CelA (also known as Cel8A or Endoglucanase A) is a secreted endo-beta-1,4-glucanase (EC 3.2.1.4) belonging to glycosyl hydrolase family 8 (GH8). It catalyzes the endohydrolysis of internal (1->4)-beta-D-glucosidic linkages in cellulose, lichenin, and cereal beta-D-glucans. The enzyme contains a GH8 catalytic domain with an open cleft/groove architecture characteristic of endo-acting glycosidases, plus a C-terminal type I dockerin domain (residues 411-477) that mediates incorporation into the cellulosome complex via cohesin-dockerin interactions with the primary scaffoldin CipA. CelA is one of the major catalytic components of the C. thermocellum cellulosome, consistently identified in proteomic surveys of purified cellulosomes grown on Avicel. The enzyme lacks its own carbohydrate-binding module (CBM) and relies on the CBM of CipA for targeting to insoluble cellulose substrates. Structural studies at atomic resolution (0.94 A) have characterized the active site, identifying residues E95 (proton donor) and D152 (nucleophile) as key catalytic residues.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000120 | ACCEPT | Summary: CelA is an endoglucanase that participates in cellulose degradation. While cellulose is a polysaccharide, the more specific term GO:0030245 (cellulose catabolic process) is already annotated and better represents the biological process. This broader term is acceptable but redundant given the more specific annotation. The deep research review confirms CelA's role in cellulose breakdown as part of the cellulosome complex [celA-deep-research-falcon.md]. Reason: This annotation is correct but overly broad. CelA specifically degrades cellulose, which is a type of polysaccharide. The term is inferred from InterPro domain annotations (IPR002105 Dockerin, IPR016134 Dockerin domain, IPR036439 Dockerin superfamily) which correctly associate the protein with polysaccharide degradation. Since GO:0030245 (cellulose catabolic process) is also annotated and is more specific, this broader term is somewhat redundant but not incorrect. For IEA annotations, broader parent terms are acceptable to retain alongside more specific ones. |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000120 | ACCEPT | Summary: CelA is a glycosyl hydrolase (GH8 family) that hydrolyzes O-glycosyl compounds, specifically (1->4)-beta-D-glucosidic linkages. This term is correct but a parent of the more specific GO:0008810 (cellulase activity) which is also annotated. Reason: This annotation is correct. CelA belongs to GH8 and catalyzes hydrolysis of beta-glucosidic bonds in cellulose. The annotation is inferred from ARBA rule ARBA00027782 and InterPro domains IPR002037 (Glyco_hydro_8) and IPR002105 (Dockerin). While GO:0008810 (cellulase activity) is more specific, this parent term is acceptable for an IEA annotation and provides a useful grouping term. |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: CelA participates in carbohydrate metabolism through its role in cellulose degradation. This is a high-level parent term of more specific annotations already present. Reason: This annotation is correct but very broad. It is inferred from InterPro records IPR002037 (Glyco_hydro_8), IPR008928 (6-hairpin_glycosidase_sf), and IPR012341 (6hp_glycosidase-like_sf). While the more specific GO:0030245 (cellulose catabolic process) better captures CelA's function, this broader term is not incorrect and acceptable for an IEA annotation derived from domain membership. |
| GO:0008810 cellulase activity | IEA GO_REF:0000003 | ACCEPT | Summary: CelA (Endoglucanase A) is definitively a cellulase with EC 3.2.1.4 activity. It catalyzes the endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose, lichenin, and cereal beta-D-glucans. This is the core molecular function of the enzyme and the most specific appropriate MF term. Reason: This annotation accurately represents the core molecular function of CelA. The enzyme is explicitly annotated as EC 3.2.1.4 (endoglucanase/cellulase) in UniProt, and the GO term definition matches exactly: "Catalysis of the endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose, lichenin and cereal beta-D-glucans." The annotation is derived from the EC number mapping (GO_REF:0000003). Proteomic studies identify CelA/Cel8A as a major catalytic component of the cellulosome with endo-beta-1,4-glucanase activity [PMID:16127726, PMID:17644599]. Structural studies confirm the open cleft architecture characteristic of endo-acting glycosidases [PMID:8805535]. Supporting Evidence: PMID:16127726 Ten of the components were previously known: the structural protein CipA, the endo-glucanases Cel8A, Cel5G, Cel9N, the cellobiohydrolases Cbh9A, Cel9K, Cel48S, the xylanases Xyn10C, Xyn10Z, and the chitinase Chi18A PMID:17644599 In total, 41 cellulosomal proteins were detected, including 36 type I dockerin-containing proteins |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | ACCEPT | Summary: CelA is a hydrolase that cleaves glycosidic bonds. This is a very broad parent term of the more specific cellulase activity already annotated. Reason: This annotation is correct but extremely broad. CelA is indeed a hydrolase (EC 3.2.1.4), but the term GO:0008810 (cellulase activity) provides much more informative functional information. This IEA annotation derived from UniProtKB keyword KW-0378 (Hydrolase) is acceptable but provides minimal functional insight beyond what is captured by more specific terms. |
