.

Process optimization for the production of high-concentration ethanol with Scenedesmus raciborskii biomass

Scenedesmus raciborskii WZKMT was subjected to fed-batch enzymatic hydrolysis and fermentation to facilitate the saccharification of high-solid-loading substrate for high-concentration ethanol. In this work, process factors affecting enzymatic hydrolysis, including enzyme loading, temperature, pH, and solid loading, were optimized. Results showed that 58.03 g L^-1 glucose, 12.57 g L^-1 xylose, and 1.45 g L^-1 cellobiose were obtained after the enzymatic hydrolysis of 330 g L^-1 substrates under the optimal conditions of 30 FPU g^-1 enzyme loading, 50 °C, and pH 5.5. Meanwhile, 89.60% yield and 30.43 g L^-1 content of ethanol were obtained after the fermentation of 330 g L^-1 hydrolysate. The maximum ethanol concentration of 79.38 g L^-1 could be achieved through repeated fed-batch process, indicating that S. raciborskii WZKMT is a promising feedstock for ethanol production.

A perspective on galactose-based fermentative hydrogen production from macroalgal biomass: Trends and opportunities

This review analyses the relevant studies which focused on hydrogen synthesis by dark fermentation of galactose from macroalgal biomass by discussing the inoculum-related pretreatments, batch fermentation and inhibition, continuous fermentation systems, bioreactor designs for continuous operation and ionic liquid-assisted catalysis. The potential for process development is also revisited and the challenges towards suppressing glucose dominance over a galactose-based hydrogen production system are presented. The key challenges in the pretreatment process aiming to achieve a maximum recovery of upgradable (fermentable) sugars from the hydrolysates and promoting the concomitant detoxification of the hydrolysates have also been highlighted. The research avenues for bioprocess intensification connected to enhance selective sugar recovery and effective detoxification constitute the critical steps to develop future red macroalgae-derived galactose-based robust biohydrogen production system.

Addition of β-galactosidase boosts the xyloglucan degradation capability of endoglucanase Cel9D from Clostridium thermocellum

BACKGROUND

Increasing the efficiency of enzymatic biomass degradation is crucial for a more economically feasible conversion of abundantly available plant feedstock. Synergistic effects between the enzymes deployed in the hydrolysis of various hemicelluloses have been demonstrated, which can reduce process costs by lowering the amount of enzyme required for the reaction. Xyloglucan is the only major hemicellulose for which no such effects have been described yet.

RESULTS

We report the beneficial combination of two enzymes for the degradation of the hemicellulose xyloglucan. The addition of β-galactosidase Bga2B from Clostridium stercorarium to an in vitro hydrolysis reaction of a model xyloglucan substrate increased the enzymatic efficiency of endoglucanase Cel9D from Clostridium thermocellum to up to 22-fold. Furthermore, the total amount of enzyme required for high hydrolysis yields was lowered by nearly 80%. Increased yields were also observed when using a natural complex substrate-tamarind kernel powder.

CONCLUSION

The findings of this study may improve the valorization of feedstocks containing high-xyloglucan amounts. The combination of the endoglucanase Cel9D and the β-galactosidase Bga2B can be used to efficiently produce the heptasaccharide XXXG. The exploitation of one specific oligosaccharide may open up possibilities for the use as a prebiotic or platform chemical in additional reactions.