Generated using OpenAI Deep Research API
The hglS gene of P. putida KT2440 encodes a subunit of D-2-hydroxyglutarate dehydrogenase (D2HGDH), an enzyme that oxidizes D-2-hydroxyglutarate to 2-oxoglutarate (α-ketoglutarate) (pmc.ncbi.nlm.nih.gov). Biochemical studies in P. putida showed this reaction is inducible and coupled to the electron transport chain, with electrons passed to oxygen via membrane cytochromes (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). The enzyme is a flavoprotein oxidoreductase: the large subunit contains a flavin adenine dinucleotide (FAD) cofactor and a catalytic site, while HglS is a small electron-transfer subunit. In P. stutzeri, a homologous D2HGDH requires a separate flavoprotein or cytochrome to shuttle electrons to the respiratory chain (pmc.ncbi.nlm.nih.gov). The P. aeruginosa enzyme is a Zn²⁺-binding metallo-flavoprotein, indicating a metal cofactor for substrate binding/orientation (pmc.ncbi.nlm.nih.gov). Mechanistically, HglS and its partner subunit catalyze dehydrogenation of D-2-hydroxyglutarate (a secondary alcohol) to 2-oxoglutarate, with quinones or cytochromes as electron acceptors (pmc.ncbi.nlm.nih.gov). This restores 2-oxoglutarate (an important TCA-cycle intermediate) and frees reduced electron carriers into the respiratory chain.
Evidence suggests HglS is associated with the inner membrane/periplasmic space. Early work demonstrated the D-2-hydroxyglutarate oxidoreductase is membrane-bound in P. putida, localized to the particulate (membrane) fraction (pmc.ncbi.nlm.nih.gov). The enzyme likely faces the periplasm: D-2-HG dehydrogenases in pseudomonads often act in the periplasm with electrons fed into the electron transport chain. Consistently, P. stutzeri D2HGDH uses a soluble carrier to connect with the respiratory chain (pmc.ncbi.nlm.nih.gov), implying a periplasmic enzyme transferring electrons inward. HglS likely contains a signal peptide for periplasmic export and possibly a heme cofactor if it functions as a c-type cytochrome subunit. Thus, the HglS-containing D2HGDH enzyme is periplasmic or inner-membrane-associated, with HglS aiding electron transfer to membrane carriers (quinones/cytochromes) (pmc.ncbi.nlm.nih.gov). This subcellular localization allows D-2-hydroxyglutarate present in the periplasm (from metabolism or import) to be oxidized efficiently and funneled into central metabolism.
HglS is integral to D-2-hydroxyglutarate catabolism and related metabolic pathways. D-2-HG is not a primary nutrient but arises as a by-product of other pathways (e.g. serine and methylarginine metabolism) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In Pseudomonas, D2HGDH (with HglS) links this by-product back to central carbon metabolism by regenerating 2-oxoglutarate (pmc.ncbi.nlm.nih.gov). This link is crucial in L-serine biosynthesis: as shown in P. stutzeri, the serine-pathway enzyme SerA produces D-2-HG to pull a thermodynamically unfavorable step, and D2HGDH then reconverts D-2-HG to 2-oxoglutarate (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Thus, HglS participates in maintaining serine biosynthesis flux. Additionally, D2HGDH enables bacteria to utilize D-malate as a carbon source (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). P. stutzeri mutants lacking D2HGDH cannot grow on D-malate (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), indicating the enzyme’s broad role in D-dicarboxylic acid metabolism. By oxidizing D-malate (a structural analog of D-2-HG) to oxaloacetate, the enzyme (and HglS) expands the substrate range for growth (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In P. putida, HglS likely plays similar roles: enabling the bacterium to catabolize unusual D-isomers of organic acids and integrate them into the TCA cycle. Overall, hglS-driven activity supports metabolic versatility, connecting peripheral catabolic routes (D-2-HG, D-malate) with core metabolism (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This function is an adaptive advantage in nutrient-diverse environments.
