Lignocellulosic saccharification is not a reaction driven by a single cellulase, but the combined result of multiple enzymes, including endoglucanase, cellobiohydrolase I (CBH I), and β-glucosidase. Endoglucanase increases internal cleavage sites within cellulose chains, CBH I processively releases cellobiose along cellulose chain ends, and β-glucosidase further converts cellobiose into glucose while alleviating product inhibition.

Keywords: cellulase; endoglucanase; CBH I; β-glucosidase; lignocellulosic saccharification; cellobiose; glucose; enzyme synergy

1 Evaluation Logic of the Synergistic Cellulase Degradation System

1.1 Structural Limitations in Lignocellulosic Saccharification

(1) Crystalline regions of cellulose

Cellulose is composed of β-1,4-linked glucose chains, and interchain hydrogen bonds form crystalline and amorphous regions. In crystalline regions, the chains are tightly packed and enzyme accessibility is low, which is an important structural factor limiting lignocellulosic saccharification efficiency. CBH I has processive hydrolytic activity toward crystalline cellulose chain ends, but its access to the substrate surface depends on exposed cellulose chain ends and effective enzyme adsorption.

(2) Hemicellulose and lignin barriers

In natural lignocellulose, hemicellulose and lignin embed cellulose microfibrils and restrict cellulase access to the substrate. Pretreatment can increase specific surface area, reduce the lignin barrier, or alter crystallinity, but it may also generate inhibitory compounds such as phenolics, furans, and organic acids. Saccharification evaluation should record substrate source, pretreatment method, particle size, lignin content, and cellulose content.

(3) Substrate accessibility

Endoglucanase acts more readily on amorphous regions, CBH I depends more strongly on accessible chain ends and crystalline surfaces, and β-glucosidase mainly acts on soluble cellobiose and short-chain cello-oligosaccharides. If substrate accessibility is insufficient, combined saccharification efficiency may remain limited even when single-enzyme activity is high.

1.2 Functional Division of Three Key Enzyme Classes

(1) Endoglucanase

Endoglucanase (EG) randomly cleaves internal β-1,4-glycosidic bonds within cellulose chains, increasing the number of chain ends and decreasing the degree of polymerization. This enzyme responds strongly to CMC, phosphoric acid-swollen cellulose (PASC), and amorphous cellulose substrates, and is an important component for initiating cellulose chain loosening and generating new chain ends.

(2) Cellobiohydrolase I

CBH I is an exo-type cellulase. Typical fungal CBH I processively releases cellobiose from the reducing ends of cellulose chains. Its activity strongly depends on adsorption to the cellulose surface, chain-end accessibility, and substrate crystalline structure. CBH I is often a key enzyme in crystalline cellulose saccharification, but it is also susceptible to inhibition by cellobiose, glucose, and nonproductive adsorption to lignin.

(3) β-Glucosidase

β-Glucosidase (BGL) catalyzes the conversion of cellobiose and short-chain cello-oligosaccharides into glucose. When BGL is insufficient, cellobiose accumulation inhibits CBH I and some endoglucanases, thereby reducing overall saccharification efficiency. Therefore, BGL not only determines final glucose production but also affects the sustained reaction capacity of upstream cellulases.

Table 1 Functional division of key enzymes in lignocellulosic saccharification systems

2 Evaluation of Single-Enzyme Activity and Substrate Response

2.1 Evaluation of Endoglucanase

(1) CMC substrate system

CMC can be used to rapidly evaluate the hydrolytic activity of endoglucanase toward soluble cellulose derivatives. The DNS method can detect reducing sugar release, while the viscosity decrease method can reflect chain cleavage. If only CMC hydrolysis is measured, the result should be defined as CMCase activity and should not be directly used to infer lignocellulosic saccharification capacity.

(2) PASC substrate system

PASC has low crystallinity and high accessibility, making it suitable for evaluating endoglucanase action on amorphous cellulose. Compared with CMC, PASC is closer to insoluble cellulose substrates, but it still cannot fully represent pretreated straw, wood powder, or other complex lignocellulosic substrates.

(3) Interpretation of activity

Increased endoglucanase activity usually increases the number of cellulose chain ends and provides more accessible starting points for CBH I. If reducing sugar release is limited in an EG-only reaction but viscosity decreases markedly, this indicates that chain cleavage has occurred but soluble sugar production is insufficient. The contribution to saccharification should therefore be evaluated together with CBH I and BGL.

