STM3697 encodes L-talarate/galactarate dehydratase (TalrD/GalrD), a magnesium-dependent acid-sugar dehydratase of the mandelate racemase subgroup of the enolase superfamily. It catalyses the elimination of water from either L-talarate (L-altrarate) or meso-galactarate to give 5-dehydro-4-deoxy-D-glucarate, using a single active site in which His328 is the L-talarate-specific base and Lys197 the galactarate-specific base, with Asp226, Glu252 and Glu278 coordinating the catalytic Mg(2+). The shared enolate intermediate also partitions into epimerization, interconverting L-talarate and galactarate. The protein is a homooctamer (tetramer of dimers) and is not active on other acid sugars. In Salmonella Typhimurium LT2 the enzyme is the sole route for growth on L-talarate as a carbon source and an accessory contributor to growth on galactarate, where the dedicated gar operon dehydratase GarD (STM3250) provides the principal route.
Definition: Catalysis of the reaction: L-altrarate (L-talarate) = galactarate. The reaction proceeds by abstraction of the C2 proton to give a metal-stabilised enolate intermediate, which is reprotonated on the opposite face of C2.
Justification: STM3697 catalyses this epimerization as a third, experimentally demonstrated activity distinct from its two dehydratase activities; UniProt curates it as a separate CATALYTIC ACTIVITY (Rhea:64172) with ECO:0000269 evidence from PMID:17649980, and the H328 and K197 mutants lose epimerization as well as dehydration. GO has no term for it: a search of OLS returns only GO:0016857 racemase and epimerase activity, acting on carbohydrates and derivatives, with no aldarate- or galactarate-specific child. Without the term the activity cannot be annotated at all, and the review has to record it in prose only.
Parent term: racemase and epimerase activity, acting on carbohydrates and derivatives
Mappings:
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000287 magnesium ion binding | IBA GO_REF:0000033 | ACCEPT | Summary: A catalytic Mg(2+) is directly established for STM3697 by crystallography and by the canonical mandelate-racemase-subgroup metal triad, so the phylogenetic inference is confirmed on the target itself. Reason: The IBD behind this row sits on an enolase-superfamily node whose members all share the Mg(2+)-stabilised enolate mechanism, so the node placement is sound; more importantly, the claim does not rest on the propagation at all. PMID:17649980 identifies Asp226, Glu252 and Glu278 as the ligands of the essential Mg(2+) at the ends of the third, fourth and fifth beta-strands, the wild-type and K197A structures (PDB 2PP0/2PP1/2PP3) were solved with magnesium bound, and UniProt curates one Mg(2+) per subunit with ECO:0000269 evidence from that paper. Magnesium is a cofactor of the core dehydration reaction rather than an unrelated metal-binding function, so this is retained as core. Supporting Evidence: PMID:17649980 Asp 226, Glu 252, and Glu 278 as ligands for the essential Mg2+ at the ends of the third, fourth, and fifth beta-strands file:SALTY/STM3697/STM3697-uniprot.txt Binds 1 Mg(2+) ion per subunit. |
| GO:0003824 catalytic activity | IEA GO_REF:0000117 | TOO GENERAL | Summary: The root catalytic-activity term is true but carries no information beyond the two specific dehydratase activities accepted elsewhere in this review. Reason: GO:0008867 galactarate dehydratase activity and GO:1990594 L-altrarate dehydratase activity are both accepted here on direct kinetic evidence, and each is a descendant of GO:0003824 via lyase / carbon-oxygen lyase / hydro-lyase activity, so this row adds nothing. The claim itself is not in doubt, which is why the action judges the term rather than the ARBA inference behind it. Supporting Evidence: PMID:17649980 it catalyzes the efficient dehydration of both l-talarate (kcat = 2.1 s-1, kcat/Km = 9.1 x 10(3) M-1 s-1) and galactarate (kcat = 3.5 s-1, kcat/Km = 1.1 x 10(4) M-1 s-1) |
| GO:0008867 galactarate dehydratase activity | EXP PMID:29248502 A continuous assay for l-talarate/galactarate dehydratase us... | ACCEPT | Summary: Independent re-measurement of galactarate dehydration by recombinant Salmonella enzyme in a second laboratory, by continuous circular dichroism, reproducing the original kinetics. Reason: Easton et al. assayed recombinant StTGD directly, following 5-KDG formation as the change in ellipticity at 323 nm, and obtained Km 0.38 mM, kcat 4.8 s-1 and kcat/Km 1.3 x 10^4 M-1 s-1, in agreement with the 2007 coupled assay. This is a second, methodologically independent demonstration of the exact reaction GO:0008867 defines (galactarate = 5-dehydro-4-deoxy-D-glucarate + H2O), on the Salmonella protein itself. Core molecular function. Supporting Evidence: PMID:29248502 the rates of StTGD-catalyzed conversion of m-galactarate to 5-KDG were determined by following the change in ellipticity at 323 nm PMID:29248502 These values are in excellent agreement with those published previously |
