Biology subjects

Nemmaru, B.

Publications and source records attributed to Nemmaru, B..

5 recordsLinked to original sources

bioRxiv2025

There is an I in team: individual improvements in supercharged cellulase cocktail facilitates cooperative cellulose degradation

Lignocellulosic biomass is a vastly abundant renewable carbon source for biofuel production but its conversion to fermentable sugars is significantly hindered by an inherent recalcitrance to enzymatic degradation. Pretreatment technologies are successful in alleviating some challenges related to substrate recalcitrance, yet enzymes like cellulases still exhibit poor activity on highly crystalline and insoluble cellulose. Both cellulose and lignin present several issues with productive enzyme binding and efficient catalytic turnover. To address these bottlenecks, we employed protein supercharging to rationally design a glycosyl hydrolase (GH) family-6 exocellulase (Cel6B) and its native fused family-2a carbohydrate binding module (CBM2a) from the thermophilic cellulolytic microbe Thermobifida fusca. A total of 16 supercharged variants were designed across both GH/CBM domains and a chimeric library of 32 constructs, including the native enzyme, were synthesized and expressed in E. coli. The entire library of supercharged enzymes was tested for activity on several cellulosic substrates to identify one key construct, D5 CBM2a - WT Cel6B, that had a positively supercharged CBM2a that showed 2-3-fold higher activity on all substrates tested at pH 5.5. Purified enzyme assays confirmed that exocellulases behave quite different from their endocellulase counterparts when supercharged using similar protocols. Still, the purified D5 CBM2a - WT Cel6B mutant showed a 2.3-fold improvement in specific activity compared to native enzyme on crystalline cellulose. Analysis of melt curves depict that, while all other constructs tested have one distinct melt peak near the expected CBM melting point, domain melting is decoupled for the D5 CBM2a mutant. This effect reveals an intrinsic melting temperature of the Cel6B CD nearly 18 {degrees}C higher than the coupled melting temperature of the full-length enzyme. This unexpected catalytic domain stabilization effect of supercharged CBM2a domain is likely the driving force for activity improvements seen for this exocellulase that is otherwise prone to stalling and denaturation on the cellulose surface during processive catalytic turnover cycles. When combining this supercharged exocellulase construct with its endocellulase counterpart, our results show that supercharged enzymes that show the highest activity alone, produced the best synergistic partners. This study highlights another successful implementation of protein supercharging strategy for cellulases and provides another key piece towards building an effective synergistic cellulase cocktail for lignocellulosic biomass deconstruction.

bioengineering↗

bioRxiv2023

Supercharging carbohydrate-binding module alone enhances endocellulase thermostability, binding, and activity on cellulosic biomass

Lignocellulosic biomass recalcitrance to enzymatic degradation necessitates high enzyme loadings incurring large processing costs for industrial-scale biofuels or biochemicals production. Manipulating surface charge interactions to minimize non-productive interactions between cellulolytic enzymes and plant cell wall components (e.g., lignin or cellulose) via protein supercharging has been hypothesized to improve biomass biodegradability, but with limited demonstrated success to date. Here we characterize the effect of introducing non-natural enzyme surface mutations and net charge on cellulosic biomass hydrolysis activity by designing a library of supercharged family-5 endoglucanase Cel5A and its native family-2a carbohydrate binding module (CBM) originally belonging to an industrially relevant thermophilic microbe Thermobifida fusca. A combinatorial library of 33 mutant constructs containing different CBM and Cel5A designs spanning a net charge range of -52 to 37 was computationally designed using Rosetta macromolecular modelling software. Activity for all mutants was rapidly characterized as soluble cell lysates and promising mutants (containing mutations either on the CBM, Cel5A catalytic domain, or both CBM and Cel5A domains) were then purified and systematically characterized. Surprisingly, often endocellulases with mutations on the CBM domain alone resulted in improved activity on cellulosic biomass, with three top-performing supercharged CBM mutants exhibiting between 2-5-fold increase in activity, compared to native enzyme, on both pretreated biomass enriched in lignin (i.e., corn stover) and isolated crystalline/amorphous cellulose. Furthermore, we were able to clearly demonstrate that endocellulase net charge can be selectively fine-tuned using protein supercharging protocol for targeting distinct substrates and maximizing biocatalytic activity. Additionally, several supercharged CBM containing endocellulases exhibited a 5-10 {degrees}C increase in optimal hydrolysis temperature, compared to native enzyme, which enabled further increase in hydrolytic yield at higher operational reaction temperatures. This study demonstrates the first successful implementation of enzyme supercharging of cellulolytic enzymes to increase hydrolytic activity towards complex lignocellulosic biomass derived substrates.

bioengineering↗

bioRxiv2021

Supercharged Cellulases Show Reduced Non-Productive Binding, But Enhanced Activity, on Pretreated Lignocellulosic Biomass

