Gene: algC · Ordered locus: PP_5288 · UniProt: Q88C93
Organism: Pseudomonas putida strain KT2440 (ATCC 47054 / DSM 6125 / NCIMB 11950)
Enzyme: Phosphomannomutase / phosphoglucomutase (PMM/PGM) · EC 5.4.2.8 / EC 5.4.2.2
Superfamily: α‑D‑phosphohexomutase (phosphohexose mutase family)
AlgC of Pseudomonas putida KT2440 is a soluble, cytoplasmic, bifunctional phosphomannomutase/phosphoglucomutase (PMM/PGM) of the α‑D‑phosphohexomutase superfamily. Its primary biochemical function is the reversible, Mg²⁺‑dependent intramolecular transfer of a phosphoryl group across a hexose sugar, interconverting the 6‑phosphate and 1‑phosphate forms of two substrates: glucose‑6‑phosphate ⇌ glucose‑1‑phosphate (PGM activity, EC 5.4.2.2) and mannose‑6‑phosphate ⇌ mannose‑1‑phosphate (PMM activity, EC 5.4.2.8). A single active‑site "hot spot" accommodates both the glucose and the mannose phosphosugars, which is the structural basis of the enzyme's dual specificity. This identity is directly consistent with the UniProt annotation for Q88C93 and is corroborated by direct enzymatic assays of AlgC orthologs across multiple gammaproteobacteria and other lineages.
The reaction proceeds by a distinctive, well‑characterized processive mechanism. A conserved catalytic serine (Ser108 in the closely studied P. aeruginosa ortholog) exists as a phosphoserine and serves as both the phosphoryl donor and acceptor. Catalysis requires two sequential phosphoryl transfers separated by a 180° reorientation of a glucose‑1,6‑bisphosphate (or mannose‑1,6‑bisphosphate) intermediate that remains bound in the active site — a textbook example of enzymatic processivity. A conserved histidine (His329) acts as the general base that deprotonates the sugar hydroxyl, and a mobile C‑terminal domain (domain 4) closes over the deep catalytic cleft to complete an efficient active site. Dephosphorylation of the catalytic serine during the cycle increases enzyme flexibility, which is thought to facilitate the intermediate's reorientation.
Biologically, AlgC is a central upstream supplier of activated‑sugar (nucleotide‑sugar) precursors. Its 1‑phosphate products are converted to GDP‑mannose (from mannose‑1‑phosphate), UDP‑glucose (from glucose‑1‑phosphate), and dTDP‑L‑rhamnose, feeding the biosynthesis of alginate, the lipopolysaccharide (LPS) core/O‑antigen, and rhamnolipid. Because it sits at this metabolic branch point, loss of AlgC simultaneously abolishes several glycoconjugate pathways. In the soil bacterium P. putida KT2440 specifically — which, unlike P. aeruginosa, encodes a stand‑alone cytoplasmic enzyme without an N‑terminal periplasmic sensor domain — AlgC's ecologically most prominent role is provisioning alginate, an exopolysaccharide induced by matric (water‑limitation) stress that creates a hydrated biofilm microenvironment and confers desiccation tolerance.
Before presenting findings, the mandatory identity checks were confirmed:
| Check | Result |
|---|---|
| Gene symbol algC matches protein description | Yes — algC is the canonical name for bacterial PMM/PGM |
| Organism correct (P. putida KT2440) | Yes — locus PP_5288, UniProt Q88C93 |
| Protein family/domains align with literature | Yes — α‑D‑phosphohexomutase superfamily (IPR005843–46, IPR016055) |
| Literature refers to the same enzyme | Yes — extensive, directly relevant primary literature |
The gene symbol is not ambiguous in this case. AlgC is well characterized, and although the deepest mechanistic and structural work was performed on the P. aeruginosa ortholog, that protein is the direct functional homolog of P. putida AlgC (same superfamily, same reaction, conserved active‑site residues). Where P. putida‑specific data exist (localization/domain architecture, alginate physiology), they are cited explicitly.