| GO:0008867 galactarate dehydratase activity | IDA PMID:17649980 Evolution of enzymatic activities in the enolase superfamily... | ACCEPT | Summary: CACAO row. The cited paper measures galactarate dehydration directly on purified STM3697 (kcat 3.5 s-1, kcat/Km 1.1 x 10^4 M-1 s-1) and identifies the galactarate-specific base; the term matches the assayed reaction exactly. Reason: What the paper shows for this gene, CACAO assessment. Yew et al. purified the STM3697 product, screened it against a physical library of acid sugars, and found efficient dehydration of galactarate alongside L-talarate; they then assigned Lys197 as the galactarate-specific base using the K197A structure with L-glucarate and solvent-deuterium incorporation into galactarate, and K197A abolishes dehydration of both substrates. The term GO:0008867 is defined as "galactarate = 5-dehydro-4-deoxy-D-glucarate + H2O", which is precisely the measured reaction, so the granularity is right and IDA is the appropriate code. The abstract calls these activities "promiscuous", but in this paper that word denotes dual substrate use within one active site, not an unvalidated screen hit: the kcat/Km values for the two substrates differ by less than 20 percent, the activity was reproduced independently by PMID:29248502, and the enzyme is not active on other acid sugars. The assertion is therefore not a promiscuous in vitro activity read as physiological function at the molecular-function level. The separate question of how much this activity contributes to galactarate catabolism in vivo is handled on the GO:0046392 row. The GONUTS wiki pages for PMID:17649980 and SALTY:TAGAD were fetched on 2026-10-29 and are offline ("temporarily offline while we perform upgrades"), so the student's recorded evidence text could not be inspected; the abstract and UniProt curation state the assay explicitly enough that no UNDECIDED is needed. Supporting Evidence: PMID:17649980 We discovered the function of STM3697 by screening a library of acid sugars PMID:17649980 we conclude that Lys 197 functions as the galactarate-specific base and His 328 functions as the l-talarate-specific base file:SALTY/STM3697/STM3697-uniprot.txt Catalyzes the efficient dehydration of both L-talarate (also called L-altrarate) and galactarate to 5-keto-4-deoxy-D-glucarate |
| GO:0008867 galactarate dehydratase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated mapping from the L-talarate dehydratase family signature, RHEA:16005 and EC 4.2.1.42; it converges on the same reaction the experimental rows establish. Reason: IPR033978 is the family-specific L-talarate dehydratase signature rather than a superfamily-wide one, and RHEA:16005 / EC 4.2.1.42 are exactly the reaction UniProt curates for this entry with ECO:0000269 evidence from PMID:17649980 and PMID:29248502. The electronic row therefore reinforces the IDA and EXP rows at the correct granularity and is kept as core. Supporting Evidence: PMID:17649980 it catalyzes the efficient dehydration of both l-talarate (kcat = 2.1 s-1, kcat/Km = 9.1 x 10(3) M-1 s-1) and galactarate (kcat = 3.5 s-1, kcat/Km = 1.1 x 10(4) M-1 s-1) |
| GO:0009063 amino acid catabolic process | IEA GO_REF:0000002 | REMOVE | Summary: Wrong process from a superfamily-wide InterPro conserved-site signature; this enzyme acts on aldarates, not amino acids. Reason: IPR018110 is the mandelate racemase / muconate lactonizing enzyme conserved site, the PROSITE PS00909 pattern that this entry matches and that is shared across the whole enolase superfamily, including mandelate racemase, muconate lactonizing enzymes and the acid-sugar dehydratases. Mapping that signature to amino acid catabolic process cannot be right for all its matches, and it is demonstrably wrong for STM3697: the substrates are the aldarates L-talarate and galactarate, the product is 5-dehydro-4-deoxy-D-glucarate, and UniProt records that the enzyme is not active on other acid sugars, let alone amino acids. No amino acid or amino acid derivative appears anywhere in the characterisation of this protein. This is a demonstrably wrong electronic mapping rather than a judgement about an experimental annotation, which is what REMOVE is for. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: FUNCTIONAL DIVERGENCE ROLE CONFLATION Sources checked: InterPro:IPR018110 SOURCE BAD Mandelate racemase/muconate lactonizing enzyme conserved site, a superfamily-wide PROSITE-pattern signature (PS00909) matched by enzymes acting on mandelate, muconate and acid sugars as well as on some amino acid derivatives. The InterPro2GO mapping of this signature to GO:0009063 is too broad for the signature's scope and does not hold for the acid-sugar dehydratase members such as STM3697. Supporting Evidence: file:SALTY/STM3697/STM3697-uniprot.txt Is not active on other acid sugars file:SALTY/STM3697/STM3697-uniprot.txt Belongs to the mandelate racemase/muconate lactonizing enzyme family. |
