CelK (also known as Cel9K, locus Cthe_0412) is a major cellulosomal cellulase belonging to glycoside hydrolase family 9 (GH9) in Acetivibrio thermocellus (formerly Clostridium thermocellum). It functions as a cellulose 1,4-beta-cellobiosidase (EC 3.2.1.91) that hydrolyzes beta-1,4-glucosidic linkages in cellulose, releasing cellobiose and cellotriose from the non-reducing ends of cellulose chains. The enzyme has a modular architecture consisting of a CBM4 carbohydrate-binding module, an Ig-like domain, the GH9 catalytic domain, and a C-terminal type I dockerin domain that mediates integration into the cellulosome complex. CelK works synergistically with Cel48S (the major GH48 cellobiohydrolase) in the cellulosome to efficiently degrade crystalline cellulose.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000272 polysaccharide catabolic process | IEA GO_REF:0000120 | MODIFY | Summary: CelK is a cellulase that degrades cellulose, which is a polysaccharide. The annotation to polysaccharide catabolic process is correct but overly broad. CelK specifically catalyzes the breakdown of cellulose (a beta-1,4-glucan), not polysaccharides in general. Reason: While cellulose is a polysaccharide and CelK does participate in polysaccharide catabolism, this term is too general. The more specific term GO:0030245 (cellulose catabolic process) already exists in the annotation set and accurately captures the biological process. This broader term provides less informative annotation. Proposed replacements: cellulose catabolic process Supporting Evidence: PMID:29075324 CBHs (exo-acting from the sugar ends) released specifically cellobiose as the only product over time (Cel48S, Cel9K, and Cbh9A) |
| GO:0004553 hydrolase activity, hydrolyzing O-glycosyl compounds | IEA GO_REF:0000120 | ACCEPT | Summary: CelK is a GH9 family glycoside hydrolase that hydrolyzes beta-1,4-glucosidic bonds in cellulose. This annotation is correct but too general - the enzyme has specific cellobiohydrolase activity (EC 3.2.1.91) rather than generic glycosyl hydrolase activity. Reason: While GO:0016162 (cellulose 1,4-beta-cellobiosidase activity) is more specific and preferred, this parent term is technically accurate. Since the more specific term is already annotated, this broader annotation is acceptable as it provides correct hierarchical coverage. IEA annotations often propagate from broader InterPro signatures and this is consistent with the enzyme's function. Supporting Evidence: PMID:29075324 exo-acting cellobiohydrolases thread the cellulose molecule from its free cellulose chain end through a tunnel built by loop structures around the active site |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | MODIFY | Summary: CelK is a secreted protein that incorporates into the extracellular cellulosome complex via its dockerin domain. The signal peptide (residues 1-27) is cleaved upon secretion. Proteomics confirms CelK is abundant in cellulosome preparations from cellulose-grown cells. Reason: While CelK is secreted to the extracellular region, a more specific cellular component annotation would be GO:0043263 (cellulosome), as CelK is a bona fide cellulosomal enzyme that integrates via dockerin-cohesin interactions. The cellulosome is a defined extracellular multi-enzyme complex in C. thermocellum. Proposed replacements: cellulosome Supporting Evidence: PMID:16127726 the cellobiohydrolases Cbh9A, Cel9K, Cel48S PMID:17644599 exoglucanases CelS and CelK |
| GO:0005975 carbohydrate metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: CelK participates in carbohydrate metabolism by degrading cellulose. This annotation is correct but extremely broad - CelK's specific role is in cellulose catabolism. Reason: This is a very high-level biological process term that is technically accurate but provides minimal information. Since more specific BP annotations exist (GO:0030245 cellulose catabolic process), this broad term is acceptable as hierarchical context but not informative as a standalone annotation. IEA from InterPro is appropriate for this level of granularity. |
| GO:0008810 cellulase activity | IEA GO_REF:0000002 | MODIFY | Summary: CelK has cellulase activity as a GH9 family enzyme. However, the GO definition of cellulase activity (GO:0008810) specifically refers to endohydrolysis, whereas CelK functions as a cellobiohydrolase (exo-acting, EC 3.2.1.91) releasing cellobiose from non-reducing ends. Reason: GO:0008810 (cellulase activity) is defined as "catalysis of the endohydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose". CelK is classified as EC 3.2.1.91 (cellulose 1,4-beta-cellobiosidase), which is an exo-acting enzyme that releases cellobiose from the non-reducing ends. The correct annotation is GO:0016162 (cellulose 1,4-beta-cellobiosidase activity), which is already present in the annotation set. Proposed replacements: cellulose 1,4-beta-cellobiosidase activity Supporting Evidence: PMID:29075324 As expected, for the CBHs Cbh9A, Cel48S, and Cel9K no or very weak activities on modified substrates were observed, whereas on PASC substantial product formation was found |
