Cellulase immobilization to enhance enzymatic hydrolysis of lignocellulosic biomass: An all-inclusive review

Cellulase-mediated lignocellulosic biorefinery plays a crucial role in the production of high-value biofuels and chemicals, with enzymatic hydrolysis being an essential component. The advent of cellulase immobilization has revolutionized this process, significantly enhancing the efficiency, stability, and reusability of cellulase enzymes. This review offers a thorough analysis of the fundamental principles underlying immobilization, encompassing various immobilization approaches such as physical adsorption, covalent binding, entrapment, and cross-linking. Furthermore, it explores a diverse range of carrier materials, including inorganic, organic, and hybrid/composite materials. The review also focuses on emerging approaches like multi-enzyme co-immobilization, oriented immobilization, immobilized enzyme microreactors, and enzyme engineering for immobilization. Additionally, it delves into novel carrier technologies like 3D printing carriers, stimuli-responsive carriers, artificial cellulosomes, and biomimetic carriers. Moreover, the review addresses recent obstacles in cellulase immobilization, including molecular-level immobilization mechanism, diffusion limitations, loss of cellulase activity, cellulase leaching, and considerations of cost-effectiveness and scalability. The knowledge derived from this review is anticipated to catalyze the evolution of more efficient and sustainable biocatalytic systems for lignocellulosic biomass conversion, representing the current state-of-the-art in cellulase immobilization techniques.

Recyclable thermoresponsive polymer-cellulase bioconjugates for biomass depolymerization

Here we report the construction and characterization of a recoverable, thermoresponsive polymer-endoglucanase bioconjugate that matches the activity of unmodified enzymes on insoluble cellulose substrates. Two copolymers exhibiting a thermoresponsive lower critical solution temperature (LCST) were created through the copolymerization of an aminooxy-bearing methacrylamide with N-isopropylacrylamide (NIPAm) or N-isopropylmethacrylamide (NIPMa). The aminooxy group provided a handle through which the LCST was adjusted through small-molecule quenching. This allowed materials with LCSTs ranging from 20.9 to 60.5 °C to be readily obtained after polymerization. The thermostable endoglucanase EGPh from the hypothermophilic Pyrococcus horikoshii was transaminated with pyridoxal-5'-phosphate to produce a ketone-bearing protein, which was then site-selectively modified through oxime linkage with benzylalkoxyamine or 5 kDa-poly(ethylene glycol)-alkoxyamine. These modified proteins showed activity comparable to the controls when assayed on an insoluble cellulosic substrate. Two polymer bioconjugates were then constructed using transaminated EGPh and the aminooxy-bearing copolymers. After 12 h, both bioconjugates produced an equivalent amount of free reducing sugars as the unmodified control using insoluble cellulose as a substrate. The recycling ability of the NIPAm copolymer-EGPh conjugate was determined through three rounds of activity, maintaining over 60% activity after two cycles of reuse and affording significantly more soluble carbohydrates than unmodified enzyme alone. When assayed on acid-pretreated Miscanthus, this bioconjugate increased the amount of reducing sugars by 2.8-fold over three rounds of activity. The synthetic strategy of this bioconjugate allows the LCST of the material to be changed readily from a common stock of copolymer and the method of attachment is applicable to a variety of proteins, enabling the same approach to be amenable to thermophile-derived cellulases or to the separation of multiple species using polymers with different recovery temperatures.