| GO:0016052 carbohydrate catabolic process | IBA GO_REF:0000033 | TOO GENERAL | Summary: True, and correctly placed phylogenetically, but entailed by the specific aldarate catabolic process terms kept in this review. Reason: The node placement is not in question: the target carries its own experimental annotations for the specific processes, and its appearance in the WITH/FROM list is the expected marker that the IBD was seeded in part on STM3697's own evidence, not a sign of circularity. The term, however, is a strict ancestor of GO:1903663 L-altrarate catabolic process (accepted here) and of GO:0046392 D-galactarate catabolic process (kept as non-core), both via aldaric acid catabolic process and galactarate metabolic process, so it restates what those rows already say. Recorded against the term, not the evidence. Supporting Evidence: PMID:17649980 we demonstrated that S. typhimurium LT2 can utilize l-talarate as carbon source. Insertional disruption of the gene encoding STM3697 abolishes this phenotype |
| GO:0016836 hydro-lyase activity | IBA GO_REF:0000033 | TOO GENERAL | Summary: Correct chemistry, but a strict ancestor of the two specific dehydratase activities accepted in this review. Reason: Hydro-lyase activity is the right family-level chemistry for an enolase-superfamily acid sugar dehydratase and the IBD node placement is sound, with the target's own experimental annotation among the descendant evidences. But GO:0008867 and GO:1990594 are both descendants of GO:0016836 and both are accepted here on direct kinetic evidence, so this row adds no information about which bond is cleaved in which substrate. This judges the term's informativeness, not the truth of the claim or the quality of the propagation. Supporting Evidence: PMID:17649980 it catalyzes the efficient dehydration of both l-talarate (kcat = 2.1 s-1, kcat/Km = 9.1 x 10(3) M-1 s-1) and galactarate (kcat = 3.5 s-1, kcat/Km = 1.1 x 10(4) M-1 s-1) |
| GO:0046392 D-galactarate catabolic process | IMP PMID:17649980 Evolution of enzymatic activities in the enolase superfamily... | KEEP AS NON CORE | Summary: CACAO row. Supported by the gene-disruption carbon-source phenotype, but the phenotype is explicitly partial and Salmonella carries a dedicated gar-operon galactarate dehydratase, so this is an accessory rather than a core route. Reason: What the paper shows for this gene, CACAO assessment. The insertional disruption of STM3697 "diminishes, but does not eliminate, the ability of the organism to utilize galactarate as carbon source". That is a genuine in vivo loss-of-function result for a process whose first committed step the enzyme performs, so the term is not too downstream (it is not a phenotype read as a process: the gene product itself catalyses galactarate dehydration, measured in the same paper) and it is at the right granularity. The term is kept. It is marked non-core because the residual growth has a definite explanation: S. Typhimurium LT2 also encodes the canonical gar operon, garR (STM3248), garL (STM3249, 5-keto-4-deoxy-D-glucarate aldolase) and garD (STM3250, galactarate dehydratase, L-threo-forming, EC 4.2.1.42), checked by UniProtKB REST query of taxon 99287. GarD is the dedicated galactarate dehydratase and provides the principal route; STM3697 contributes a partially redundant one, making the same product enantiomer (TalrD/GalrD gives 2-keto-L-threo-4,5-dihydroxyadipate, and UniProt names GarD L-threo-forming). The molecular function GO:0008867 is nevertheless accepted as core on this gene: the activity is fully established, and it is only its share of the organism's galactarate flux that is secondary. Minor ontology note, not a reason to change the action: galactarate is a meso compound (ChEBI:16537) and GO:0046392's label retains a "D-" prefix; this is a naming inheritance in GO, and GO:0046392 is the term used across GO for galactarate catabolism, including for GarD-type enzymes. GONUTS was offline when checked (2026-10-29), so the student's own evidence note could not be read. Supporting Evidence: PMID:17649980 this disruption also diminishes, but does not eliminate, the ability of the organism to utilize galactarate as carbon source file:SALTY/STM3697/STM3697-uniprot.txt Cells lacking this gene lose the ability to grow on L-talarate as carbon source, and are impaired in the ability to utilize galactarate as carbon source. |
