Functional Annotation Report: hisI (PP_5014, Q88D15) — Phosphoribosyl-AMP Cyclohydrolase from Pseudomonas putida KT2440

Summary

The gene hisI (locus PP_5014; UniProt Q88D15) of Pseudomonas putida KT2440 encodes phosphoribosyl-AMP cyclohydrolase (PRA-CH; EC 3.5.4.19), the enzyme that catalyzes the third step of the de novo L-histidine biosynthetic pathway. Its identity is unambiguous: the UniProt/HAMAP annotation (rule MF_01021), the InterPro/Pfam domain architecture (PRA-CH family, PF01502), and the primary and structural literature on orthologous PRA-CH enzymes all converge on the same function. The gene symbol hisI is used consistently across bacteria for this cyclohydrolase activity, and no conflicting gene-identity issues were encountered during the investigation.

The enzyme performs a chemically distinctive reaction: the hydrolytic opening of the adenine (purine) ring of the intermediate N1-(5′-phospho-β-D-ribosyl)-AMP (PR-AMP) by cleaving the N1–C6 bond, producing the ring-opened formimino compound ProFAR (N-(5′-phospho-D-ribosylformimino)-5-amino-1-(5″-phospho-D-ribosyl)-4-imidazolecarboxamide), which is then handed to the downstream isomerase HisA. This ring-opening chemistry is unique to histidine biosynthesis and represents the committed conversion of a purine-derived precursor into the histidine-specific portion of the pathway. HisI is a Zn²⁺/Mg²⁺-dependent metalloenzyme: it carries one essential, tightly bound catalytic Zn²⁺ (coordinated by three cysteines, Cys78/Cys95/Cys102 in the P. putida sequence) plus a lower-affinity Mg²⁺ requirement, and it functions as an obligate cytoplasmic homodimer with a shared, inter-subunit active site.

A notable organism-specific feature is that in P. putida KT2440, hisI is a standalone monofunctional gene (PP_5014, 130 aa) sitting adjacent to a separate monofunctional hisE gene (PP_5015, phosphoribosyl-ATP pyrophosphohydrolase, EC 3.6.1.31). This contrasts with the enteric bacteria (E. coli, Salmonella) and plants, where the two activities are fused into a single bifunctional HisIE/HISN2 polypeptide. This organization reflects the "piecewise" evolutionary assembly of the proteobacterial histidine operon into scattered mini-operons. Below, each of these conclusions is developed with its supporting evidence, followed by a mechanistic model, an evaluation of the evidence base, limitations, and proposed follow-up work.

Key Findings

Finding 1 — HisI catalyzes step 3 of histidine biosynthesis: hydrolytic opening of the purine ring of PR-AMP

The core function of PP_5014 is defined by its EC number and pathway annotation. UniProt Q88D15 assigns EC 3.5.4.19 and the reaction:

1-(5-phospho-β-D-ribosyl)-5′-AMP + H₂O → 1-(5-phospho-β-D-ribosyl)-5-[(5-phospho-β-D-ribosylamino)methylideneamino]imidazole-4-carboxamide (ProFAR)

This is annotated as step 3 of 9 in L-histidine biosynthesis. The protein is small (130 aa, ~15 kDa) and belongs to the PRA-CH family (Pfam PF01502). Mechanistically, the enzyme opens the adenine (purine) ring by hydrolyzing the N1–C6 bond, a transformation that occurs nowhere else in metabolism, marking this as the histidine-committed step.

The primary literature directly corroborates this assignment. A structural study of the orthologous archaeal enzyme states plainly that "Hydrolysis of phosphoribosyl-AMP, the third step in the histidine biosynthetic pathway, is carried out by PR-AMP cyclohydrolase, the product of the hisI gene" (PMID: 16042384). The biochemical characterization of the monofunctional enzyme specifies the exact chemistry, describing the enzyme as "catalyzing the hydrolysis of the N1-C6 bond of the purine substrate, a reaction unique to this pathway" (PMID: 9931020). Taken together, these establish HisI as the PRA-CH of P. putida, catalyzing the ring-opening hydrolysis that defines the third step of histidine biosynthesis.