2.2 Evaluation of CBH I

(1) Crystalline cellulose substrates

CBH I should preferably be evaluated using insoluble substrates such as microcrystalline cellulose, filter paper, Avicel, or pretreated lignocellulose. These substrates can reflect the processive hydrolytic capacity of CBH I toward chain ends and crystalline surfaces, and are more suitable than CMC for evaluating exo-type cellulase activity.

(2) Cellobiose release

The main product of CBH I reaction is cellobiose. If only the DNS method is used to detect total reducing sugar, product composition can be underestimated or confused. HPLC, ion chromatography, or enzymatic coupled assays should preferably be added to detect the ratio between cellobiose and glucose.

(3) Adsorption and desorption

The adsorption capacity of CBH I to cellulose surfaces affects hydrolysis efficiency. High adsorption does not necessarily indicate efficient saccharification, because nonproductive adsorption may retain enzymes on lignin or inaccessible surfaces. When evaluating CBH I, free enzyme remaining in solution, substrate-bound enzyme amount, and hydrolysis product formation rate can be measured.

2.3 Evaluation of β-Glucosidase

(1) pNPG substrate system

pNPG is commonly used for rapid detection of β-glucosidase activity, and the reaction product p-nitrophenol can be easily detected colorimetrically. This method is sensitive and convenient, making it suitable for preliminary screening and enzyme activity comparison. However, pNPG is an artificial substrate and cannot fully represent cellobiose hydrolysis capacity.

(2) Cellobiose substrate system

Cellobiose is the more direct BGL substrate in lignocellulosic saccharification. Detecting glucose production with cellobiose as the substrate can more accurately evaluate the ability of BGL to alleviate cellobiose inhibition and improve final glucose yield.

(3) Glucose inhibition

Some β-glucosidases are strongly inhibited by the product glucose. In high-substrate-load and long-term saccharification systems, glucose accumulation may reduce BGL reaction rate. When screening and formulating BGL, glucose tolerance should be evaluated rather than comparing only initial pNPG activity.

Table 2 Substrate selection and readout design in single-enzyme evaluation

3 Evaluation of Multi-Enzyme Synergy

3.1 Dual-Enzyme Combination Analysis

(1) Endoglucanase and CBH I

The EG and CBH I combination can be used to evaluate the synergy between internal chain cleavage and processive chain-end hydrolysis. EG increases new chain ends, while CBH I releases cellobiose along the chain ends. If the sugar yield of the combination is significantly higher than the sum of the two single enzymes, effective synergy exists between chain-end generation and exo-type hydrolysis.

(2) CBH I and β-glucosidase

The CBH I and BGL combination can be used to evaluate how cellobiose removal promotes exo-type hydrolysis. If the addition of BGL decreases cellobiose, increases glucose, and maintains a higher continuous product formation rate by CBH I, this indicates that BGL alleviates cellobiose inhibition.

(3) Endoglucanase and β-glucosidase

The EG and BGL combination usually cannot fully complete crystalline cellulose saccharification, but it can be used to determine whether soluble oligosaccharides produced by EG can be further converted by BGL. If glucose production is limited, this may indicate a lack of CBH I contribution to chain-end cellobiose release.

3.2 Three-Enzyme Combination Analysis

(1) Basic ratio

In the three-enzyme combination, EG increases chain ends, CBH I continuously releases cellobiose, and BGL performs terminal hydrolysis and relieves cellobiose inhibition. Experiments should include EG-only, CBH I-only, and BGL-only groups, dual-enzyme groups, and a three-enzyme group. The comparison logic should be consistent in terms of total protein amount or total enzyme activity units.

(2) Ratio gradient

Different substrates require different enzyme ratios. Substrates with a high amorphous fraction may depend more on EG, substrates with high crystalline cellulose content may depend more on CBH I, and systems with obvious cellobiose accumulation require a higher BGL ratio. It is recommended to vary the EG:CBH I:BGL ratio based on fixed total protein or fixed total enzyme activity and monitor the reaction over time.

(3) Synergy coefficient

Multi-enzyme synergy can be evaluated by the ratio of the combined hydrolysis effect to the sum of the single-enzyme effects. A synergy coefficient greater than 1 indicates positive synergy, a value close to 1 indicates mainly additive effects, and a value less than 1 suggests inter-enzyme competition, nonproductive adsorption, or limited substrate accessibility. This indicator should be calculated using the same substrate, reaction time, and normalization method.

3.3 Time-Course and Product Profile Analysis

(1) Early-stage reaction

Early-stage reactions more readily reflect enzyme adsorption, EG-mediated internal chain cleavage, and CBH I initiation efficiency. If reducing sugar or cellobiose production is low at the early stage, insufficient substrate accessibility, limited enzyme adsorption, or insufficient chain-end number may be involved.