| GO:1903663 L-altrarate catabolic process | IMP PMID:17649980 Evolution of enzymatic activities in the enolase superfamily... | ACCEPT | Summary: CACAO row, and the best-supported process annotation on this gene: insertional disruption abolishes growth on L-talarate (L-altrarate) as sole carbon source. Reason: What the paper shows for this gene, CACAO assessment. Yew et al. first had to establish that the growth substrate existed at all, since L-talarate was "a previously unknown metabolite"; they showed LT2 grows on it as a carbon source and that insertional disruption of STM3697 abolishes that phenotype. Complete loss, not partial, which makes STM3697 the sole known route of L-altrarate catabolism in this organism. The gene product performs the first step of the process itself (L-altrarate dehydration to 5-KDG, kcat 2.1 s-1), so this is participation and not merely necessity, and the term matches the process the mutant lost at exactly the right granularity, neither a downstream phenotype term nor a generic sugar-catabolism term. The CACAO student's choice of GO:1903663 over the broader GO:0016052 is the correct one. GONUTS was offline when checked (2026-10-29). Core biological process. Supporting Evidence: PMID:17649980 we demonstrated that S. typhimurium LT2 can utilize l-talarate as carbon source. Insertional disruption of the gene encoding STM3697 abolishes this phenotype file:SALTY/STM3697/STM3697-uniprot.txt Is required for the utilization of L-talarate as a carbon source. |
| GO:1990594 L-altrarate dehydratase activity | IDA PMID:17649980 Evolution of enzymatic activities in the enolase superfamily... | ACCEPT | Summary: CACAO row. Direct kinetic measurement of L-talarate (= L-altrarate) dehydration on purified STM3697; the term's defined reaction is the assayed one. Reason: What the paper shows for this gene, CACAO assessment. The acid-sugar library screen gave kcat 2.1 s-1 and kcat/Km 9.1 x 10^3 M-1 s-1 for L-talarate on purified enzyme, and the mechanism was worked out to the level of the specific base (His328, with H328N and H328A abolishing dehydration of both substrates). GO:1990594 is defined as "L-altrarate = 5-dehydro-4-deoxy-D-glucarate + H2O" and UniProt's own entry states that L-talarate is also called L-altrarate, so term and assay correspond exactly; the synonymy is worth stating because the GO label and the paper's substrate name differ. Unlike the galactarate activity, this one also has an unambiguous in vivo counterpart (growth abolished in the disruptant), so it is the core molecular function. GONUTS was offline when checked (2026-10-29). Supporting Evidence: PMID:17649980 it catalyzes the efficient dehydration of both l-talarate (kcat = 2.1 s-1, kcat/Km = 9.1 x 10(3) M-1 s-1) file:SALTY/STM3697/STM3697-uniprot.txt Catalyzes the efficient dehydration of both L-talarate (also called L-altrarate) and galactarate to 5-keto-4-deoxy-D-glucarate file:SALTY/STM3697/STM3697-uniprot.txt H->N,A: Loss of dehydration activity on both L-talarate and galactarate and loss of epimerization activity. |
| GO:1990594 L-altrarate dehydratase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Automated mapping from the family-specific L-talarate dehydratase signature and RHEA:44028, converging on the experimentally established activity. Reason: IPR033978 is the L-talarate dehydratase family signature, not a superfamily-wide site, and RHEA:44028 is the reaction UniProt curates with ECO:0000269 evidence from PMID:17649980 (with PhysiologicalDirection left-to-right). The electronic row therefore agrees with the IDA row at the correct granularity. Supporting Evidence: PMID:17649980 it catalyzes the efficient dehydration of both l-talarate (kcat = 2.1 s-1, kcat/Km = 9.1 x 10(3) M-1 s-1) |
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Download this section (compressed HTML)Q: Should the InterPro2GO mapping from IPR018110 (mandelate racemase / muconate lactonizing enzyme, conserved site) to GO:0009063 (amino acid catabolic process) be withdrawn or restricted? The signature is a superfamily-wide PROSITE pattern matched by enzymes acting on mandelate, muconate and acid sugars, and the resulting annotation is wrong for every acid-sugar dehydratase member, STM3697 included.