Finding 2 — HisI is a Zn²⁺/Mg²⁺-dependent metalloenzyme with three cysteine zinc ligands

HisI requires two metal cofactors. The UniProt/HAMAP annotation (MF_01021) for Q88D15 lists both Zn²⁺ and Mg²⁺. The P. putida sequence contains exactly three cysteines — Cys78, Cys95, Cys102 — which serve as the annotated Zn²⁺ ligands, while the Mg²⁺ site is built from aspartate side chains (Asp77, Asp79, Asp81).

The functional roles of these metals are established biochemically. In the monofunctional PRA-CH, "Bound to the enzyme is 1 equiv of Zn²⁺ that can be removed only by extended dialysis with 1,10-phenanthroline (Kd ≤ 10⁻⁹ M). Removal of the Zn²⁺ correlates with the loss of enzyme activity" — demonstrating that the tightly bound Zn²⁺ is catalytically essential (PMID: 9931020). The same study shows that, in addition, "free Mg²⁺ (Ks = 4.9 ± 0.7 µM) is required for catalytic activity," matching the dual-cofactor UniProt annotation (PMID: 9931020).

The structural basis of the zinc site is resolved in the 1.7 Å crystal structure of the Methanobacterium thermoautotrophicum enzyme, in which "These metal binding sites include three cysteine ligands, two from one monomer and the third from the second monomer" (PMID: 16042384). This three-cysteine coordination — spanning the dimer interface — is consistent with the three cysteines (Cys78/Cys95/Cys102) present in P. putida HisI. The Zn²⁺ almost certainly serves to activate a water molecule (or polarize the substrate) for the hydrolytic ring-opening attack, while Mg²⁺ likely stabilizes the anionic phosphoribosyl groups of the substrate.

Finding 3 — HisI is an obligate cytoplasmic homodimer with a shared inter-subunit active site

The enzyme's quaternary structure and localization are well defined. UniProt Q88D15 (via HAMAP MF_01021) annotates the subunit as a homodimer and the subcellular location as cytoplasm. Because histidine biosynthesis is a soluble metabolic pathway using phosphorylated, highly polar intermediates, cytoplasmic localization is expected and consistent with the enzyme carrying no signal peptide or transmembrane segments.

Structural analysis shows the dimerization is not incidental but functionally required. In the M. thermoautotrophicum structure, "The putative substrate-binding cleft is formed at the interface between the two monomers of the dimer. This fact, combined with the localization of the Zn²⁺-binding site, indicates that the enzyme is an obligate dimer" (PMID: 16042384). Because one of the three zinc ligands is contributed by the partner subunit (Finding 2), the active site literally cannot be assembled by a monomer. HisI therefore carries out its function as a soluble cytoplasmic homodimer, with each of the two composite active sites built from residues of both protomers.

Finding 4 — In P. putida, hisI (PRA-CH) and hisE (PRA-PH) are separate monofunctional genes, not a fused bifunctional enzyme

A genome-level distinction of P. putida is that the two consecutive activities of the pathway — phosphoribosyl-ATP pyrophosphohydrolase (PRA-PH; step 2) and phosphoribosyl-AMP cyclohydrolase (PRA-CH; step 3) — are encoded by two separate genes. hisI = PP_5014 (Q88D15, 130 aa, EC 3.5.4.19, PRA-CH only) lies immediately adjacent to hisE = PP_5015 (Q88D14, 111 aa, EC 3.6.1.31, PRA-PH), forming a hisIE gene pair of two standalone polypeptides.

This organization differs from the enteric bacteria and plants, where PRA-PH and PRA-CH are fused into a single bifunctional polypeptide. The Arabidopsis study that mapped the two domains notes that "the functional domains for the PRA-CH (hisI) and PRA-PH (hisE) resided in the N-terminal and the C-terminal halves" of the fused enzyme (PMID: 9733547) — a fusion that in P. putida corresponds instead to two distinct genes. Evolutionary analyses explain this pattern: across proteobacteria the histidine genes were assembled "piecewise" from scattered monofunctional units into mini-operons, and in P. putida KT2440 the his genes remain in three separate genomic clusters — hisGDC, the hisBHAF core, and hisIE — "three mini-operons (hisGDC, hisBHAF, hisIE) transcribed from independent promoters" (PMID: 15871048). That hisI can encode a purely unifunctional PRA-CH is directly precedented in Rhodobacter sphaeroides, where "the R. sphaeroides hisI gene encodes a unifunctional protein" (PMID: 8760919). P. putida HisI therefore represents the standalone, monofunctional cyclohydrolase form of this enzyme.