UniProt Q88C93 annotates PP_5288/algC as a phosphomannomutase/phosphoglucomutase carrying two EC activities, EC 5.4.2.8 (PMM) and EC 5.4.2.2 (PGM), and assigns it to the phosphohexose mutase family with the four signature α‑D‑phosphohexomutase domains (IPR005844/45/46, IPR005843, IPR016055). This annotation is not merely computational: multiple orthologs are experimentally confirmed to be bifunctional. In P. aeruginosa, "the algC gene … is involved in alginate production through its phosphomannomutase activity and in LPS synthesis through its phosphoglucomutase activity" (PMID: 10481091). The Stenotrophomonas maltophilia ortholog SpgM "was shown to encode a bifunctional enzyme with both PGM and phosphomannomutase activities" (PMID: 12761084). Further homologs — Sphingomonas paucimobilis PgmG (PMID: 10788412) and Prochlorothrix hollandica PmmA, which is 37% identical to AlgC and possesses "both PGM and PMM activities as judged by both enzyme assays and complementation analysis" (PMID: 8765122) — confirm that bifunctionality is a conserved family trait. Evolutionary‑trace analysis of 71 superfamily members demonstrated that "key residues in the active site, including many of those involved in substrate contacts … are conserved throughout the enzyme family" (PMID: 15238632), placing AlgC firmly within a superfamily that shares a conserved catalytic apparatus.
The catalytic mechanism has been resolved in structural and kinetic detail in the P. aeruginosa ortholog. "The reaction entails two phosphoryl transfers, with an intervening 180° reorientation of the reaction intermediate (e.g. glucose 1,6‑bisphosphate) during catalysis" (PMID: 16595672). Critically, this reorientation happens "without dissociation from the active site of the enzyme and is, thus, a simple example of processivity" (PMID: 16595672). Transient‑state kinetic studies confirmed that "glucose 1,6‑bisphosphate is formed as an intermediate in the reaction" and behaves as an obligatory enzyme‑bound species, partitioning forward to product roughly 14‑fold more often than it dissociates (PMID: 15865428). Site‑directed mutagenesis identified active‑site residues that are critical for retaining the bisphosphate during its reorientation (PMID: 16595672). This mechanism is why the enzyme is often used as a model of "simple processivity."
AlgC sits at the top of several glycoconjugate biosynthetic pathways. Its PMM activity (mannose‑6‑P → mannose‑1‑P) feeds GDP‑mannose synthesis for alginate, while its PGM activity (glucose‑6‑P → glucose‑1‑P) feeds UDP‑glucose for the LPS core and dTDP‑L‑rhamnose for rhamnolipid. In P. aeruginosa, "the AlgC protein plays a central role in the production of the three … virulence‑associated saccharides: alginate, LPS and rhamnolipid" (PMID: 10481091). Genetic evidence shows that "the P. aeruginosa algC gene is required for biosynthesis of alginate and lipopolysaccharide" (PMID: 7558335). Substrate specificity of an AlgC ortholog was quantified in Sphingomonas PgmG: "the catalytic efficiency was about 50‑fold higher for G1P than it was for mannose‑1‑phosphate (M1P). The estimated apparent Kₘ values for G1P and M1P were … 0.33 and 1.27 mM" (PMID: 10788412), indicating a kinetic preference for the glucose substrate. In P. putida KT2440, "alginate, an exopolysaccharide (EPS) produced by P. putida, is known to create hydrated environments and alleviate the effect of water limitation" (PMID: 24912454) — a downstream AlgC‑dependent product — while alginate and other EPS also stabilize KT2440 biofilms (PMID: 21507178).
| AlgC product | Activated to | Feeds pathway | Cellular product |
|---|---|---|---|
| Mannose‑1‑phosphate | GDP‑mannose | Alginate biosynthesis | Alginate EPS |
| Glucose‑1‑phosphate | UDP‑glucose | LPS core assembly | Complete LPS core / O‑antigen |
| Glucose‑1‑phosphate | dTDP‑L‑rhamnose | Rhamnolipid biosynthesis | Rhamnolipid (biosurfactant) |
A key organism‑specific distinction: some gammaproteobacterial AlgC‑type PMMs carry a ~200‑amino‑acid N‑terminal periplasmic dCache sensor domain anchored by two transmembrane segments. Qian, Fei & Galperin found that these "previously overlooked N‑terminal periplasmic sensor domains were detected in the well‑characterized PMMs of Pseudomonas aeruginosa and Xanthomonas campestris, albeit not in the enzymes from Pseudomonas fluorescens, Pseudomonas putida or Azotobacter vinelandii" (PMID: 30938049). This directly establishes that P. putida AlgC is the stand‑alone, soluble PMM/PGM enzymatic module — a cytoplasmic protein — without the membrane anchor or extracytoplasmic sensor found in some relatives. Consequently, while mechanistic inferences from the P. aeruginosa ortholog's catalytic core transfer directly to P. putida AlgC, the regulatory/sensory features associated with the membrane‑bound form (e.g., the requirement of the membrane‑bound form for twitching motility in Lysobacter enzymogenes) do not apply to the KT2440 enzyme.