Non-productive adsorption of cellulolytic enzymes to various plant cell wall components, such as lignin and cellulose, necessitates high enzyme loadings to achieve efficient conversion of pretreated lignocellulosic biomass to fermentable sugars. Carbohydrate-binding modules (CBMs), appended to various catalytic domains (CDs), promote lignocellulose deconstruction by increasing targeted substrate-bound CD concentration but often at the cost of increased non-productive enzyme binding. Here, we demonstrate how a computational protein design strategy can be applied to a model endocellulase enzyme (Cel5A) from Thermobifida fusca to allow fine-tuning its CBM surface charge, which led to increased hydrolytic activity towards pretreated lignocellulosic biomass (e.g., corn stover) by up to [~]330% versus the wild-type Cel5A control. We established that the mechanistic basis for this improvement arises from reduced non-productive binding of supercharged Cel5A mutants to cell wall components such as crystalline cellulose (up to 1.7-fold) and lignin (up to 1.8-fold). Interestingly, supercharged Cel5A mutants that showed improved activity on various forms of pretreated corn stover showed increased reversible binding to lignin (up to 2.2-fold) while showing no change in overall thermal stability remarkably. In general, negative supercharging led to increased hydrolytic activity towards both pretreated lignocellulosic biomass and crystalline cellulose whereas positive supercharging led to a reduction of hydrolytic activity. Overall, selective supercharging of protein surfaces was shown to be an effective strategy for improving hydrolytic performance of cellulolytic enzymes for saccharification of real-world pretreated lignocellulosic biomass substrates. Future work should address the implications of supercharging cellulases from various families on inter-enzyme interactions and synergism.

bioengineering↗

bioRxiv2020

Reduced type-A carbohydrate-binding module interactions to cellulose leads to improved endocellulase activity

Dissociation of non-productively bound cellulolytic enzymes from cellulose is hypothesized to be a key rate-limiting factor impeding cost-effective biomass conversion to fermentable sugars. However, the role of carbohydrate-binding modules (CBMs) in enabling non-productive enzyme binding is not well understood. Here, we examine the subtle interplay of CBM binding and cellulose hydrolysis activity for three model Type-A CBMs (families 1, 3a, and 64) tethered to a multifunctional endoglucanase (CelE) on two distinct cellulose allomorphs (i.e., cellulose I and III). We generated a small-library of mutant CBMs with varying cellulose affinity, as determined by equilibrium binding assays, followed by monitoring cellulose hydrolysis activity of CelE-CBM fusion constructs. Finally, kinetic binding assays using quartz crystal microbalance with dissipation (QCM-D) were employed to measure CBM adsorption and desorption rate constants Kon and Koff, respectively, towards nanocrystalline cellulose derived from both allomorphs. Overall, our results indicate that reduced CBM equilibrium binding affinity towards cellulose I alone, resulting from increased desorption rates (Koff) and reduced effective adsorption rates (nKon), is correlated to overall improved endocellulase activity. Future studies could employ similar approaches to unravel the role of CBMs in non-productive enzyme binding and develop improved cellulolytic enzymes for industrial applications.Competing Interest StatementShishir P.S. Chundawat declares a conflict of interest and competing financial interest(s) having filed two patent applications on pretreatment processes to produce cellulose-III enriched cellulosic biomass for biofuels production (US20130244293A1and WO2011133571A2). All other authors declare that they have no competing interests.View Full Text

biochemistry↗

bioRxiv2020

Multiscale Characterization of Complex Binding Interactions of Cellulolytic Enzymes Highlights Limitations of Classical Approaches

Cellulolytic microorganisms, like Trichoderma reesei or Clostridium thermocellum, frequently have non-catalytic carbohydrate-binding modules (CBMs) associated with secreted or cell surface bound multidomain carbohydrate-active enzymes (CAZymes) like cellulases. Mostly type-A family CBMs are known to promote cellulose deconstruction by increasing the substrate-bound concentration of cognate cellulase catalytic domains. However, due to the interfacial nature of cellulose hydrolysis and the structural heterogeneity of cellulose, it has been challenging to fully understand the role of CBMs on cellulase activity using classical protein-ligand binding assays. Here, we report a single-molecule CAZyme assay for an industrially relevant processive cellulase Cel7A (from T. reesei) to reveal how subtle CBM1 binding differences can drastically impact cellulase motility/velocity and commitment to initial processive motion for deconstruction of two well-studied crystalline cellulose allomorphs (namely cellulose I and III). We take a multifaceted approach to characterize the complex binding interactions of all major type-A family representative CBMs including CBM1, using an optical-tweezers based single-molecule CBM-cellulose bond rupture assay to complement several classical bulk ensemble protein-ligand binding characterization methods. While our work provides a basis for the cautious use of Langmuir-type adsorption models to characterize classical protein-ligand binding assay data, we highlight the critical limitations of using such overly simplistic models to gain a truly molecular-level understanding of interfacial protein binding interactions at heterogeneous solid-liquid interfaces. Finally, molecular dynamics simulations provided a theoretical basis for the complex binding behavior seen for CBM1 towards two distinct cellulose allomorphs reconciling experimental findings from multiscale analytical methods. Significance StatementMultimodal biomolecular binding interactions involving carbohydrate polymers (e.g., cellulose, starch, chitin, glycosaminoglycans) are fundamental molecular processes relevant to the recognition, biosynthesis, and degradation of all major terrestrial and aquatic biomass. Protein-carbohydrate binding interactions are also critical to industrial biotechnology operations such as enzymatically-catalyzed bioconversion of starch and lignocellulose into biochemicals like ethanol. However, despite the ubiquitous importance of such interfacial processes, we have a poor molecular-level understanding of protein-polysaccharide binding interactions. Here, we provide a comprehensive experimental and theoretical analysis of bulk ensemble versus single-molecule binding interactions of enzyme motors and associated non-catalytic binding domains with cellulosic polysaccharides to highlight the critical limitations of applying classical biochemical assay techniques alone to understanding protein adsorption or biological activity at solid-liquid interfaces.

biochemistry↗