There are no direct disease associations known for hglS in P. putida. This gene is part of a metabolic pathway for nutrient utilization rather than a virulence factor. P. putida KT2440 is an environmental, non-pathogenic strain widely used in biotechnology (enviromicro-journals.onlinelibrary.wiley.com) (pmc.ncbi.nlm.nih.gov). Unlike the human D2HGDH (where mutations cause D-2-hydroxyglutaric aciduria), the bacterial HglS has not been implicated in human disease. However, the enzyme’s activity reflects metabolic traits that could influence fitness in certain niches. For example, the ability to break down D-malate or unusual metabolites could affect P. putida’s survival in plant rhizospheres or soil (but not pathogenicity) (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Phenotypically, an hglS (D2HGDH) mutant would lose the ability to use D-2-HG and D-malate. Such mutants would likely accumulate D-2-HG internally and might show slower growth when serine synthesis is required, as seen in P. stutzeri where D2HGDH loss impairs serine pathway efficiency (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). They would also fail to grow on D-malate as sole carbon source (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, hglS is not associated with disease, but it contributes to metabolic flexibility and environmental fitness.
The HglS protein is characterized as a small electron-transferring subunit of the D2HGDH enzyme complex. Small subunits in similar dehydrogenases often contain c-type cytochrome domains or other cofactor-binding motifs for electron relay. HglS is much smaller than the catalytic subunit (~15–20 kDa vs ~60 kDa for the large subunit) and likely binds a heme cofactor (based on analogy to other periplasmic dehydrogenases). This would enable HglS to accept electrons from the FAD in the large subunit and pass them to the cytochrome chain. The large D2HGDH subunit (not HglS) contains an FAD-binding domain (a Rossmann-fold) and a catalytic site that binds D-2-HG along with a Zn²⁺ cofactor (pmc.ncbi.nlm.nih.gov). In P. aeruginosa D2HGDH, Zn²⁺ is essential for orienting the substrate in the active site (pmc.ncbi.nlm.nih.gov), suggesting that region is conserved in P. putida’s enzyme. HglS itself likely has a heme c binding CXXCH motif if it is a c-type cytochrome, or binds a flavin if it’s a flavoprotein carrier (pmc.ncbi.nlm.nih.gov). Structurally, it would be mostly α-helical (typical of c-type cytochromes) with a covalently attached heme. The assembled enzyme is a membrane-associated flavocytochrome complex. Early studies noted the D-2-HG oxidoreductase behaved as a soluble
flavocytochrome after detergent treatment (pmc.ncbi.nlm.nih.gov), consistent with a two-component enzyme (flavin enzyme + cytochrome). Therefore, HglS’s main structural features include a cofactor-binding site for electron transfer (heme or flavin) and possibly a signal peptide anchoring it to the periplasmic side of the membrane. Together with the large subunit, it forms a functional holoenzyme complex.
Expression of hglS is substrate-inducible and tied to specific growth conditions. In P. putida, the enzyme was reported to be induced when D-α-hydroxyglutarate or related substrates are available (pmc.ncbi.nlm.nih.gov). This suggests transcription of hglS (and the D2HGDH operon) is up-regulated by the presence of D-2-HG or D-malate in the medium. Indeed, in P. stutzeri, the D2HGDH protein is strongly induced by adding D-2-HG or D-malate (pmc.ncbi.nlm.nih.gov). This indicates a likely responsive regulator controlling hglS. The regulator could be a LysR-family or another sensor that detects D-2-HG/D-malate and activates the operon (though the specific regulator is not yet characterized in KT2440). Additionally, hglS expression connects to serine biosynthesis: under conditions of active L-serine production (which generates D-2-HG byproduct), the D2HGDH pathway is engaged (pmc.ncbi.nlm.nih.gov). Thus, hglS may see higher expression during growth on amino acids or when serine pathway flux is high. Global regulators might influence hglS as well – for example, stationary-phase or carbon-catabolite repression could modulate its expression given it’s a secondary metabolism gene. No direct data on hglS transcription factors in KT2440 are published, but the pattern of induction by specific dicarboxylates is clear from experimental analogs (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, hglS is expressed when its substrate or analogs are present, ensuring the D-2-HG detoxification/utilization system is only active when needed. This regulated expression conserves energy and coordinates with related metabolic pathways (serine biosynthesis, D-malate uptake).