The most direct experimental proof that a single AlgC polypeptide carries both activities comes from Coyne et al. Genetically defined P. aeruginosa algC mutants "had no detectable phosphomannomutase activity and … neither algC strain had detectable phosphoglucomutase (PGM) activity" (PMID: 7515870), and the cloned intact algC gene complemented an E. coli pgm mutant — showing that one enzyme provides both activities. Functionally, the "algC mutants of a serotype O5 strain (PAO1) and a serotype O3 strain (PAC1R) did not express lipopolysaccharide (LPS) O side chains or the A‑band (common antigen) polysaccharide," and their LPS migrated like glucose‑deficient rough mutants (PMID: 7515870), demonstrating that "the synthesis of glucose 1‑phosphate is necessary in the biosynthesis of the P. aeruginosa LPS core" (PMID: 7515870). This is direct enzymatic + genetic‑complementation evidence, superseding pure sequence inference.
The ecological role of AlgC's output in P. putida has been defined physiologically. Chang et al. showed that "total exopolysaccharide (EPS) and alginate production increased with increasing matric, but not solute, stress severity" (PMID: 17601783), and that "alginate deficiency decreased survival of desiccation not only by P. putida but also by Pseudomonas aeruginosa PAO1 and Pseudomonas syringae pv. syringae B728a" (PMID: 17601783). Independent matric‑stress‑controlled gene screens in P. putida recovered alginate‑biosynthesis and cell‑envelope genes as contributors to desiccation tolerance (PMID: 15101980), and alginate acts as a biofilm structural stabilizer alongside other KT2440 EPS systems (PMID: 21507178). Because alginate synthesis depends on GDP‑mannose derived from the mannose‑1‑phosphate that AlgC produces, AlgC is the essential upstream metabolic gateway to this protective response.
Structural and mutational studies of the P. aeruginosa ortholog complete the atomic picture. The conserved catalytic serine acts as both phosphoryl donor and acceptor: "the S108C substitution of the phosphoryl donor and acceptor slowed transformation of the glucose 1‑phosphate substrate by impairing kcat" (PMID: 22242625). The serine's phosphorylation state governs enzyme compactness and flexibility — dephosphorylation makes the enzyme "less compact in solution," and increased flexibility is proposed to facilitate reorientation of the reaction intermediate (PMID: 24403075). A histidine general base was identified: "a histidine (His329) in the active site is critical for enzyme activity in a well‑studied member of the superfamily, phosphomannomutase/phosphoglucomutase from Pseudomonas aeruginosa" (PMID: 23517223), positioned to abstract a proton from the O1/O6 hydroxyl of the phosphosugar (a structurally analogous lysine performs this role in the PGM subgroup). The enzyme's four‑domain architecture includes a mobile C‑terminal domain 4 that closes over the catalytic cleft and is required for full catalytic efficiency (PMID: 23893395, PMID: 20512975, PMID: 20589904). A single active‑site hot spot underlies bifunctionality: "one of the most important hot spots is in the active site, consistent with the ability of the enzyme to bind both glucose and mannose phosphosugar substrates" (PMID: 20589904).
Substrate: glucose-6-P (or mannose-6-P)
│
▼
┌─────────────────────────────────────────────────────────┐
│ Enzyme-Ser108-phosphate (phosphoenzyme, Mg2+ bound) │
│ │
│ STEP 1: phosphoryl transfer from P-Ser108 to sugar 1-OH │
│ → glucose-1,6-bisphosphate (bound intermediate)│
│ Enzyme now dephosphorylated → more flexible │
│ │
│ STEP 2: 180° REORIENTATION of the bisphosphate │
│ intermediate WITHIN the active site (processive)│
│ His329 general base positions/deprotonates OH │
│ │
│ STEP 3: phosphoryl transfer from sugar 6-P back to Ser108│
│ → regenerates P-Ser108 phosphoenzyme │
└─────────────────────────────────────────────────────────┘
│
▼
Product: glucose-1-P (or mannose-1-P)
(C-terminal domain 4 closes over cleft during turnover)
The elegance of this system is that a single catalytic serine performs two phosphoryl transfers on opposite ends of the sugar, requiring the bisphosphate intermediate to flip 180° between steps. The enzyme holds the intermediate throughout (processivity), and modulates its own conformational flexibility through the phosphorylation state of Ser108 to permit the reorientation. His329 provides the general‑base chemistry; a Mg²⁺ ion coordinates the phosphoryl groups.
