CelE (CtCel5C-CE2) is a multifunctional enzyme involved in plant cell wall degradation. It contains an N-terminal GH5 cellulase domain (CtCel5C), a central type I dockerin module for cellulosome integration, and a C-terminal CE2 esterase domain (CtCE2). The GH5 domain displays endo-1,4-beta-glucanase activity against cellulose, beta-glucan, and CMC. The CE2 domain exhibits both acetylxylan esterase and glucomannan deacetylase activities, with strong preference for glucomannan. Uniquely, the CE2 domain also functions as a cellulose-binding module through its catalytic site region, potentiating the cellulase activity of the appended GH5 domain on recalcitrant substrates.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: This broad biological process annotation is supported by the established functions of CelE in degrading cellulose, xylan, and glucomannan - all plant cell wall polysaccharides (PMID:19338387, PMID:3066698). Reason: CelE functions in the catabolism of multiple polysaccharides including cellulose, xylan, and glucomannan. This general term correctly captures the enzyme's role in polysaccharide breakdown, though more specific terms (cellulose catabolic process, xylan catabolic process, glucomannan catabolic process) provide greater specificity and are also annotated. Supporting Evidence: PMID:19338387 CtCE2 catalyses deacetylation of noncellulosic plant structural polysaccharides to deprotect these substrates for attack by other enzymes PMID:3066698 The complete nucleotide sequence of the Clostridium thermocellum celE gene, coding for an endo-beta-1,4-glucanase (endoglucanase E; EGE) with xylan-hydrolysing activity has been determined. file:ACET2/P10477/P10477-deep-research-falcon.md CelE is a bifunctional enzyme with both cellulase and esterase domains |
| GO:0003824 catalytic activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This is a very general molecular function term that is true but uninformative. CelE has well-characterized specific catalytic activities including cellulase (EC 3.2.1.4) and acetylxylan esterase (EC 3.1.1.72) activities. Reason: While technically correct, this term is too general to be informative. The specific catalytic activities (cellulase activity GO:0008810, acetylxylan esterase activity GO:0046555) are far more useful annotations. This annotation can be retained as a parent term but should not be considered a core annotation. |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000120 | ACCEPT | Summary: This annotation correctly captures the glycoside hydrolase activity of the GH5 domain. The enzyme hydrolyzes beta-1,4-glucosidic linkages in cellulose and beta-glucan (PMID:3066698, PMID:1991028). Reason: The GH5 cellulase domain of CelE catalyzes hydrolysis of O-glycosyl bonds in cellulose. This is an accurate parent term for the more specific cellulase activity. Supporting Evidence: PMID:3066698 coding for an endo-beta-1,4-glucanase (endoglucanase E; EGE) |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: CelE is a secreted protein that functions as part of the extracellular cellulosome complex. The protein has a signal peptide (residues 1-34) and integrates into the cellulosome via its dockerin domain. Reason: The UniProt record indicates the protein is secreted (signal peptide residues 1-34), and the protein functions in the extracellular cellulosome complex. The dockerin domain mediates integration into this extracellular multi-enzyme complex. Supporting Evidence: PMID:19338387 The enzyme also contains a type I dockerin module that, by binding to cohesin modules in the scaffoldin protein, incorporates CtCel5C-CE2 into the multienzyme plant cell-wall-degrading complex known as the cellulosome |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This very broad biological process term is accurate but provides minimal specificity. The enzyme is involved in carbohydrate catabolism, specifically of polysaccharides. Reason: While technically correct, this is a very high-level term. The more specific terms (cellulose catabolic process, xylan catabolic process, glucomannan catabolic process) provide much better functional annotation. |
| GO:0008810 cellulase activity | IEA GO_REF:0000003 | ACCEPT | Summary: Cellulase activity is a core function of CelE, demonstrated through biochemical assays showing activity against CMC and barley beta-glucan (PMID:3066698, PMID:1991028). This activity is localized to the N-terminal GH5 domain (CtCel5C). Reason: This is a core molecular function of CelE. The enzyme has EC 3.2.1.4 activity confirmed by direct experimental evidence. The GH5 domain catalyzes endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose. Supporting Evidence: PMID:3066698 coding for an endo-beta-1,4-glucanase (endoglucanase E; EGE) |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: CelE is a hydrolase with both glycosidase and esterase activities. This broad term is accurate but not specific. Reason: This is a parent term that is technically correct but uninformative. The specific hydrolase activities (cellulase, acetylxylan esterase) are better annotations. |
| GO:0016788 hydrolase activity, acting on ester bonds | IEA GO_REF:0000002 | ACCEPT | Summary: The CE2 domain of CelE has well-characterized esterase activity, catalyzing deacetylation of xylan and glucomannan (PMID:19338387). Reason: This correctly captures the esterase activity of the CE2 domain. The enzyme hydrolyzes acetyl ester bonds in acetylated polysaccharides. More specific terms (acetylxylan esterase activity) provide additional detail. Supporting Evidence: PMID:19338387 All of these CE2 enzymes act as acetyl esterases, releasing acetate from activated artificial substrates such as 4-nitrophenyl acetate (4-NPAc; see Table 1) and, to different extents, the acetylated plant polysaccharides xylan and glucomannan |
