Lintao Bu

Identification of amino acids responsible for processivity in a Family 1 carbohydrate-binding module from a fungal cellulase.

We probe the molecular-level behavior of the Family 1 carbohydrate-binding module (CBM) from a commonly studied fungal cellulase, the Family 7 cellobiohydrolase (Cel7A) from Trichoderma reesei, on the hydrophobic face of crystalline cellulose. With a fully atomistic model, we predict that the CBM alone exhibits regions of thermodynamic stability along a cellulose chain corresponding to a cellobiose unit, which is the catalytic product of the entire Cel7A enzyme. In addition, we determine which residues and the types of interactions that are responsible for the observed processivity length scale of the CBM: Y5, Q7, N29, and Y32. These results imply that the CBM can anchor the Cel7A enzyme at discrete points along a cellulose chain and thus aid in both recognizing cellulose chain ends for initial attachment to cellulose as well as aid in enzymatic catalysis by diffusing between stable wells on a length scale commensurate with the catalytic, processive cycle of Cel7A during cellulose hydrolysis. Comparison of other Family 1 CBMs show high functional homology to the four amino acids responsible for the processivity length scale on the surface of crystalline cellulose, which suggests that Family 1 CBMs may generally employ this type of approach for translation on the cellulose surface. Overall, this work provides further insight into the molecular-level mechanisms by which a CBM recognizes and interacts with cellulose.

The Energy Landscape for the Interaction of the Family 1 Carbohydrate-Binding Module and the Cellulose Surface is Altered by Hydrolyzed Glycosidic Bonds

A multiscale simulation model is used to construct potential and free energy surfaces for the carbohydrate-binding module [CBM] from an industrially important cellulase, Trichoderma reesei cellobiohydrolase I, on the hydrophobic face of a coarse-grained cellulose Ibeta polymorph. We predict from computation that the CBM alone exhibits regions of stability on the hydrophobic face of cellulose every 5 and 10 A, corresponding to a glucose unit and a cellobiose unit, respectively. In addition, we predict a new role for the CBM: specifically, that in the presence of hydrolyzed cellulose chain ends, the CBM exerts a thermodynamic driving force to translate away from the free cellulose chain ends. This suggests that the CBM is not only required for binding to cellulose, as has been known for two decades, but also that it has evolved to both assist the enzyme in recognizing a cellulose chain end and exert a driving force on the enzyme during processive hydrolysis of cellulose.

Binding preferences, surface attachment, diffusivity, and orientation of a family 1 carbohydrate-binding module on cellulose

Background: Family 1 carbohydrate-binding modules (CBMs) bind selectively to the hydrophobic surfaces of cellulose. Results: Simulations have shown that the planar face of the CBM binds preferentially to the hydrophobic face. Conclusion: Thermodynamic driving forces enable transfer of the CBM from the hydrophilic to hydrophobic surfaces. Significance: Selectivity of CBM provides access of cellulases to active surfaces of cellulose. Cellulase enzymes often contain carbohydrate-binding modules (CBMs) for binding to cellulose. The mechanisms by which CBMs recognize specific surfaces of cellulose and aid in deconstruction are essential to understand cellulase action. The Family 1 CBM from the Trichoderma reesei Family 7 cellobiohydrolase, Cel7A, is known to selectively bind to hydrophobic surfaces of native cellulose. It is most commonly suggested that three aromatic residues identify the planar binding face of this CBM, but several recent studies have challenged this hypothesis. Here, we use molecular simulation to study the CBM binding orientation and affinity on hydrophilic and hydrophobic cellulose surfaces. Roughly 43 μs of molecular dynamics simulations were conducted, which enables statistically significant observations. We quantify the fractions of the CBMs that detach from crystal surfaces or diffuse to other surfaces, the diffusivity along the hydrophobic surface, and the overall orientation of the CBM on both hydrophobic and hydrophilic faces. The simulations demonstrate that there is a thermodynamic driving force for the Cel7A CBM to bind preferentially to the hydrophobic surface of cellulose relative to hydrophilic surfaces. In addition, the simulations demonstrate that the CBM can diffuse from hydrophilic surfaces to the hydrophobic surface, whereas the reverse transition is not observed. Lastly, our simulations suggest that the flat faces of Family 1 CBMs are the preferred binding surfaces. These results enhance our understanding of how Family 1 CBMs interact with and recognize specific cellulose surfaces and provide insights into the initial events of cellulase adsorption and diffusion on cellulose.