Q: Should GO represent the two regiochemically distinct galactarate dehydratase reactions separately? TalrD/GalrD and GarD dehydrate meso-galactarate from one end to give 2-keto-L-threo-4,5-dihydroxyadipate, whereas the Oceanobacillus iheyensis GalrD-II enzyme Ob2843 gives the enantiomeric D-threo product; GO:0008867 currently covers both, so the distinction established in PMID:19883118 is not expressible.
Suggested experts: John A. Gerlt
Q: Is STM3697 transcriptionally induced by L-talarate or galactarate, and which transporter delivers L-talarate to the cytoplasm? No regulator, promoter or uptake system has been reported for this gene, and it lies far from the gar operon (STM3248-STM3250) in the genome.
Q: Does the epimerization of L-talarate to galactarate have any physiological role, for example feeding L-talarate-derived carbon into the GarD route, or is it purely a consequence of partitioning of the shared enolate intermediate?
Suggested experts: Stephen L. Bearne
Experiment: Construct single and double deletions of STM3697 and garD (STM3250) in Salmonella Typhimurium LT2 and compare growth rates and final yields on minimal medium with meso-galactarate, L-talarate or glucose as sole carbon source, complementing each mutation in trans. This directly partitions the organism's galactarate flux between the dedicated gar route and the accessory TalrD/GalrD route, and so settles whether the GO:0046392 annotation on STM3697 should be read as core or accessory.
Hypothesis: The residual galactarate utilisation of a STM3697 disruptant is due to GarD (STM3250), so a STM3697 garD double mutant should be unable to grow on galactarate at all.
Type: bacterial genetics and growth phenotyping
Experiment: Search for L-talarate by LC-MS/MS against an authentic standard in candidate sources (mouse caecal contents, plant cell wall hydrolysates, soil extracts), and test whether a STM3697 mutant is outcompeted by wild type in those matrices in a tagged competitive-fitness assay. A positive result would convert the L-talarate dehydratase activity from a demonstrated laboratory function to a selected physiological one.
Hypothesis: L-talarate is a real metabolite of a niche Salmonella occupies.
Type: metabolomics and competitive fitness assay
Experiment: Build a mutant series around the His328-Asp301 dyad and the Lys197-x-Lys motif that preferentially impairs departure of the 3-OH group while retaining alpha-proton abstraction, using the continuous circular dichroism assay of PMID:29248502 for dehydration and NMR or chiral chromatography for epimerization, then express such variants in a STM3697 garD background to ask whether epimerase activity alone supports any growth on L-talarate.
Hypothesis: Epimerization and dehydration can be separated at the level of single active-site residues, allowing the epimerase activity to be tested in isolation.
Type: site-directed mutagenesis with coupled enzymology and genetics
Experiment: Measure STM3697 transcript and protein abundance by RT-qPCR and targeted proteomics in LT2 grown on glucose, galactarate and L-talarate, and screen a transposon library for mutants that fail to induce a STM3697 promoter-reporter fusion on L-talarate, to identify the regulator and any cognate transporter.
Hypothesis: STM3697 expression is substrate-inducible.
Type: expression profiling and transposon screen
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The biological source of L-talarate is unknown: no pathway, host compound or environmental polysaccharide has been shown to yield it, so it is not known under what circumstances Salmonella encounters the substrate its only dedicated catabolic enzyme is specific for.
BIOLOGY
What is known: What is established is that the enzyme dehydrates L-talarate efficiently, that LT2 can grow on purified L-talarate as sole carbon source, and that STM3697 is required for that growth. What is missing is any demonstration that L-talarate occurs in a niche Salmonella inhabits.
Significance: Whether the L-talarate activity is the enzyme's selected function or an incidental consequence of an active site evolved for galactarate turns on whether the substrate exists in vivo; the enolase superfamily is a standard test case for how promiscuous activities become physiological ones, and this gene is one of the few where the in vivo question has been posed at all.
What would resolve it: Metabolomic detection of L-talarate (or of a hydrolysable precursor) in gut contents, plant or soil material, together with competitive-fitness assays of a STM3697 mutant in those niches.
Provenance (the field's own admissions):
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