| GO:0016162 cellulose 1,4-beta-cellobiosidase activity | IEA GO_REF:0000003 | ACCEPT | Summary: This is the most accurate molecular function annotation for CelK. The enzyme is classified as EC 3.2.1.91 (cellulose 1,4-beta-cellobiosidase), which matches this GO term exactly. UniProt catalytic activity annotation confirms: "Hydrolysis of (1->4)-beta-D-glucosidic linkages in cellulose and cellotetraose, releasing cellobiose from the non-reducing ends of the chains." Reason: This is the correct and most specific molecular function annotation for CelK. The annotation is derived from EC:3.2.1.91 mapping, which is the experimentally determined enzyme classification for this protein. This represents the core catalytic function of CelK. Supporting Evidence: PMID:29075324 CBHs (exo-acting from the sugar ends) released specifically cellobiose as the only product over time (Cel48S, Cel9K, and Cbh9A) |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | ACCEPT | Summary: CelK is indeed a hydrolase - it catalyzes the hydrolytic cleavage of beta-1,4-glucosidic bonds. This annotation is correct but extremely general. Reason: This is the root molecular function term for hydrolases. While technically accurate, it provides minimal information. More specific annotations (GO:0016162, GO:0004553, GO:0016798) are already present and preferred. This broad term is acceptable as it provides hierarchical consistency but is not informative as a standalone annotation. |
| GO:0016798 hydrolase activity, acting on glycosyl bonds | IEA GO_REF:0000120 | ACCEPT | Summary: CelK hydrolyzes glycosyl bonds (specifically beta-1,4-glucosidic linkages in cellulose). This annotation is correct and at an appropriate intermediate level of specificity. Reason: This term accurately describes CelK's molecular function at an intermediate level of specificity. While GO:0016162 is more specific, this parent term is correctly applied. IEA annotation from InterPro domain signatures appropriately captures this level of function. |
| GO:0030245 cellulose catabolic process | IEA GO_REF:0000043 | ACCEPT | Summary: CelK is directly involved in cellulose catabolism as a major cellulosomal cellulase. The enzyme degrades crystalline cellulose to cellobiose and cellotriose as primary products. This is the most appropriate biological process annotation for CelK. Reason: This is the correct and most specific biological process annotation for CelK. The enzyme's primary biological role is cellulose degradation as part of the cellulosome complex. This annotation accurately represents the core biological process in which CelK participates. Supporting Evidence: PMID:29075324 After prolonged incubation times (overnight), the final products are mainly cellobiose and cellotriose (DP 2 to DP 3) |
| GO:0043263 cellulosome | IEA GO_REF:0000044 | NEW | Summary: CelK contains a C-terminal type I dockerin domain (residues 828-894) that mediates integration into the cellulosome via interaction with cohesin domains on the CipA scaffoldin protein. Proteomics confirms CelK is one of the most abundant cellulosomal enzyme subunits. Reason: This annotation is not currently in the GOA file but is strongly supported by the domain architecture (dockerin domain) and proteomics data showing CelK as a major cellulosomal component. This is the most specific and accurate cellular component annotation. Supporting Evidence: PMID:17644599 exoglucanases CelS and CelK PMID:16127726 the cellobiohydrolases Cbh9A, Cel9K, Cel48S file:ACET2/celK/celK-deep-research-falcon.md CelK is a secreted, extracellular cellulosomal subunit that incorporates into the cellulosome via type-I dockerin interactions |
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Download this section (compressed HTML)Q: What is the precise processivity of CelK - does it remain attached to the cellulose chain after each catalytic event or does it dissociate and rebind?
Suggested experts: Wolfgang H. Schwarz, Vladimir V. Zverlov
Q: What are the kinetic parameters (kcat, Km) for CelK on various cellulosic substrates (Avicel, PASC, CMC)?
Suggested experts: Benedikt Leis, Wolfgang Liebl
Experiment: Perform single-molecule fluorescence microscopy to track individual CelK molecules on cellulose nanocrystals. Measure processivity (number of catalytic events per encounter) and compare to other GH9 family members with known processivities.
Hypothesis: CelK exhibits processive cellobiohydrolase activity on crystalline cellulose
Type: Single-molecule enzymology
Experiment: Measure hydrolysis rates of crystalline cellulose (Avicel) using CelK alone, Cel48S alone, and equimolar combinations. Quantify synergy as the ratio of combined activity to the sum of individual activities. Test at various enzyme ratios to determine optimal stoichiometry.
Hypothesis: CelK and Cel48S show synergistic enhancement of cellulose hydrolysis
Type: Enzyme kinetics / synergy assay
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