Finding 5 — Catalytic mechanism: adenine ring-opening followed by rate-limiting proton transfer, yielding ProFAR

The chemical mechanism has been probed by pre-steady-state kinetics of the bifunctional HisIE enzyme, whose HisI-like cyclohydrolase domain is homologous to P. putida HisI. In the pathway, the upstream HisE/PRA-PH converts PR-ATP to PR-AMP, and HisI/PRA-CH then hydrolyzes PR-AMP to ProFAR, which the downstream HisA isomerizes. Kinetic dissection revealed a lag followed by a burst in ProFAR formation, a large solvent deuterium kinetic isotope effect that decreases at basic pH, and no solvent viscosity effect — a pattern that pinpoints a slow chemical step: "These observations are consistent with a rate-limiting unimolecular step involving a proton transfer following adenine ring opening" (PMID: 37288093). The same study identifies the product and the responsible domain: "cyclohydrolysis of PRAMP to N-(5′-phospho-D-ribosylformimino)-5-amino-1-(5″-phospho-D-ribosyl)-4-imidazolecarboxamide (ProFAR) in the N-terminal HisI-like domain" (PMID: 37288093).

This mechanistic picture is complemented by the pH dependence of the monofunctional enzyme, which "shows a single titration event in the kcat/Km profile with a pKa of 7.3 that is consistent with the functional role of a metal site in catalysis" (PMID: 9931020). Together these data support a mechanism in which the active-site metal (Zn²⁺) activates water/hydroxide to attack the N1–C6 bond and open the purine ring, after which a rate-limiting unimolecular proton-transfer step completes formation of the ring-opened ProFAR product.

Finding 6 — HisI adopts the conserved HisI-like α/β fold diagnostic of the PRA-CH family

Domain and fold annotations independently corroborate the functional assignment. InterPro annotation of Q88D15 includes family IPR026660 (Phosphoribosyl-AMP cyclohydrolase), domain IPR002496, and superfamily IPR038019; Pfam PF01502; HAMAP MF_01021 (hisI); NCBIfam NF000768; PANTHER PTHR42945 (Histidine biosynthesis PRA-CH/PRA-PH); CATH-Gene3D G3DSA:3.10.20.810; and SCOP/SUPERFAMILY SSF141734 ("HisI-like"). This set of orthogonal classifications reinforces the UniProt PRA-CH call, and the fold matches the experimentally determined M. thermoautotrophicum HisI structure (PDB, 1.7 Å; PMID: 16042384). The consistency of family, domain, superfamily, and fold assignments provides high confidence that Q88D15 is a bona fide member of the PRA-CH family and shares the catalytic architecture characterized in the structural studies.

Finding 7 — HisI is required for de novo histidine biosynthesis and links the pathway to purine/nucleotide metabolism

The biological necessity of the PRA-CH step is established genetically by complementation. Bacterial hisI mutants are histidine auxotrophs that can be rescued by heterologous PRA-CH genes or domains. The Arabidopsis study reports that "By complementing a hisI mutation of Escherichia coli with an Arabidopsis cDNA library, we isolated an Arabidopsis cDNA (At-IE)" (PMID: 9733547), demonstrating that the PRA-CH activity is essential for histidine prototrophy and is functionally interchangeable across large evolutionary distances.

Histidine biosynthesis is also metabolically interwoven with nucleotide metabolism. The pathway consumes PRPP and ATP to build the imidazole ring, and downstream the HisHF (imidazole-glycerol-phosphate synthase) step releases AICAR (5-aminoimidazole-4-carboxamide ribotide), an intermediate that re-enters de novo purine biosynthesis. Evolutionary analyses note this interconnection, describing "the his 'core,' a cluster consisting of four genes (hisBHAF) whose products interconnect histidine biosynthesis to both de novo synthesis of purine metabolism" (PMID: 15871048). HisI sits at the entry to this histidine-committed branch, converting a purine-derived (AMP-containing) precursor into the first histidine-specific intermediate.

