LicB (also known as Lam1) is a thermostable beta-glucanase (lichenase, EC 3.2.1.73) from Acetivibrio thermocellus (formerly Clostridium thermocellum). It is a GH16 family glycoside hydrolase that catalyzes the endohydrolysis of (1->4)-beta-D-glucosidic linkages in mixed-linkage beta-D-glucans containing both (1->3) and (1->4) bonds, such as lichenan and barley beta-glucan. The enzyme contains a C-terminal type-I dockerin domain that enables its incorporation into the cellulosome, the large extracellular enzyme complex characteristic of this bacterium that efficiently degrades plant cell wall polysaccharides.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000002 | MODIFY | Summary: This IEA annotation is derived from InterPro domain matches (IPR002105, IPR016134, IPR036439 - dockerin domains). While LicB does participate in polysaccharide catabolism, this term is too general. The enzyme specifically degrades mixed-linkage beta-glucans. A more specific term like 'beta-glucan catabolic process' (GO:0051275) would be more accurate. Reason: The term 'polysaccharide catabolic process' is overly broad. LicB is a lichenase that specifically hydrolyzes beta-glucans containing (1->3) and (1->4) linkages. UniProt confirms the catalytic activity with EC 3.2.1.73. A more specific biological process term should be used. Proposed replacements: beta-glucan catabolic process Supporting Evidence: UniProtKB:Q84C00 Hydrolysis of (1->4)-beta-D-glucosidic linkages in beta-D- glucans containing (1->3)- and (1->4)-bonds. file:ACET2/Q84C00/Q84C00-deep-research-falcon.md LicB is a GH16 family lichenase that degrades beta-glucans |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000002 | ACCEPT | Summary: This annotation is correct but represents a high-level parent term. LicB does indeed have O-glycosyl hydrolase activity, but the specific term 'licheninase activity' (GO:0042972) already captures this more precisely. This annotation can be retained as it is not incorrect, but it provides less information than the more specific term. Reason: While this is a general parent term, it is not incorrect for LicB. The enzyme hydrolyzes O-glycosyl bonds in beta-glucans. Since the more specific 'licheninase activity' (GO:0042972) is also annotated, this broader term provides hierarchical context and is acceptable to retain. Supporting Evidence: UniProtKB:Q84C00 Belongs to the glycosyl hydrolase 16 family. |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | MODIFY | Summary: This is a very high-level biological process term. While technically correct (lichenase participates in carbohydrate metabolism), this term is too general to be informative. It should be replaced with a more specific term. Reason: 'Carbohydrate metabolic process' is the grandparent term for nearly all sugar-related processes. For a specific enzyme like lichenase, a more informative term such as 'beta-glucan catabolic process' (GO:0051275) should be used to accurately reflect the enzyme's biological role. Proposed replacements: beta-glucan catabolic process Supporting Evidence: UniProtKB:Q84C00 Hydrolysis of (1->4)-beta-D-glucosidic linkages in beta-D- glucans containing (1->3)- and (1->4)-bonds. |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | MARK AS OVER ANNOTATED | Summary: This annotation is derived from UniProtKB keyword mapping (KW-0378 Hydrolase). While LicB is indeed a hydrolase, this is the root molecular function term for all hydrolases and is too general to be informative when more specific terms are available. Reason: The term 'hydrolase activity' is the most general hydrolase term in GO. Since LicB already has the specific annotation 'licheninase activity' (GO:0042972), retaining this very general term adds no informational value and clutters the annotation set. Supporting Evidence: UniProtKB:Q84C00 Glycosidase; Hydrolase; Signal. |
| GO:0016798 hydrolase activity, acting on glycosyl bonds | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation comes from UniProtKB keyword mapping (KW-0326 Glycosidase). It is a parent term of 'hydrolase activity, hydrolyzing O-glycosyl compounds' (GO:0004553). While correct, it is redundant given the presence of more specific glycosidase terms. Reason: This term correctly captures the glycosidase activity of LicB. Although it is a mid-level term in the hierarchy (parent of GO:0004553), it is informative enough to retain alongside the specific 'licheninase activity' term. It provides appropriate hierarchical context. Supporting Evidence: UniProtKB:Q84C00 Belongs to the glycosyl hydrolase 16 family. |
| GO:0042972 licheninase activity | IEA GO_REF:0000003 | ACCEPT | Summary: This is the most specific and accurate molecular function annotation for LicB. Licheninase activity (EC 3.2.1.73) catalyzes the hydrolysis of (1->4)-beta-D- glucosidic linkages in beta-D-glucans containing (1->3) and (1->4) bonds. This precisely matches the documented function of LicB. Reason: This annotation is derived from EC number mapping (EC:3.2.1.73) and accurately represents the core molecular function of LicB. UniProt explicitly assigns EC 3.2.1.73 to this enzyme, and the catalytic activity described matches the GO term definition exactly. This is the primary functional annotation for this enzyme. Supporting Evidence: UniProtKB:Q84C00 RecName: Full=Beta-glucanase; |
| GO:1990311 type-I cohesin domain binding | ISS UniProtKB:Q84C00 | NEW | Summary: LicB contains a type-I dockerin domain (residues 267-334) that mediates binding to type-I cohesin domains on the CipA scaffoldin protein of the cellulosome. This domain architecture is characteristic of cellulosomal enzymes in A. thermocellus. Reason: The UniProt entry clearly documents a dockerin domain (PROSITE pattern PS51766). The dockerin domain binds to cohesin domains on the scaffoldin protein, enabling incorporation of the enzyme into the cellulosome. This is a core function of the protein enabling its biological role in extracellular polysaccharide degradation. Supporting Evidence: UniProtKB:Q84C00 InterPro; IPR002105; Dockerin_1_rpt. |
| GO:0043263 cellulosome | ISS UniProtKB:Q84C00 | NEW | Summary: LicB is a component of the cellulosome based on its dockerin domain, which mediates attachment to the scaffoldin. The cellulosome is the characteristic extracellular enzyme complex of A. thermocellus. Reason: The presence of a type-I dockerin domain (IPR002105, PS51766) strongly indicates that LicB is a cellulosomal enzyme. The dockerin domain is the hallmark of cellulosomal enzymes in Clostridia, enabling their incorporation into the multi-enzyme complex via binding to cohesin domains on the scaffoldin. Supporting Evidence: UniProtKB:Q84C00 PROSITE; PS00448; CLOS_CELLULOSOME_RPT; 2. |
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Download this section (compressed HTML)Q: What is the substrate specificity of LicB compared to other GH16 enzymes in A. thermocellus? Does it preferentially cleave lichenan vs barley beta-glucan?
Q: Is LicB constitutively expressed or is its expression regulated by carbon source availability?
Experiment: Kinetic characterization of recombinant LicB with different beta-glucan substrates (lichenan, barley beta-glucan, laminarin) to determine substrate specificity and kinetic parameters.
Hypothesis: Would provide quantitative data to better annotate substrate specificity.
Experiment: Pulldown or co-IP experiments to confirm cellulosome incorporation and identify specific cohesin binding partners.
Hypothesis: Would provide direct experimental evidence for cellulosome localization annotation.
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