Verification outcome — CONFIRMED (unambiguous). The gene symbol ispG (synonym gcpE), the EC number, the annotated reaction, the [4Fe-4S] IspG protein family, and the TIM-barrel domain architecture are mutually consistent and match the primary experimental and structural literature. ispG is a highly conserved, universally single-copy enzyme of the bacterial/plastidial MEP pathway; there is no competing gene bearing this symbol. Mechanistic and structural studies were performed in orthologs (E. coli, Thermus thermophilus, Aquifex aeolicus); given the enzyme's near-invariant catalytic core, these findings transfer directly to the P. putida protein.
ispG encodes (E)-4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase (HMBPP synthase / GcpE), an oxygen-sensitive [4Fe-4S] cluster metalloenzyme that catalyzes the penultimate step of the methylerythritol phosphate (MEP / non-mevalonate) pathway of isoprenoid precursor biosynthesis. It performs the reductive dehydroxylation (ring-opening) of the cyclic substrate 2‑C‑methyl‑D‑erythritol‑2,4‑cyclodiphosphate (MEcPP) to the linear allylic product (E)‑4‑hydroxy‑3‑methylbut‑2‑enyl diphosphate (HMBPP), consuming two electrons supplied by a physiological electron-donor system (flavodoxin/flavodoxin reductase; ferredoxin in some organisms). The enzyme functions in the cytoplasm and is essential for supplying IPP and DMAPP, the universal five-carbon isoprenoid building blocks.
Reaction (EC 1.17.7.3):
2‑C‑methyl‑D‑erythritol‑2,4‑cyclodiphosphate (MEcPP) + 2 e⁻ + 2 H⁺ → (E)‑4‑hydroxy‑3‑methylbut‑2‑enyl diphosphate (HMBPP) + H₂O
This was established biochemically by Kollas et al. (2002): recombinant GcpE "was enzymatically active in converting 2‑C‑methyl‑D‑erythritol‑2,4‑cyclodiphosphate (MEcPP) into (E)‑4‑hydroxy‑3‑methyl‑but‑2‑enyl diphosphate (HMBPP) in the presence of dithionite as reductant," with a k_cat ≈ 0.4 s⁻¹ and specific activity 0.6 µmol·min⁻¹·mg⁻¹ (PMID 12482607). The transformation is formally a 2‑electron reductive dehydroxylation: the substrate's five-membered 2,4-cyclodiphosphate ring is opened and the C3 hydroxyl is eliminated, generating the (E)-configured allylic double bond of HMBPP (Witjaksono et al. 2025, PMID 41122037).
Substrate specificity. IspG is highly specific for MEcPP, the product of the upstream enzyme IspF (MEcPP synthase). Substrate analogues such as monofluoromethyl-erythritol cyclodiphosphate have been synthesized specifically to probe/inhibit the active site, underscoring the tight steric/electronic requirements of the funnel-shaped pocket (Witjaksono et al. 2025, PMID 41122037).
IspG is a [4Fe-4S] metalloenzyme (Witjaksono et al. 2025, PMID 41122037). Anaerobically purified enzyme "contains solely [4Fe-4S] clusters," and, upon addition of MEcPP plus a reductant, EPR spectroscopy reveals a transient iron-sulfur signal whose similarity to that of ferredoxin:thioredoxin reductase indicates that "an intermediate is directly bound to the active-site cluster" (Adedeji et al. 2007, PMID 17214985). Thus catalysis proceeds through inner-sphere coordination of the substrate to the unique ("apical") fourth iron of the cluster, which lacks a cysteine ligand and is instead available to bind the substrate oxygen. Two electrons, delivered one at a time by the electron-donor system, drive C–O bond cleavage, likely via organometallic/radical intermediates (a ferraoxetane/allyl-anion or Fe-bound radical intermediate has been proposed).