(2) Middle- and late-stage reaction

Middle- and late-stage reactions are more susceptible to product inhibition, decreased substrate accessibility, and nonproductive adsorption. If cellobiose continuously accumulates while glucose production is insufficient, BGL activity, BGL ratio, and glucose tolerance should be evaluated first.

(3) Product composition

Total reducing sugar cannot distinguish glucose, cellobiose, and cello-oligosaccharides. Lignocellulosic saccharification evaluation should detect glucose, cellobiose, and low-molecular-weight oligosaccharide composition whenever possible. High glucose yield combined with low cellobiose accumulation more clearly indicates sufficient terminal hydrolysis by the three-enzyme system.

Table 3 Multi-enzyme combination evaluation design

4 Lignocellulosic Substrates and Reaction Condition Design

4.1 Model Substrate Selection

(1) CMC

CMC is suitable for evaluating endoglucanase activity and hydrolysis of soluble cellulose derivatives, but it is not suitable for evaluating processive hydrolysis of crystalline cellulose by CBH I. When using CMC results, CMCase activity should not be directly equated with lignocellulosic saccharification capacity.

(2) Microcrystalline cellulose and filter paper

Microcrystalline cellulose and filter paper can be used to evaluate crystalline cellulose hydrolysis and multi-enzyme synergistic saccharification. Filter paper activity (FPase) reflects the overall degradation capacity of a composite cellulase system, but it cannot distinguish the individual contributions of EG, CBH I, and BGL.

(3) PASC

PASC has relatively low crystallinity and can be used to evaluate enzymatic hydrolysis of amorphous cellulose. This substrate is sensitive to EG action and is suitable for analyzing the effect of internal chain cleavage on subsequent saccharification.

(4) Cellobiose

Cellobiose is a key substrate for evaluating the actual substrate conversion capacity of BGL. Compared with pNPG, cellobiose more directly reflects the effect of BGL on cellobiose accumulation and glucose yield in saccharification systems.

4.2 Real Lignocellulosic Substrates

(1) Agricultural residues

Corn stover, wheat straw, rice straw, and sugarcane bagasse are commonly used in lignocellulosic saccharification studies. For such substrates, cellulose, hemicellulose, lignin, and ash contents should be recorded, and grinding particle size and pretreatment method should be controlled.

(2) Woody materials

Wood powder, hardwood, and softwood substrates differ markedly in lignin structure, which may change enzyme adsorption and accessibility. Softwood is usually more difficult to saccharify, and evaluation should focus on the lignin barrier and nonproductive enzyme adsorption.

(3) Pretreated substrates

Acid pretreatment, alkali pretreatment, steam explosion, ionic liquid treatment, and organosolv pretreatment can change substrate accessibility, crystallinity, and inhibitor composition. Saccharification rates among different pretreated substrates should not be compared directly without considering composition and structural parameters.

Table 4 Applicability of common substrates in cellulase synergy evaluation

4.3 Reaction Condition Control

(1) Substrate loading

Low substrate loading facilitates reaction uniformity and kinetic comparison, while high substrate loading is closer to practical saccharification processes but introduces mass transfer limitations, product inhibition, and system viscosity problems. Low to moderate substrate loading can be used during ratio optimization, and substrate concentration can be increased during scale-up validation.

(2) Enzyme dosage

Enzyme dosage can be normalized by protein amount, total cellulase activity, single-enzyme activity unit, or substrate dry weight. When comparing EG, CBH I, and BGL ratios, the normalization basis must be clearly defined; otherwise, it is impossible to determine whether differences arise from enzyme ratio or total enzyme dosage.

(3) pH and temperature

The optimal pH and temperature of a cellulase system depend on enzyme source. In combination reactions, conditions should be selected under which all enzymes can retain activity rather than being set only according to the optimum of a single enzyme. If one enzyme component has poor stability, it may become the limiting factor during long-term saccharification.

(4) Reaction time

Short reaction times are suitable for evaluating initial rate and enzyme adsorption, while long reaction times are suitable for evaluating final saccharification rate and product inhibition. Multiple time points are recommended to distinguish early synergistic enhancement from late reaction stagnation.

5 Detection of Saccharification Results and Data Interpretation

5.1 Reducing Sugar and Glucose Detection

(1) DNS method

The DNS method can rapidly detect total reducing sugar and is suitable for preliminary comparison of hydrolysis levels. Because glucose, cellobiose, and various oligosaccharides can all contribute reducing signals, the DNS method cannot distinguish product composition.