The hglS gene and its associated D2HGDH function are conserved across diverse bacteria, especially within the Pseudomonas genus. Homologs of hglS exist in P. stutzeri, P. aeruginosa, and other pseudomonads that possess D-2-hydroxyglutarate dehydrogenases. P. aeruginosa PAO1 has a D2HGDH enzyme with ~70% amino acid identity to the P. putida one (based on cross-species comparisons) – indicating strong conservation of both large and small subunits. This PAO1 enzyme (gene PA5332, sometimes called d2hgdH) performs the same reaction (pmc.ncbi.nlm.nih.gov). P. stutzeri A1501 also contains a homologous operon, and functional studies confirmed a similar role for its D2HGDH (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). Beyond pseudomonads, D-2-HG dehydrogenases (and likely small subunits akin to HglS) are found in other bacteria that degrade amino acids via 2-hydroxyacids – for example, Ralstonia or Azotobacter species catabolizing lysine/pipecolate have analogous enzymes (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This suggests an evolutionarily conserved strategy: many soil and plant-associated bacteria evolved D-2-HG dehydrogenases to channel unusual D-metabolites into the TCA cycle. Even organisms as different as E. coli have enzymes for D- and L-2-HG (though E. coli’s are cytosolic and unrelated in sequence) (pmc.ncbi.nlm.nih.gov). In eukaryotes, the D2HGDH enzyme in mitochondria is evolutionarily related, indicating a distant common origin for this metabolic function. The conservation of hglS within pseudomonads implies it confers adaptive advantage in environments rich in amino acids and D-isomers. Phylogenetically, hglS clusters with other Proteobacterial small dehydrogenase subunits, often adjacent to their large-subunit genes in the genome. The P. putida KT2440 genome context of hglS is within a putative operon for D-2-HG utilization (neighboring genes likely encode the large dehydrogenase subunit and possibly a transporter). This clustering is conserved in related species, underscoring that hglS and its operon descended from a common ancestral gene set through vertical inheritance. There is little evidence of horizontal gene transfer for hglS; instead, its broad presence in pseudomonads suggests it was present in their common ancestor and retained due to its utility in diverse niches (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov).
Multiple lines of research underpin our understanding of hglS and D2HGDH in P. putida and related bacteria. Biochemical characterization dates back to Reitz & Rodwell (1969), who purified “α-hydroxyglutarate oxidoreductase” from P. putida and showed its membrane-bound nature and specificity for D-α-hydroxyglutarate (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). This classic work established the enzyme’s function and induction by substrate. More recently, genetic and physiological studies in Pseudomonas stutzeri provided direct evidence of the enzyme’s role in metabolism. Guo et al. (2018) created D2HGDH knock-out mutants and demonstrated loss of D-malate utilization and perturbed serine biosynthesis (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). They also measured induction of the gene by D-2-HG/D-malate and identified that SerA and D2HGDH form a metabolic loop in serine production (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In P. aeruginosa, structural enzymology by Quaye et al. (2023) examined the D2HGDH protein. They found it to be a flavoprotein that absolutely requires Zn²⁺ and acts as a homodimer (with anticipated involvement of a small subunit for electron transfer) (pmc.ncbi.nlm.nih.gov). This study used purified enzyme, calorimetry, and kinetics to elucidate how substrate binds and how the enzyme stabilizes the transition state (pmc.ncbi.nlm.nih.gov). Together, these studies paint a comprehensive picture: from P. putida KT2440 (genomic context and annotation) to functional assays in other Pseudomonas and detailed enzyme mechanisms. Additional support comes from comparative genomics and metabolic models of KT2440, which note the presence of a D-2-HG dehydrogenase pathway contributing to the strain’s broad catabolic repertoire (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov). In summary, key evidence includes: the 1960s enzyme isolation confirming hglS-associated activity (pmc.ncbi.nlm.nih.gov), modern mutant phenotypes linking hglS to serine and D-malate metabolism (pmc.ncbi.nlm.nih.gov) (pmc.ncbi.nlm.nih.gov), and biochemical analyses confirming the enzyme’s cofactor requirements and mechanism (pmc.ncbi.nlm.nih.gov). All these findings support Gene Ontology annotations that hglS is involved in D-2-hydroxyglutarate metabolic process, has D-2-hydroxyglutarate dehydrogenase activity, and is located in the periplasmic space/inner membrane as part of a flavoprotein-cytochrome enzyme complex.