Probing Carbohydrate Product Expulsion from a Processive Cellulase with Multiple Absolute Binding Free Energy Methods

Understanding the enzymatic mechanism that cellulases employ to degrade cellulose is critical to efforts to efficiently utilize plant biomass as a sustainable energy resource. A key component of cellulase action on cellulose is product inhibition from monosaccharide and disaccharides in the product site of cellulase tunnel. The absolute binding free energy of cellobiose and glucose to the product site of the catalytic tunnel of the Family 7 cellobiohydrolase (Cel7A) of Trichoderma reesei (Hypocrea jecorina) was calculated using two different approaches: steered molecular dynamics (SMD) simulations and alchemical free energy perturbation molecular dynamics (FEP/MD) simulations. For the SMD approach, three methods based on Jarzynskis equality were used to construct the potential of mean force from multiple pulling trajectories. The calculated binding free energies, −14.4 kcal/mol using SMD and −11.2 kcal/mol using FEP/MD, are in good qualitative agreement. Analysis of the SMD pulling trajectories suggests that several protein residues (Arg-251, Asp-259, Asp-262, Trp-376, and Tyr-381) play key roles in cellobiose and glucose binding to the catalytic tunnel. Five mutations (R251A, D259A, D262A, W376A, and Y381A) were made computationally to measure the changes in free energy during the product expulsion process. The absolute binding free energies of cellobiose to the catalytic tunnel of these five mutants are −13.1, −6.0, −11.5, −7.5, and −8.8 kcal/mol, respectively. The results demonstrated that all of the mutants tested can lower the binding free energy of cellobiose, which provides potential applications in engineering the enzyme to accelerate the product expulsion process and improve the efficiency of biomass conversion.

Computational Investigation of Glycosylation Effects on a Family 1 Carbohydrate-binding Module

Background: Family 1 carbohydrate-binding modules (CBMs) are often components of cellulases for binding to cellulose. Results: Family 1 CBM binding affinity is dramatically affected by the presence of O-glycosylation near the CBM binding face. Conclusion: Glycosylation should be accounted for in CBM binding affinity studies. Significance: Glycosylation can be harnessed to tune cellulase binding affinity, which is known to affect activity. Carbohydrate-binding modules (CBMs) are ubiquitous components of glycoside hydrolases, which degrade polysaccharides in nature. CBMs target specific polysaccharides, and CBM binding affinity to cellulose is known to be proportional to cellulase activity, such that increasing binding affinity is an important component of performance improvement. To ascertain the impact of protein and glycan engineering on CBM binding, we use molecular simulation to quantify cellulose binding of a natively glycosylated Family 1 CBM. To validate our approach, we first examine aromatic-carbohydrate interactions on binding, and our predictions are consistent with previous experiments, showing that a tyrosine to tryptophan mutation yields a 2-fold improvement in binding affinity. We then demonstrate that enhanced binding of 3–6-fold over a nonglycosylated CBM is achieved by the addition of a single, native mannose or a mannose dimer, respectively, which has not been considered previously. Furthermore, we show that the addition of a single, artificial glycan on the anterior of the CBM, with the native, posterior glycans also present, can have a dramatic impact on binding affinity in our model, increasing it up to 140-fold relative to the nonglycosylated CBM. These results suggest new directions in protein engineering, in that modifying glycosylation patterns via heterologous expression, manipulation of culture conditions, or introduction of artificial glycosylation sites, can alter CBM binding affinity to carbohydrates and may thus be a general strategy to enhance cellulase performance. Our results also suggest that CBM binding studies should consider the effects of glycosylation on binding and function.