(2) Glucose-specific detection

The glucose oxidase method or HPLC can be used to determine glucose yield. In the three-enzyme system, glucose is an important indicator for evaluating BGL sufficiency and final saccharification performance. If total reducing sugar increases but the proportion of glucose remains low, cellobiose or oligosaccharides may still be accumulating.

(3) Cellobiose detection

Cellobiose accumulation indicates insufficient BGL, BGL inhibition, or inadequate removal of CBH I products. Cellobiose detection can be used to determine whether the BGL ratio should be increased or whether a BGL with better glucose tolerance should be screened.

5.2 Solid Residue Analysis

(1) Residual cellulose

The decrease in residual cellulose content can be used to calculate saccharification efficiency. If liquid-phase sugar production is low while residual cellulose decreases markedly, sample recovery, sugar degradation, or methodological bias should be examined.

(2) Crystallinity change

During saccharification, amorphous regions are usually degraded first, and the relative crystallinity of the residual substrate may increase. Crystallinity changes can help interpret the effects of EG and CBH I on different structural regions, but they cannot independently represent saccharification degree.

(3) Surface morphology

Scanning electron microscopy, atomic force microscopy, or particle size analysis can be used to observe substrate surface disruption, pore formation, and fiber bundle dissociation. Morphological changes should be interpreted together with sugar production and residue results, rather than relying on micrographs alone to judge enzymatic hydrolysis efficiency.

5.3 Calculation of Saccharification Rate and Conversion Rate

(1) Glucose yield

Glucose yield should be calculated based on theoretical glucose production. Because the structural unit of cellulose is anhydroglucose, there is a molecular weight conversion relationship when cellulose is converted into glucose. The calculation should clearly state whether the denominator is glucan content in the substrate or total substrate mass.

(2) Cellulose conversion rate

Cellulose conversion rate is more suitable for evaluating substrate utilization. This indicator should integrate substrate glucan content, liquid-phase glucose, cellobiose, and residual solid results to avoid underestimating the contribution of incompletely hydrolyzed oligosaccharides.

(3) Synergistic effect

Synergistic effects should be calculated based on the same substrate, enzyme dosage, and reaction time. If the yield of the three-enzyme combination is higher than the sum of single-enzyme or dual-enzyme effects, synergy can be inferred. If the combined effect is lower than expected, product inhibition, enzyme inactivation, substrate competition, and nonproductive adsorption should be examined.

Table 5 Detection methods for saccharification results and interpretation points

6 Common Inhibitory Factors and System Optimization

6.1 Product Inhibition

(1) Cellobiose inhibition

Cellobiose can inhibit CBH I and some endoglucanases. If cellobiose accumulation is obvious during the reaction, the BGL ratio should be increased, a BGL with better glucose tolerance should be used, or stepwise BGL supplementation should be adopted.

(2) Glucose inhibition

Glucose can inhibit some BGLs. In high-solid saccharification or long-term reactions, glucose accumulation may limit terminal hydrolysis. Glucose inhibition can be reduced through fed-batch saccharification, product removal, screening of glucose-tolerant BGLs, or simultaneous fermentation.

(3) Oligosaccharide accumulation

Accumulation of short-chain cello-oligosaccharides may indicate insufficient BGL or other auxiliary enzymes. If total reducing sugar is high but the glucose proportion is low, oligosaccharide distribution should be further analyzed instead of directly concluding that saccharification is complete.

6.2 Nonproductive Adsorption to Lignin

(1) Enzyme adsorption loss

Lignin can adsorb cellulases through hydrophobic and electrostatic interactions, preventing enzymes from acting efficiently on cellulose. Because CBH I contains a cellulose-binding module, it may be more prone to nonproductive adsorption.

(2) Effect of substrate pretreatment

Alkali treatment, surface modification, or lignin removal can reduce nonproductive adsorption and increase cellulose accessibility. Different pretreatments simultaneously change hemicellulose, lignin, and cellulose structures, so the interpretation should include substrate compositional analysis.

(3) Additive regulation

Some surfactants, protein protectants, or polymer additives can reduce nonproductive adsorption. When additives are used, blank groups should be included to confirm that the additive itself does not interfere with sugar detection, enzyme activity, or downstream fermentation.