| GO:0016798 hydrolase activity, acting on glycosyl bonds | IEA GO_REF:0000043 | ACCEPT | Summary: CelA hydrolyzes glycosyl bonds, specifically (1->4)-beta-D-glucosidic linkages. This term is a parent of GO:0004553 (hydrolase activity, hydrolyzing O-glycosyl compounds) and GO:0008810 (cellulase activity). Reason: This annotation is correct. CelA acts on glycosyl bonds (specifically beta-1,4 glucosidic bonds in cellulose). The annotation is inferred from UniProtKB keyword KW-0326 (Glycosidase). While more specific terms are available and already annotated, this intermediate-level term is acceptable for an IEA. |
| GO:0030245 cellulose catabolic process | IEA GO_REF:0000043 | ACCEPT | Summary: CelA is a cellulose-degrading endoglucanase that directly participates in cellulose catabolism as part of the cellulosome complex. This is the appropriate biological process term for this enzyme. Reason: This annotation accurately represents CelA's biological role. The enzyme catalyzes the endohydrolysis of cellulose chains, producing new chain ends for exoglucanases and contributing directly to cellulose breakdown. The annotation is derived from UniProtKB keyword KW-0136 (Cellulose degradation). Proteomic analyses confirm CelA is a major component of the cellulosome, the multi-enzyme complex responsible for cellulose catabolism in C. thermocellum [PMID:16127726, PMID:17644599, PMID:38234915]. Supporting Evidence: PMID:38234915 the anchoring of various cellulolytic enzymes, i.e., endoglucanases, cellobiohydrolases, xylanases, to a scaffoldin makes it a complex structure in which different enzymes work synergistically to attack heterogeneous, insoluble cellulose substrates file:ACET2/celA/celA-deep-research-falcon.md CelA/Cel8A is an endo-beta-1,4-glucanase (EC 3.2.1.4) that cleaves internal bonds in cellulose chains, producing new chain ends for exoglucanases |
| GO:0043263 cellulosome | IEA GO_REF:0000120 | NEW | Summary: CelA contains a type I dockerin domain that mediates its incorporation into the cellulosome complex. The cellulosome (GO:0043263) is the appropriate cellular component annotation for this protein. Reason: CelA has a well-characterized dockerin domain (residues 411-477) that binds to type I cohesins on the CipA scaffoldin. Proteomic surveys consistently identify CelA as a component of purified cellulosomes. This cellular component annotation should be added, as the protein functions as part of the cellulosome complex. The InterPro domains IPR002105 (Dockerin_1_rpt), IPR016134 (Dockerin_dom), and IPR036439 (Dockerin_dom_sf) all indicate cellulosome localization. Note: This annotation may already exist via IEA but was not present in the provided GOA file. Proposed replacements: cellulosome Supporting Evidence: PMID:16127726 To identify the predominant catalytic components, cellulosomes were purified and the components were separated by an adapted two-dimensional gel electrophoresis technique. The apparent major spots were identified by MALDI-TOF/TOF PMID:38234915 Cellulosome comprises two major subunits: long flexible scaffoldin, which forms the center part containing specific binding sites cohesions, and the enzymes containing a dockerin module that binds to cohesion |
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Download this section (compressed HTML)Q: What is the precise substrate specificity of CelA compared to other GH8 endoglucanases in the cellulosome? Does it show any preference for specific regions of crystalline vs amorphous cellulose?
Q: How does CelA synergize with exoglucanases (like CelS/Cel48A) and other endoglucanases in the cellulosome to achieve efficient cellulose degradation?
Experiment: Compare the kinetic parameters (kcat, Km) of CelA in free form versus when bound to mini-cellulosomes containing different combinations of other cellulases. Use defined cellulose substrates (Avicel, filter paper, CMC, phosphoric acid swollen cellulose) to assess potential synergistic effects.
Hypothesis: CelA shows optimal activity when incorporated into the cellulosome complex compared to free enzyme, due to proximity effects with other cellulases.
Type: enzyme kinetics
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