AlgG is a periplasmic mannuronan C5-epimerase that converts beta-D-mannuronate residues in alginate to alpha-L-guluronate. Independently of this catalytic modification, AlgG helps protect nascent polymer from periplasmic degradation during export; catalytically defective AlgG can retain this structural role.
Definition: Catalysis of C5 epimerization of beta-D-mannuronate residues within a (1->4)-linked mannuronan chain to alpha-L-guluronate residues, producing alginate without cleaving the glycosidic backbone.
Justification: GO:0016857 covers alteration of chiral centers in carbohydrate molecules, including polymer-bound residues, and is the valid current annotation. A specific term would preserve the polymer-level mannuronan substrate and C5 stereochemistry of EC 5.1.3.37 (RHEA:45572), distinguishing this reaction from small-molecule sugar epimerases. The precise term is an ontology proposal, not an additional redundant NEW annotation.
Parent term: racemase and epimerase activity, acting on carbohydrates and derivatives
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005737 cytoplasm | IBA GO_REF:0000033 | MODIFY | Summary: Mature AlgG functions in the periplasm. Reason: The IBA at PANTHER:PTN007723485 assigns a compartment inconsistent with signal-peptide processing and direct localization of mature AlgG. Replace cytoplasm with periplasmic space; this is a lateral compartment correction. Propagation Review Root cause: PROPAGATION BAD Failure modes: COMPARTMENT OR COMPLEX MISMATCH Sources checked: PANTHER:PTN007723485 UNRESOLVED The proximate ancestral node is identified from seeded WITH/FROM. Its complete PAINT assertion history has not been reconstructed; source-node status remains unresolved. Target-specific primary evidence supports the functional/compartment distinction explained in the review reason, rather than a donor-count or circularity argument. Proposed replacements: periplasmic space Supporting Evidence: |
| GO:0042121 alginic acid biosynthetic process | IDA PMID:8144447 Pseudomonas aeruginosa AlgG is a polymer level alginate C5-m... | ACCEPT | Summary: AlgG modifies alginate and supports intact polymer export. Reason: AlgG epimerizes mannuronate residues and has a separable polymer-protection role. The S272N allele loses epimerase activity while retaining polymer production, demonstrating that epimerization is not required for the structural export contribution. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
| GO:0042121 alginic acid biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: AlgG modifies alginate and supports intact polymer export. Reason: AlgG epimerizes mannuronate residues and has a separable polymer-protection role. The S272N allele loses epimerase activity while retaining polymer production, demonstrating that epimerization is not required for the structural export contribution. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
| GO:0042121 alginic acid biosynthetic process | IMP PMID:12581364 The dual roles of AlgG in C-5-epimerization and secretion of... | ACCEPT | Summary: AlgG modifies alginate and supports intact polymer export. Reason: AlgG epimerizes mannuronate residues and has a separable polymer-protection role. The S272N allele loses epimerase activity while retaining polymer production, demonstrating that epimerization is not required for the structural export contribution. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
| GO:0042597 periplasmic space | EXP PMID:8144447 Pseudomonas aeruginosa AlgG is a polymer level alginate C5-m... | ACCEPT | Summary: Processed AlgG is periplasmic. Reason: Biochemical processing/localization experiments establish the mature periplasmic protein; catalytic epimerization and polymer protection occur during periplasmic transit. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. |
| GO:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: Processed AlgG is periplasmic. Reason: Biochemical processing/localization experiments establish the mature periplasmic protein; catalytic epimerization and polymer protection occur during periplasmic transit. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. |
| GO:0043161 proteasome-mediated ubiquitin-dependent protein catabolic process | IBA GO_REF:0000033 | REMOVE | Summary: AlgG is a bacterial polysaccharide epimerase/export accessory, not a ubiquitin-system adaptor. Reason: The IBA at PANTHER:PTN007723485 propagates a eukaryotic ubiquitin/proteasome-related role onto a secreted bacterial carbohydrate-active protein. Direct AlgG chemistry, periplasmic localization and separation-of-function genetics contradict this compartment and pathway assignment. The challenge concerns propagation scope, not similarity or donor counts. Propagation Review Root cause: PROPAGATION BAD Failure modes: LINEAGE OR TAXON MISMATCH Sources checked: PANTHER:PTN007723485 UNRESOLVED The proximate ancestral node is identified from seeded WITH/FROM. Its complete PAINT assertion history has not been reconstructed; source-node status remains unresolved. Target-specific primary evidence supports the functional/compartment distinction explained in the review reason, rather than a donor-count or circularity argument. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
| GO:1990756 ubiquitin-like ligase-substrate adaptor activity | IBA GO_REF:0000033 | REMOVE | Summary: AlgG is a bacterial polysaccharide epimerase/export accessory, not a ubiquitin-system adaptor. Reason: The IBA at PANTHER:PTN007723485 propagates a eukaryotic ubiquitin/proteasome-related role onto a secreted bacterial carbohydrate-active protein. Direct AlgG chemistry, periplasmic localization and separation-of-function genetics contradict this compartment and pathway assignment. The challenge concerns propagation scope, not similarity or donor counts. Propagation Review Root cause: PROPAGATION BAD Failure modes: LINEAGE OR TAXON MISMATCH Sources checked: PANTHER:PTN007723485 UNRESOLVED The proximate ancestral node is identified from seeded WITH/FROM. Its complete PAINT assertion history has not been reconstructed; source-node status remains unresolved. Target-specific primary evidence supports the functional/compartment distinction explained in the review reason, rather than a donor-count or circularity argument. Supporting Evidence: PMID:12581364 AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
| GO:0016857 racemase and epimerase activity, acting on carbohydrates and derivatives | IDA PMID:8144447 Pseudomonas aeruginosa AlgG is a polymer level alginate C5-m... | NEW | Summary: Recombinant AlgG directly epimerizes polymer-bound mannuronate. Reason: GO:0016857 is defined as changing one or more chiral centers in a carbohydrate molecule; it is not restricted to free monosaccharides. Direct recombinant target epimerization establishes this MF independently of the noncatalytic polymer-protection role. Existing annotations contain no epimerase MF or replacement and therefore do not duplicate this proposal. The narrower mannuronan C5-epimerase term is requested separately, not asserted as a second NEW annotation. Supporting Evidence: PMID:8144447 These results demonstrate that AlgG has polymer-level mannuronan C5-epimerase activity. |
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