| GO:0016798 hydrolase activity, acting on glycosyl bonds | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation correctly captures the glycoside hydrolase activity of the GH5 domain, which cleaves beta-1,4-glucosidic bonds in cellulose. Reason: The GH5 domain of CelE is a glycoside hydrolase that cleaves glycosyl bonds. This is a parent term for the more specific cellulase activity annotation. Supporting Evidence: PMID:3066698 coding for an endo-beta-1,4-glucanase |
| GO:0030245 cellulose catabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: CelE participates in cellulose degradation through its GH5 cellulase domain, which hydrolyzes beta-1,4-glucosidic linkages in cellulose. The CE2 domain further enhances this activity by binding to cellulose and bringing the enzyme into contact with its substrate (PMID:19338387, PMID:1991028). Reason: This is a core biological process annotation. CelE is a cellulosome component that directly participates in cellulose catabolism through its cellulase activity and cellulose-binding function. Supporting Evidence: PMID:19338387 it also acts as a cellulose-binding domain, which promotes the activity of the appended cellulase on recalcitrant substrates |
| GO:0046555 acetylxylan esterase activity | IEA GO_REF:0000120 | ACCEPT | Summary: The CE2 domain of CelE has acetylxylan esterase activity (EC 3.1.1.72), catalyzing deacetylation of xylan (PMID:19338387). Kinetic parameters show KM of 2.7 mM and kcat of 12 min-1 for acetylated birchwood xylan. Reason: This is a core molecular function of the CE2 domain. The enzyme catalyzes deacetylation of xylan, removing acetyl groups that protect the xylan backbone from degradation by other enzymes. Supporting Evidence: PMID:19338387 CtCE2 and CjCE2B exhibit a significant preference for acetylated glucomannan over xylan |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | MODIFY | Summary: The dockerin domain of CelE contains calcium-binding sites that are essential for its function in binding to cohesin domains of the scaffoldin protein. Multiple calcium-binding residues are annotated in the UniProt record (residues 415-462). Reason: While this annotation is technically correct, it would be more informative to use the more specific term GO:0005509 (calcium ion binding), as the metal binding is specifically to calcium ions in the dockerin domain. Proposed replacements: calcium ion binding |
| GO:0052689 carboxylic ester hydrolase activity | IEA GO_REF:0000002 | ACCEPT | Summary: The CE2 domain hydrolyzes carboxylic esters (acetyl groups) from polysaccharides. This is consistent with its acetylxylan esterase and glucomannan deacetylase activities. Reason: This is an accurate parent term for the esterase activity. The CE2 domain catalyzes hydrolysis of acetyl ester bonds, releasing acetate from polysaccharide substrates. Supporting Evidence: PMID:19338387 The esterases appear specific for acetyl groups |
| GO:2000884 glucomannan catabolic process | IEA GO_REF:0000117 | ACCEPT | Summary: CelE participates in glucomannan catabolism through deacetylation of acetylated glucomannan by its CE2 domain. The enzyme shows preference for glucomannan over xylan, with KM of 0.019 mM (PMID:19338387). Reason: This is a core biological process annotation. The CE2 domain deacetylates glucomannan, removing acetyl groups that protect the polysaccharide from degradation. The very low KM indicates high affinity for this substrate. Supporting Evidence: PMID:19338387 CtCE2 and CjCE2B exhibit a significant preference for acetylated glucomannan over xylan |
| GO:0045493 xylan catabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: CelE participates in xylan catabolism through both its xylanase activity (noted in PMID:3066698) and its acetylxylan esterase activity that deacetylates xylan to make it accessible to other enzymes (PMID:19338387). Reason: This is a core biological process annotation. The original characterization noted xylan-hydrolyzing activity, and the CE2 domain deacetylates xylan, contributing to xylan degradation. Supporting Evidence: PMID:3066698 an endo-beta-1,4-glucanase (endoglucanase E; EGE) with xylan-hydrolysing activity PMID:19338387 CtCE2 catalyses deacetylation of noncellulosic plant structural polysaccharides to deprotect these substrates for attack by other enzymes |
| GO:0008810 cellulase activity | IDA PMID:3066698 Conserved reiterated domains in Clostridium thermocellum end... | ACCEPT | Summary: Direct experimental evidence from the original characterization of CelE demonstrates cellulase activity against cellulose substrates (PMID:3066698). Reason: This IDA annotation is strongly supported by the original biochemical characterization of CelE as an endoglucanase. This is a core molecular function. Supporting Evidence: PMID:3066698 coding for an endo-beta-1,4-glucanase (endoglucanase E; EGE) |
| GO:0030245 cellulose catabolic process | IDA PMID:3066698 Conserved reiterated domains in Clostridium thermocellum end... | ACCEPT | Summary: The original characterization established CelE's role in cellulose degradation. Reason: This IDA annotation is appropriate based on the biochemical evidence for cellulase activity and the enzyme's role in the cellulosome complex. Supporting Evidence: PMID:3066698 coding for an endo-beta-1,4-glucanase |