The reaction's obligatory reductant is a redox protein system. Mössbauer and reconstitution studies established that the bacterial enzyme utilizes NADPH/flavodoxin/flavodoxin reductase as a reducing shuttle system, reducing MEcPP to HMBPP via two consecutive one-electron transfers (Seemann et al. 2005, PMID 15650872). This is the origin of the UniProt name "…(flavodoxin)": in P. putida, electrons flow NADPH → flavodoxin reductase → flavodoxin → IspG [4Fe-4S] cluster. (Notably, the Arabidopsis plastidial ortholog cannot use this bacterial shuttle and required 5-deazaflavin semiquinone in vitro, indicating organism-specific redox partners — PMID 15650872.) The enzyme's oxygen sensitivity (PMID 41122037) reflects the lability of the reduced [4Fe-4S] cluster and confines function to the reducing intracellular environment.
Organometallic intermediates. Detailed EPR/ENDOR/HYSCORE spectroscopy with ²H, ¹³C and ¹⁷O labeling shows the mechanism is genuinely organometallic: reduction of MEcPP generates an Fe–C–H-containing intermediate, most likely a ferraoxetane, which is reduced to a ferracyclopropane in which HMBPP forms an η²-alkenyl π-complex with the 4th (apical) iron of the [4Fe-4S] cluster; alkyne (propargyl) diphosphate analogues exploit this by forming a comparable metallacycle and inhibit potently (Kᵢ ≈ 300 nM) (Wang et al. 2010, PMID 20534554). This explains the mechanistic necessity of the non-cysteinyl 4th iron identified structurally.
The crystal structure of the GcpE(IspG)–MEcPP complex from Thermus thermophilus (1.55 Å; Rekittke et al. 2012, PMID 22967895) reveals a two-domain architecture:
"MEcPP binding inside the TIM-barrel funnel induces a 60° rotation of the [4Fe-4S] cluster-containing domain onto the TIM-barrel entrance," closing a clamshell over the substrate; "the apical iron of the [4Fe-4S] cluster ligates with the C3 oxygen atom of MEcPP" (PMID 22967895). This directly visualizes how the metallocenter is positioned to abstract the C3 hydroxyl. In physiological assemblies the enzyme is typically dimeric, and in some organisms the two domains are contributed in trans.
IspG is a soluble cytoplasmic enzyme. Multiple lines of evidence support this: (i) it lacks signal peptides or transmembrane segments; (ii) its oxygen-labile reduced [4Fe-4S] cluster requires the reducing cytosolic milieu; (iii) its substrate (MEcPP) and product (HMBPP) are polar, doubly-phosphorylated small molecules generated and consumed by other cytosolic MEP-pathway enzymes; and (iv) its electron donors (flavodoxin/ferredoxin) are cytoplasmic. In plants/algae the ortholog is plastid-localized, consistent with the cyanobacterial/endosymbiotic origin of the pathway; in the free-living bacterium P. putida the reaction occurs in the cytoplasm.
Target-specific sequence support (this study). Kyte–Doolittle hydropathy analysis of Q88PJ7 (window 19) finds no transmembrane-like segment (maximum window hydropathy 1.51, below the ~1.6 TM threshold; 0 windows exceed it) and an overall GRAVY of −0.03 (hydrophilic, i.e., soluble/globular). The N-terminus (MHGESPIKRRESRKIW…) is highly charged (8 charged residues in the first 30) and lacks the hydrophobic core of a cleavable signal peptide. These features are fully consistent with a soluble cytoplasmic enzyme.
ispG operates in the MEP (2‑C‑methyl‑D‑erythritol‑4‑phosphate) / DOXP / non-mevalonate pathway, the route Pseudomonas putida uses to synthesize isoprenoid precursors. This is confirmed for the target organism: "In P. putida and most other bacteria, these precursors are produced from pyruvate and glyceraldehyde 3-phosphate by the methylerythritol 4-phosphate (MEP) pathway," and KT2440 has been engineered via MEP-pathway genes (dxs, dxr, idi) for isoprenoid (lycopene) production (Hernandez-Arranz et al. 2019, PMID 31500633). More broadly, many Gram-negative bacteria including Pseudomonas rely exclusively on the MEP pathway rather than the mevalonate pathway (Qabar et al. 2026, PMID 41700894).
