AlgG is the predicted periplasmic mannuronan C5-epimerase of Pseudomonas putida KT2440. It converts polymeric beta-D-mannuronate residues into alpha-L-guluronate during alginate maturation. The specific family assignment and characterized Pseudomonas orthologs support this activity and a separable role in protecting nascent polymer during export. Typical native Pseudomonas polymer composition does not define an absolute catalytic restriction: purified P. aeruginosa AlgG can generate adjacent guluronates in vitro. Target-specific residue requirements and product distributions have not been measured.
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:0042597 periplasmic space | IEA GO_REF:0000044 | ACCEPT | Summary: AlgG acts on polymer in the periplasm. Reason: The target curated by-similarity localization and signal sequence agree with experimentally characterized Pseudomonas orthologs; no native target fractionation is asserted. Supporting Evidence: file:PSEPK/algG/algG-deep-research-falcon.md In pseudomonads, alginate is synthesized initially as a polymannuronate (poly-M) chain and is subsequently modified in the periplasm by epimerization (AlgG) file:PSEPK/algG/algG-uniprot.txt SUBCELLULAR LOCATION: Periplasm |
| GO:0042121 alginic acid biosynthetic process | IEA GO_REF:0000041 | ACCEPT | Summary: AlgG contributes polymer epimerization and protection to alginate production. Reason: Comparative catalytic and noncatalytic functions are supported by the target specific family and separate ortholog deletion versus inactive-variant studies. Protection is not assigned an invented adaptor MF. Supporting Evidence: file:PSEPK/algG/algG-uniprot.txt Belongs to the D-mannuronate C5-epimerase family. PMID:24398681 the structure of Pseudomonas syringae AlgG has been determined PMID:12581364 AlgG is bifunctional with (i) epimerase activity and |
| GO:0016857 racemase and epimerase activity, acting on carbohydrates and derivatives | ISS PMID:8144447 Pseudomonas aeruginosa AlgG is a polymer level alginate C5-m... | NEW | Summary: The pre-existing NEW carbohydrate epimerase term captures a directly characterized ortholog activity. Reason: For this retained ISS proposal, the explicitly identified sequence comparator is Pseudomonas aeruginosa AlgG Q51371, whose source-linked FRD1 work in PMID:8144447 demonstrates recombinant polymer-level C5 epimerization. E. coli expression/extract assays and P. aeruginosa complementation in that study do not constitute direct KT2440 enzymology. Specific target family and alginate-locus context support transfer to Q88NC9. This is a reviewer-authored NEW ISS proposal, with the characterized donor named in supporting_entities and the primary experiment as its reference. GO:0016857 includes carbohydrate polymer epimerization, and physiological polymer composition does not impose a universal ban on adjacent guluronates. Propagation Review Root cause: NO FAILURE CORE Sources checked: UniProtKB:Q51371 · AlgG (Pseudomonas aeruginosa) SUPPORTS TRANSFER Specific experimentally characterized comparator identified for this review; supports the stated KT2440 homology inference. The same donor is recorded in the reviewer-authored proposal's supporting_entities field; this is a homology inference, not a direct target assay. Supporting Evidence: file:PSEPK/algG/algG-uniprot.txt Belongs to the D-mannuronate C5-epimerase family. PMID:24398681 the structure of Pseudomonas syringae AlgG has been determined PMID:16401084 tracts of adjacent guluronate residues were readily formed PMID:8144447 Epimerization was also detected in the in vitro reaction between recombinant AlgG and poly-D-mannuronate, using high-performance anion-exchange chromatography. |
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Download this section (compressed HTML)Q: How important is AlgG's non-catalytic scaffolding role in KT2440 relative to its epimerase activity during native alginate production?
Suggested experts: Svein Valla, Dennis E. Ohman
Q: Under which physiological conditions does KT2440 deploy AlgG-dependent alginate remodeling most strongly?
Suggested experts: P. Lynne Howell, Martin Gimmestad
Experiment: Create an algG deletion strain and catalytic-site mutants in the DPHD motif, then compare polymer yield and uronate composition under alginate-inducing conditions.
Hypothesis: Catalytic-dead mutants will separate guluronate formation from the structural role of AlgG in polymer protection/export.
Experiment: Measure secreted alginate composition by NMR or linkage-sensitive enzymology in wild type, algG-null, and complemented strains.
Hypothesis: AlgG activity and its biosynthetic context jointly determine guluronate content and block distribution; compare native polymers with purified-enzyme products rather than assume a catalytic ban on adjacent guluronates.
Experiment: Test physical or genetic interactions between AlgG and AlgK, AlgX, AlgE, and AlgL in KT2440.
Hypothesis: AlgG participates in a periplasmic biosynthetic complex that channels alginate away from AlgL during export.
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