Fructose-6-P
/ \
(PMI) (glycolysis)
│
Mannose-6-P Glucose-6-P
│ ⇅ AlgC (PMM) │ ⇅ AlgC (PGM)
Mannose-1-P Glucose-1-P
│ / \
GDP-mannose UDP-glucose dTDP-L-rhamnose
│ │ │
ALGINATE LPS CORE / RHAMNOLIPID
(desiccation, O-ANTIGEN (biosurfactant)
biofilm EPS)
AlgC is the shared node feeding all of these. This explains why algC loss is pleiotropic (alginate⁻, LPS⁻, rhamnolipid⁻ simultaneously) yet the primary molecular function is singular and precise: a sugar‑phosphate mutase. In P. putida KT2440, the branch of greatest ecological consequence is the alginate branch, because alginate production is triggered by water/matric stress and protects the soil bacterium against desiccation.
All evidence points to a cytoplasmic site of action: the substrates (hexose‑6/1‑phosphates) and downstream nucleotide‑sugar synthesis are cytoplasmic processes, and the P. putida enzyme specifically lacks the membrane anchor / periplasmic sensor domain present in some orthologs (PMID: 30938049). AlgC therefore performs its catalytic function in the cytosol, upstream of the membrane‑associated polymerization/export machineries that build alginate, LPS, and rhamnolipid.
| PMID | Study focus | How it supports the findings |
|---|---|---|
| 10481091 | P. aeruginosa AlgC in rhamnolipid biosynthesis | Establishes bifunctional PMM/PGM roles and central supply of alginate/LPS/rhamnolipid precursors (F001, F003) |
| 7515870 | algC encodes PGM; LPS core | Direct enzymatic proof of both activities; glucose‑1‑P required for LPS core (F005) |
| 7558335 | Avirulence of algC mutant | Genetic requirement of AlgC for alginate + LPS (F003) |
| 12761084 | S. maltophilia SpgM | Ortholog confirmed bifunctional; LPS/virulence role (F001) |
| 10788412 | Sphingomonas PgmG | Ortholog bifunctional; quantifies G1P vs M1P specificity (~50× higher for G1P) (F001, F003) |
| 8765122 | Prochlorothrix PmmA | 37% identical to AlgC, dual PMM/PGM activity confirmed (F001) |
| 15238632 | Evolutionary trace of superfamily | Conserved active‑site residues across 71 members (F001) |
| 16595672 | Processive mechanism | Two phosphoryl transfers + 180° intermediate reorientation (F002) |
| 15865428 | Transient‑state kinetics | Glucose‑1,6‑bisphosphate is an obligatory bound intermediate (F002) |
| 22242625 | NMR / phosphoryl transfer | Ser108 is the phosphoryl donor/acceptor; S108C impairs kcat (F007) |
| 24403075 | HDX‑MS / SAXS flexibility | Dephosphorylation increases flexibility to aid reorientation (F007) |
| 23517223 | Essential active‑site residue | His329 identified as critical general base (F007) |
| 20589904 | Domain motion / hot spots | Single active site binds both glucose & mannose sugars (F007) |
| 23893395 | Domain 4 NMR | C‑terminal domain closes over active site; needed for efficiency (F007) |
| 20512975 | Fragment complementation | Chain connectivity of domain 4 optimizes catalysis (F007) |
| 30938049 | Two forms of PMM | P. putida AlgC lacks periplasmic sensor → stand‑alone cytoplasmic enzyme (F004) |
| 17601783 | Alginate & water limitation | Matric‑stress induction; desiccation protection in P. putida (F006) |
| 15101980 | Cell envelope / desiccation | Alginate/envelope genes contribute to desiccation tolerance (F006) |
| 21507178 | KT2440 EPS biofilm | Alginate/EPS act as biofilm structural stabilizers (F003, F006) |
| 24912454 | KT2440 colony/transcriptome | Alginate creates hydrated environment under water limitation (F003) |
| 25186153 | ErsA sRNA regulation of algC | algC is post‑transcriptionally regulated (context for expression control) |
Note on organism scope: The deep structural/mechanistic evidence (F002, F007) derives from the P. aeruginosa PMM/PGM. Because that enzyme is the direct functional ortholog of P. putida AlgC — same superfamily, same reaction, conserved active site (PMID: 15238632) — these mechanistic conclusions transfer with high confidence to the KT2440 enzyme. Organism‑specific claims about localization/domain architecture (F004) and alginate physiology (F006) are supported by direct P. putida data.