Product Binding Varies Dramatically between Processive and Nonprocessive Cellulase Enzymes

Background: Experimentally measured product inhibition constants of cellobiose for cellulases vary significantly. Results: Cellodextrin bound as substrate to cellulases increases product binding to processive cellulases, whereas it does not affect nonprocessive cellulases. Conclusion: The increased binding affinity correlates with hydrogen bonds between the substrate and cellobiose in processive cellulase tunnels. Significance: The results offer an interpretation for the discrepancy in measured inhibition constants. Cellulases hydrolyze β-1,4 glycosidic linkages in cellulose, which are among the most prevalent and stable bonds in Nature. Cellulases comprise many glycoside hydrolase families and exist as processive or nonprocessive enzymes. Product inhibition negatively impacts cellulase action, but experimental measurements of product-binding constants vary significantly, and there is little consensus on the importance of this phenomenon. To provide molecular level insights into cellulase product inhibition, we examine the impact of product binding on processive and nonprocessive cellulases by calculating the binding free energy of cellobiose to the product sites of catalytic domains of processive and nonprocessive enzymes from glycoside hydrolase families 6 and 7. The results suggest that cellobiose binds to processive cellulases much more strongly than nonprocessive cellulases. We also predict that the presence of a cellodextrin bound in the reactant site of the catalytic domain, which is present during enzymatic catalysis, has no effect on product binding in nonprocessive cellulases, whereas it significantly increases product binding to processive cellulases. This difference in product binding correlates with hydrogen bonding between the substrate-side ligand and the cellobiose product in processive cellulase tunnels and the additional stabilization from the longer tunnel-forming loops. The hydrogen bonds between the substrate- and product-side ligands are disrupted by water in nonprocessive cellulase clefts, and the lack of long tunnel-forming loops results in lower affinity of the product ligand. These findings provide new insights into the large discrepancies reported for binding constants for cellulases and suggest that product inhibition will vary significantly based on the amount of productive binding for processive cellulases on cellulose.

Computational Investigation of the pH Dependence of Loop Flexibility and Catalytic Function in Glycoside Hydrolases

Background: Solution pH affects cellulase enzyme activity. Results: We used simulation to predict pH-dependent behavior in cellulases, including pKa values for the catalytic machinery, ring distortion, and loop flexibility. Conclusion: pH increases active site tunnel flexibility and affects the −1 carbohydrate ring distortion, suggesting pH-dependent mechanisms for complexation and catalysis of cellulose chains. Significance: These results provide molecular-level understanding of pH effects on cellulases. Cellulase enzymes cleave glycosidic bonds in cellulose to produce cellobiose via either retaining or inverting hydrolysis mechanisms, which are significantly pH-dependent. Many fungal cellulases function optimally at pH ∼5, and their activities decrease dramatically at higher or lower pH. To understand the molecular-level implications of pH in cellulase structure, we use a hybrid, solvent-based, constant pH molecular dynamics method combined with pH-based replica exchange to determine the pKa values of titratable residues of a glycoside hydrolase (GH) family 6 cellobiohydrolase (Cel6A) and a GH family 7 cellobiohydrolase (Cel7A) from the fungus Hypocrea jecorina. For both enzymes, we demonstrate that a bound substrate significantly affects the pKa values of the acid residues at the catalytic center. The calculated pKa values of catalytic residues confirm their proposed roles from structural studies and are consistent with the experimentally measured apparent pKa values. Additionally, GHs are known to impart a strained pucker conformation in carbohydrate substrates in active sites for catalysis, and results from free energy calculations combined with constant pH molecular dynamics suggest that the correct ring pucker is stable near the optimal pH for both Cel6A and Cel7A. Much longer molecular dynamics simulations of Cel6A and Cel7A with fixed protonation states based on the calculated pKa values suggest that pH affects the flexibility of tunnel loops, which likely affects processivity and substrate complexation. Taken together, this work demonstrates several molecular-level effects of pH on GH enzymes important for cellulose turnover in the biosphere and relevant to biomass conversion processes.