| GO:0030248 cellulose binding | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | ACCEPT | Summary: The CE2 domain of CelE binds cellulose through a unique mechanism involving its catalytic site region. ITC measurements show KD of 33 uM for cellohexaose. This binding potentiates the activity of the appended cellulase domain on recalcitrant substrates (PMID:19338387, PMID:1991028). Reason: This is a key molecular function that distinguishes CelE from other CE2 family members. The cellulose-binding function is mediated through the active site of the CE2 domain, representing a novel dual-function in a single domain. Pull-down assays and AGE demonstrated binding to insoluble cellulose. Supporting Evidence: PMID:19338387 CtCE2 was previously characterized as a carbohydrate-binding module (CBM) by virtue of its cellulose-binding capacity and its ability to potentiate the cellulase activity of the linked CtCel5C catalytic module PMID:19338387 Isothermal titration calorimetry (ITC) revealed that CtCE2 binds to cellooligosaccharides with a K |
| GO:0045493 xylan catabolic process | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | ACCEPT | Summary: PMID:19338387 demonstrates that CelE (CtCE2 domain) deacetylates xylan, contributing to xylan catabolism by removing protective acetyl groups. Reason: Direct experimental evidence shows the CE2 domain catalyzes deacetylation of acetylated birchwood xylan, contributing to xylan degradation. Supporting Evidence: PMID:19338387 All of these CE2 enzymes act as acetyl esterases, releasing acetate from activated artificial substrates such as 4-nitrophenyl acetate (4-NPAc; see Table 1) and, to different extents, the acetylated plant polysaccharides xylan and glucomannan |
| GO:0046555 acetylxylan esterase activity | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | ACCEPT | Summary: PMID:19338387 provides extensive biochemical characterization of the acetylxylan esterase activity of the CE2 domain, including kinetic parameters (KM 2.7 mM, kcat 12 min-1 for acetylated birchwood xylan). Reason: Strong experimental evidence supports this annotation. The CE2 domain was shown to deacetylate xylan with defined kinetic parameters. Supporting Evidence: PMID:19338387 All of these CE2 enzymes act as acetyl esterases, releasing acetate from activated artificial substrates such as 4-nitrophenyl acetate (4-NPAc; see Table 1) and, to different extents, the acetylated plant polysaccharides xylan and glucomannan |
| GO:2000884 glucomannan catabolic process | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | ACCEPT | Summary: PMID:19338387 demonstrates that the CE2 domain preferentially deacetylates glucomannan over xylan, with KM of 0.019 mM showing very high affinity for this substrate. Reason: Direct experimental evidence establishes CelE's role in glucomannan catabolism. The very low KM for glucomannan indicates this may be a preferred physiological substrate for the CE2 domain. Supporting Evidence: PMID:19338387 Based on their catalytic efficiencies, CtCE2 and CjCE2B exhibit a significant preference for acetylated glucomannan over xylan |
| GO:0043263 cellulosome | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | NEW | Summary: CelE contains a type I dockerin domain that mediates its integration into the cellulosome complex. This localization is essential for its function in plant cell wall degradation. Reason: The literature clearly establishes that CelE is a cellulosome component via its dockerin domain, but this cellular component annotation is missing from the current annotation set. This should be added as it represents a key aspect of the protein's localization and function. Supporting Evidence: PMID:19338387 The enzyme also contains a type I dockerin module that, by binding to cohesin modules in the scaffoldin protein, incorporates CtCel5C-CE2 into the multienzyme plant cell-wall-degrading complex known as the cellulosome |
| GO:1990311 type-I cohesin domain binding | IDA PMID:19338387 The active site of a carbohydrate esterase displays divergen... | NEW | Summary: The dockerin domain of CelE binds to type-I cohesin domains in the scaffoldin protein, enabling integration into the cellulosome. Reason: The type I dockerin domain of CelE binds to cohesin domains, which is essential for cellulosome assembly. This molecular function annotation would complete the annotation set. Supporting Evidence: PMID:19338387 The enzyme also contains a type I dockerin module that, by binding to cohesin modules in the scaffoldin protein, incorporates CtCel5C-CE2 into the multienzyme plant cell-wall-degrading complex |
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Download this section (compressed HTML)Q: What is the relative contribution of the cellulase vs esterase activities to plant cell wall degradation?
Q: Does the cellulose-binding function of the CE2 domain have regulatory significance beyond substrate targeting?
Q: Are there other CE2 family members that have gained cellulose-binding function through similar mechanisms?
Experiment: Quantify the enhancement of cellulase activity on natural plant cell wall substrates when CtCE2 cellulose binding is intact vs abolished
Hypothesis: The cellulose-binding function of CtCE2 significantly enhances cellulase activity on crystalline cellulose
Experiment: Determine whether CtCE2 cellulose binding affects cellulosome assembly or localization
Hypothesis: Cellulose binding by CtCE2 may help anchor the cellulosome to its substrate
Experiment: Characterize the deacetylation activity on native plant cell wall polysaccharides from different plant sources
Hypothesis: The preference for glucomannan may reflect the substrate composition of natural plant cell walls
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