Pathway order and IspG's position (penultimate step):
The product HMBPP is the direct substrate of IspH, which yields isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) — the universal C5 building blocks for all downstream isoprenoids (ubiquinone/quinones for respiration, bactoprenol/undecaprenyl phosphate for cell-wall/LPS assembly, carotenoids, and prenylated products). IspG is therefore essential for viability in P. putida (Kollas et al. 2002 describe GcpE as "an essential enzyme of the non-mevalonate pathway"; PMID 12482607).
Downstream significance of the product. HMBPP is the most potent known microbial phosphoantigen, sensed by human Vγ9Vδ2 T cells through intracellular binding to butyrophilin BTN3A1/BTN2A1 (Yang et al. 2019, PMID 30902636; Zhang et al. 2025, PMID 40505658). Thus IspG activity in bacteria couples directly to host immune surveillance. Because humans use the mevalonate pathway and lack IspG, the enzyme is a validated, selective antimicrobial drug target (Witjaksono et al. 2025, PMID 41122037).
To ensure the mechanistic/structural conclusions (derived from orthologs) apply to the actual P. putida protein, I retrieved the Q88PJ7 sequence (369 aa) and aligned it to the biochemically characterized E. coli IspG (P62620, 372 aa):
This target-specific evolutionary/structural inference confirms an intact, canonical IspG active site in Q88PJ7 and justifies transferring the enzymology and mechanism established in E. coli and T. thermophilus to the P. putida enzyme.
| Claim | Evidence type | Source |
|---|---|---|
| Catalyzes MEcPP → HMBPP | Direct in vitro enzymology, kinetics (k_cat 0.4 s⁻¹) | Kollas 2002 (PMID 12482607) |
| Essential MEP-pathway enzyme | Functional/genetic characterization | Kollas 2002 (PMID 12482607) |
| [4Fe-4S] enzyme; 2-e⁻ reductive dehydroxylation; O₂-sensitive | Biochemistry + mechanistic review | Witjaksono 2025 (PMID 41122037) |
| Substrate binds apical Fe of the cluster | EPR spectroscopy of intermediate | Adedeji 2007 (PMID 17214985) |
| TIM-barrel + mobile Fe-S domain; Fe–C3-O ligation | 1.55 Å X-ray structure | Rekittke 2012 (PMID 22967895) |
| Pseudomonas uses MEP (not mevalonate) pathway | Comparative genomics review | Qabar 2026 (PMID 41700894) |
| Product HMBPP is a phosphoantigen; validated drug target | Structural immunology; med-chem | Yang 2019 (PMID 30902636); Witjaksono 2025 (PMID 41122037) |
| Domain/family assignment | Bioinformatics (InterPro/UniProt) | UniProt Q88PJ7; InterPro |
| Target conserves [4Fe-4S] cysteines; 73.8% id to E. coli IspG | Sequence alignment (this study) | Q88PJ7 vs P62620 |
| Electron donor = NADPH/flavodoxin/flavodoxin reductase; two 1-e⁻ transfers | Mössbauer + reconstitution | Seemann 2005 (PMID 15650872) |
| Organometallic ferraoxetane/ferracyclopropane intermediates on 4th Fe | EPR/ENDOR/HYSCORE | Wang/Oldfield 2010 (PMID 20534554) |
| P. putida KT2440 uses the MEP pathway for isoprenoid precursors | Metabolic engineering | Hernandez-Arranz 2019 (PMID 31500633) |
| Target has no TM helix/signal peptide; soluble (GRAVY −0.03) → cytoplasmic | Hydropathy analysis (this study) | Q88PJ7 |