Loop Motions Important to Product Expulsion in the Thermobifida fusca Glycoside Hydrolase Family 6 Cellobiohydrolase from Structural and Computational Studies

Background: Family 6 glycoside hydrolases represent an important, diverse enzyme class in cellulolytic organisms. Results: We solved structures of two Thermobifida fusca Cel6B (TfuCel6B) cellobiohydrolase mutants and examined ligand dynamics and product release with simulation. Conclusion: These results suggest mechanisms for product release in TfuCel6B. Significance: This study further elucidates the mechanism of a unique cellobiohydrolase with an extended, enclosed active site tunnel. Cellobiohydrolases (CBHs) are typically major components of natural enzyme cocktails for biomass degradation. Their active sites are enclosed in a tunnel, enabling processive hydrolysis of cellulose chains. Glycoside hydrolase Family 6 (GH6) CBHs act from nonreducing ends by an inverting mechanism and are present in many cellulolytic fungi and bacteria. The bacterial Thermobifida fusca Cel6B (TfuCel6B) exhibits a longer and more enclosed active site tunnel than its fungal counterparts. Here, we determine the structures of two TfuCel6B mutants co-crystallized with cellobiose, D274A (catalytic acid), and the double mutant D226A/S232A, which targets the putative catalytic base and a conserved serine that binds the nucleophilic water. The ligand binding and the structure of the active site are retained when compared with the wild type structure, supporting the hypothesis that these residues are directly involved in catalysis. One structure exhibits crystallographic waters that enable construction of a model of the α-anomer product after hydrolysis. Interestingly, the product sites of TfuCel6B are completely enclosed by an “exit loop” not present in fungal GH6 CBHs and by an extended “bottom loop”. From the structures, we hypothesize that either of the loops enclosing the product subsites in the TfuCel6B active site tunnel must open substantially for product release. With simulation, we demonstrate that both loops can readily open to allow product release with equal probability in solution or when the enzyme is engaged on cellulose. Overall, this study reveals new structural details of GH6 CBHs likely important for functional differences among enzymes from this important family.

The molecular origins of twist in cellulose I-beta.

The observation of twisted microfibrils in cellulose Iβ both in imaging and in molecular simulations has been reported and studied for years. This article reports a computational modeling study of cellulose Iβ twist showing its strong dependence on fibril diameter and no dependence on fibril length. We report that an important contribution to the twist in the model, empirically and analytically, is the hydrogen bonding that spans the glycosidic linkage, and that the characteristics of the chiral centers involved in the trans-glycosidic-linkage hydrogen bonding determine the directions if those interactions and cause observed right-handed twist. Other crystalline forms of cellulose show evidence of twisting at the microfibril scale, but less than Iβ. The minimal twist in other forms of cellulose was shown previously to be due to inter-layer hydrogen bonds; this study shows it is also partially due to the primary alcohol not occurring in the TG orientation in those forms. Thus, only cellulose I has the primary alcohol in TG orientation, which leads to formation of the twist-causing hydrogen bonds.

Cell wall targeted in planta iron accumulation enhances biomass conversion and seed iron concentration in Arabidopsis and rice.

Summary Conversion of nongrain biomass into liquid fuel is a sustainable approach to energy demands as global population increases. Previously, we showed that iron can act as a catalyst to enhance the degradation of lignocellulosic biomass for biofuel production. However, direct addition of iron catalysts to biomass pretreatment is diffusion‐limited, would increase the cost and complexity of biorefinery unit operations and may have deleterious environmental impacts. Here, we show a new strategy for in planta accumulation of iron throughout the volume of the cell wall where iron acts as a catalyst in the deconstruction of lignocellulosic biomass. We engineered CBM‐IBP fusion polypeptides composed of a carbohydrate‐binding module family 11 (CBM11) and an iron‐binding peptide (IBP) for secretion into Arabidopsis and rice cell walls. CBM‐IBP transformed Arabidopsis and rice plants show significant increases in iron accumulation and biomass conversion compared to respective controls. Further, CBM‐IBP rice shows a 35% increase in seed iron concentration and a 40% increase in seed yield in greenhouse experiments. CBM‐IBP rice potentially could be used to address iron deficiency, the most common and widespread nutritional disorder according to the World Health Organization.