6.3 Enzyme Ratio Optimization

(1) Insufficient EG

When EG is insufficient, new chain-end formation is limited, and accessible sites for CBH I are insufficient, which may lead to a low early saccharification rate. Increasing the EG ratio can improve amorphous-region cleavage and chain-end supply, but excessive EG does not necessarily improve final glucose yield.

(2) Insufficient CBH I

When CBH I is insufficient, crystalline cellulose chain-end hydrolysis is limited, and cellobiose production is insufficient. For microcrystalline cellulose, filter paper, and highly crystalline pretreated substrates, the CBH I ratio is usually an important limiting factor for saccharification efficiency.

(3) Insufficient BGL

When BGL is insufficient, cellobiose accumulates, glucose production is inadequate, and sustained CBH I reaction may be inhibited. To determine whether BGL is sufficient, residual cellobiose, glucose yield, and the increase in sugar production after BGL supplementation should be compared.

Table 6 Problems and optimization strategies in synergistic enzymatic saccharification systems

7 Related Product and Material Selection

Table 7 Products and materials related to synergistic cellulase saccharification evaluation

8 Frequently Asked Questions

8.1 Can CMC enzyme activity alone be used to judge lignocellulosic saccharification capacity?

No. CMC enzyme activity mainly reflects the hydrolytic capacity of endoglucanase toward soluble cellulose derivatives. Lignocellulosic saccharification also depends on processive hydrolysis of crystalline cellulose chain ends by CBH I, conversion of cellobiose by BGL, and substrate accessibility after pretreatment.

8.2 Why is CBH I important in saccharification systems?

CBH I can processively release cellobiose along cellulose chain ends and is a key exo-type enzyme in crystalline cellulose hydrolysis. If CBH I is insufficient, even when EG generates new chain ends, the substrate may still fail to be efficiently converted into soluble sugars.

8.3 What happens when β-glucosidase is insufficient?

Insufficient BGL leads to cellobiose accumulation, which further inhibits CBH I and some endoglucanases. This is reflected by limited increase in total reducing sugar, low glucose yield, and high residual cellobiose. Increasing the BGL ratio or using glucose-tolerant BGL can improve this problem.

8.4 Does high pNPG activity indicate that BGL is suitable for lignocellulosic saccharification?

Not necessarily. pNPG is an artificial substrate and is suitable for rapid BGL activity screening, but it does not fully reflect cellobiose hydrolysis capacity or glucose tolerance. BGL evaluation in saccharification systems should also detect cellobiose conversion and stability under glucose accumulation conditions.

8.5 Why do three-enzyme combinations sometimes show no obvious synergy?

Possible reasons include unreasonable enzyme ratio, low substrate accessibility, nonproductive adsorption to lignin, cellobiose or glucose inhibition, poor enzyme stability, and substrate structural limitations. Single-enzyme, dual-enzyme, three-enzyme, and time-course analyses should be used to locate the limiting step.

8.6 Should saccharification results prioritize total reducing sugar or glucose?

Total reducing sugar is suitable for preliminary evaluation of hydrolysis degree, while glucose is more suitable for evaluating final saccharification performance. If total reducing sugar is high but glucose is low, cellobiose or oligosaccharides may still be accumulating, and BGL may be insufficient. A complete evaluation should detect total reducing sugar, glucose, and cellobiose together.

8.7 Why is enzyme ratio optimization still needed after lignocellulose pretreatment?

Pretreatment can increase substrate accessibility or reduce structural barriers, but it does not automatically ensure a matched ratio of EG, CBH I, and BGL. Different pretreatments change amorphous regions, crystalline regions, chain-end numbers, and lignin adsorption characteristics. Therefore, enzyme ratios and reaction conditions need to be re-screened.

Evaluation of synergistic cellulase saccharification should focus on the continuous process of “internal chain cleavage—cellobiose release from chain ends—conversion of cellobiose into glucose.” Experiments should include single-enzyme, dual-enzyme, and three-enzyme combinations and should integrate substrate structure, product profile, residual solids, and enzyme ratio analysis. Only then can the real contributions of endoglucanase, CBH I, and β-glucosidase in lignocellulosic saccharification be determined.

For more related articles, please see below:

[1] Cellulase: Composition of Multicomponent Enzyme Systems, Mechanisms of Action, and Key Application Considerations

[2] From Natural Cellulose to Nanocrystals: Structural Characteristics, Application Directions, and Industrialization Challenges of CNC

[3] Practical Guide to Sodium Carboxymethyl Cellulose (CMC-Na): Thickening/Stabilizing Mechanisms, Key Controls for Solution Preparation, and Selection Navigation (including Table 1 and Tables A–C)