Absence of direct KT2440 enzyme kinetics. No crystal structure or purified‑enzyme kinetic characterization exists specifically for P. putida KT2440 AlgC (Q88C93). The mechanism, catalytic residues (Ser108, His329), and processivity are inferred from the highly conserved P. aeruginosa ortholog and other family members. Residue numbering (Ser108, His329) is that of the P. aeruginosa protein; the equivalent positions in Q88C93 should be confirmed by alignment.
Substrate‑specificity ratios are from orthologs. The ~50‑fold kinetic preference for glucose‑1‑phosphate over mannose‑1‑phosphate is measured in Sphingomonas PgmG (PMID: 10788412); the exact PMM:PGM activity ratio for KT2440 AlgC is not experimentally established and may differ.
Localization is inferred. Cytoplasmic localization is strongly supported by domain architecture and substrate chemistry but has not been directly demonstrated (e.g., by fractionation) for KT2440 AlgC.
Regulation in P. putida is less defined. The ErsA/σ²² post‑transcriptional regulatory circuit (PMID: 25186153) was characterized in P. aeruginosa; whether an equivalent circuit operates in KT2440 is unknown.
Relative importance of the three downstream pathways in KT2440 (alginate vs LPS vs rhamnolipid) has not been dissected with an algC‑specific mutant series in this strain; the alginate/desiccation link is the best‑supported.
Purify and assay KT2440 AlgC (Q88C93). Express the recombinant protein and directly measure PMM and PGM activities, Kₘ/kcat for glucose‑1‑P and mannose‑1‑P, Mg²⁺ dependence, and the PMM:PGM ratio — closing the largest gap (organism‑specific kinetics).
Structural determination or confident homology model. Solve a crystal/cryo‑EM structure of KT2440 AlgC, or build an AlphaFold model, and map the catalytic serine and histidine to their Q88C93 residue numbers; validate the four‑domain architecture and the mobile C‑terminal domain.
Targeted active‑site mutagenesis in KT2440. Alanine/cysteine substitutions at the predicted catalytic Ser and His to confirm their roles in the native enzyme via loss of PMM/PGM activity.
Clean algC deletion + complementation in KT2440. Quantify effects on alginate, LPS core/O‑antigen, and rhamnolipid, and on desiccation/matric‑stress survival, to establish the relative contribution of each downstream branch in this strain.
Localization assay. Cell fractionation or fluorescent fusion to confirm cytoplasmic localization and the absence of membrane association predicted from the domain architecture.
Regulatory circuit mapping. Test whether a σ²²/sRNA‑type post‑transcriptional control (analogous to ErsA in P. aeruginosa) modulates algC/PP_5288 under envelope or water stress in KT2440.
algC (PP_5288, Q88C93) encodes AlgC, a soluble cytoplasmic bifunctional phosphomannomutase/phosphoglucomutase (EC 5.4.2.8 / 5.4.2.2) of the α‑D‑phosphohexomutase superfamily. It reversibly interconverts glucose‑6‑P ⇌ glucose‑1‑P and mannose‑6‑P ⇌ mannose‑1‑P by a Mg²⁺‑dependent processive mechanism using a catalytic phosphoserine as phosphoryl donor/acceptor, a histidine general base, and an enzyme‑bound sugar‑1,6‑bisphosphate intermediate that reorients 180° while a C‑terminal domain closes over the active site. The hexose‑1‑phosphates it produces are activated to GDP‑mannose, UDP‑glucose, and dTDP‑L‑rhamnose, supplying precursors for alginate, the LPS core/O‑antigen, and rhamnolipid; in P. putida KT2440 its ecologically primary output is alginate, an exopolysaccharide induced by matric water stress that protects cells against desiccation through a hydrated biofilm microenvironment.