Boosting Hydrolysis of Cellulose at High Temperature by β‐Glucosidase Induced Metal–Organic Framework In‐Situ Co‐Precipitation Encapsulation

Abstract

Due to the poor enzyme thermal stability, the efficient conversion of high crystallinity cellulose into glucose in aqueous phase over 50 °C is challenging. Herein, an enzymeinduced MOFs encapsulation of β-glucosidase (β-G) strategy was proposed for the first time. By using various methods, including SEM, XRD, XPS, NMR, FTIR and BET, the successful preparation of a porous channel-type flower-like enzyme complex (β-G@MOFs) was confirmed. The prepared enzyme complex (β-G@MOFs) materials showed improved thermal stability (from 50 °C to 100 °C in the aqueous phase) and excellent resistance to ionic liquids (the reaction temperature was as high as 110 °C) compared to the free enzyme (β-G). Not only the catalytic hydrolysis of cellulose by single enzyme (β-G) in ionic liquid was realized, but also the high-temperature continuous reaction performance of the enzyme was significantly improved. Benefiting from the significantly improved heat resistance, the β-G@MOFs exhibited 32.1 times and 34.2 times higher enzymatic hydrolysis rate compared to β-G for cellobiose and cellulose substrates, respectively. Besides, the catalytic activity of β-G@MOFs was retained up to 86 % after five cycles at 110 °C. This was remarkable because the fixation of the enzyme by the MOFs ensured that the folded structure of the enzyme would not expand at high temperatures, allowing the native conformation of the encapsulated protein wellmaintained. Furthermore, we believe that this structural stability was caused by the confinement of flower-like porous MOFs.

Construction of Macroporous β-Glucosidase@MOFs by a Metal Competitive Coordination and Oxidation Strategy for Efficient Cellulose Conversion at 120 °C

“…Metal–organic frameworks (MOFs) are a class of porous framework materials formed by orderly coordination of metal nodes and organic ligand units. , They have high specific surface area, , tailorable pore structure, and rigid organic framework. − However, until now, owing to the microporous structure of MOFs, only a few enzymes can be encapsulated in situ by MOFs. Moreover, micropores smaller than 2 nm not only affect the accessibility and diffusivity of large macromolecules into the pores but also the enzymes “armored” by the microporosity MOFs suffer compromising activity (<10% of the original activity of the enzyme). , In previous work, our team has successfully prepared an ionic liquid-resistant and high-temperature-resistant flower-like enzyme complex β-G@MOF­(Cu-PABA) by the enzyme-induced MOF method and achieved rapidly hydrolysis of cellulose and cellobiose . However, due to the wide distribution of molecular weight of microcrystalline cellulose, some are concentrated in 2900–3600 g/mol (about 68–84 nm in diameter); this large molecular weight cellulose is difficult to enter the pore of the MOF (Cu-PABA), indicating that the enzyme lysis rate could only reach 50% .…”

“…Moreover, micropores smaller than 2 nm not only affect the accessibility and diffusivity of large macromolecules into the pores but also the enzymes “armored” by the microporosity MOFs suffer compromising activity (<10% of the original activity of the enzyme). , In previous work, our team has successfully prepared an ionic liquid-resistant and high-temperature-resistant flower-like enzyme complex β-G@MOF­(Cu-PABA) by the enzyme-induced MOF method and achieved rapidly hydrolysis of cellulose and cellobiose . However, due to the wide distribution of molecular weight of microcrystalline cellulose, some are concentrated in 2900–3600 g/mol (about 68–84 nm in diameter); this large molecular weight cellulose is difficult to enter the pore of the MOF (Cu-PABA), indicating that the enzyme lysis rate could only reach 50% . Therefore, it is very necessary to construct a MOF with macroporous structure to solve the above problems.…”

“…Simultaneously, under this strategy, MOF-Fe enabled in situ encapsulation of β-glucosidase (β-G), resulting in the construction of a macroporous enzyme complex (β-G@MOF-Fe) possessing superior synergistic catalysis ability. MOF­(Cu-PABA), a framework that is enabled to be assembled in a compatible way, was selected as the model MOF matrix . Specifically, during the formation of MOF-Fe, the addition of Fe 2+ created competitive coordination with the original metal Cu 2+ , and the doped Fe 2+ would preferentially coordinate with the amino-formed Fe 2+ –N bond according to the coordination characteristics of Fe 2+ . , Owing to the strong reducibility of Fe 2+ , Fe 2+ –N would convert into Fe 3+ –N under the action of oxygen.…”

“…30 However, due to the wide distribution of molecular weight of microcrystalline cellulose, some are concentrated in 2900−3600 g/mol (about 68−84 nm in diameter); this large molecular weight cellulose is difficult to enter the pore of the MOF (Cu-PABA), indicating that the enzyme lysis rate could only reach 50%. 30 Therefore, it is very necessary to construct a MOF with macroporous structure to solve the above problems.…”

Encapsulating β-Glucosidase within Nanoporous ZIF-8-Cu Nanoparticles Enables High-Temperature Activity

“…19−21 In the previous study, our research group have successfully structured floral complexes β-G@MOF(Cu-PABA) with resistance to ionic liquids, as well as high-temperature resistance by the enzyme-induced MOF method. 22 However, owing to the longer mass-transfer distance of the prepared 3D enzyme complex (the particle size was 22 μm), the enzyme catalytic performance was not significantly improved. As a result, it is very meaningful to prepare MOFs with a mesoporous structure, thinner crystal thickness, and more exposed active sites to solve the above problems.…”