Finding 8 — The catalytic Zn-binding cysteine cluster is conserved between P. putida HisI and the E. coli HisIE cyclohydrolase domain

Sequence comparison confirms that the zinc-ligating cysteine set is conserved across the monofunctional and bifunctional forms. P. putida HisI (Q88D15, 130 aa) carries exactly three cysteines at positions 78, 95, 102, embedded in the conserved PRA-CH motif ...GHIACHTGRESC... (residues ~91–102). The N-terminal HisI (cyclohydrolase) domain of the E. coli bifunctional HisIE (P06987, 355 aa) carries a matching N-terminal cysteine cluster at positions 93, 101, 103, whereas its remaining cysteines (254, 265, 269) belong to the separate C-terminal HisE/PRA-PH domain. Thus the three-cysteine Zn²⁺ ligand set is conserved between the P. putida monofunctional enzyme and the E. coli bifunctional enzyme, and matches the structurally defined zinc site of the M. thermoautotrophicum HisI, where "These metal binding sites include three cysteine ligands, two from one monomer and the third from the second monomer" (PMID: 16042384). This conservation strongly implies that P. putida HisI uses the same zinc-dependent catalytic machinery.

Mechanistic Model / Interpretation

Position in the histidine biosynthetic pathway

HisI operates at the third of nine steps in the de novo L-histidine biosynthetic pathway. The immediate upstream and downstream context is:

 Step 1: HisG (ATP-PRT)      PRPP + ATP ──────────────► PR-ATP
 Step 2: HisE (PRA-PH)       PR-ATP + H2O ────────────► PR-AMP + PPi
 Step 3: HisI (PRA-CH)  ***  PR-AMP + H2O ────────────► ProFAR      <-- THIS ENZYME
 Step 4: HisA (isomerase)    ProFAR ───────────────────► PRFAR
 Step 5: HisHF (IGPS)        PRFAR + glutamine ────────► ImGP + AICAR (+ Glu)
   ...   HisB, HisC, HisD    ──────────────────────────► L-histidine

HisI catalyzes the committed ring-opening that converts the AMP-derived, purine-ring-containing intermediate PR-AMP into the open-chain formimino compound ProFAR. The imidazole ring of PR-AMP is preserved and ultimately becomes the imidazole side chain of histidine, while the pyrimidine portion of the former adenine ring, once opened at N1–C6, is later cleaved off as AICAR at the HisHF step and recycled into purine biosynthesis. HisI thus marks the metabolic branch point where a purine-nucleotide precursor is committed to histidine.

Catalytic architecture and chemistry

Subunit A                         Subunit B
     ┌───────────────┐                ┌───────────────┐
     │ Cys78  Cys95  │   Zn2+ site    │               │
     │ Cys102 ───────┼──── Zn2+ ──────┼─── (3rd Cys   │
     │               │   (shared)     │    from        │
     │ Asp77/79/81 ──┼──── Mg2+       │    partner)    │
     └───────┬───────┘                └───────┬───────┘
     │      shared active-site cleft  │
     └──────── at dimer interface ────┘

   Chemistry:  Zn2+-activated water/OH- attacks N1-C6 bond
       → adenine (purine) ring opens
       → rate-limiting unimolecular proton transfer
       → ProFAR released

The enzyme is an obligate homodimer: each active site is assembled at the interface, drawing zinc ligands from both subunits. The catalytically essential Zn²⁺ (Kd ≤ 10⁻⁹ M) is best interpreted as a Lewis acid that activates a water molecule for nucleophilic attack on the N1–C6 bond, consistent with the single kcat/Km titration (pKa 7.3) attributed to a metal-water site. Kinetics of the homologous HisIE enzyme indicate that ring opening is followed by a rate-limiting unimolecular proton-transfer step; the large solvent deuterium isotope effect that diminishes at basic pH is the diagnostic signature of this proton transfer. A lower-affinity Mg²⁺ (Ks ≈ 4.9 µM), coordinated by the Asp77/Asp79/Asp81 cluster, likely stabilizes the two anionic 5′-phosphoribosyl moieties of the substrate/product.

Genomic and evolutionary organization

This comparison highlights that the P. putida enzyme is one polypeptide with one job — a "clean" model system for studying PRA-CH catalysis without the confounding presence of a fused PRA-PH domain. The scattered, three-cluster genomic layout in P. putida reflects the "piecewise" assembly model of the proteobacterial histidine operon, in which small units (some with their own promoters) were accreted over evolutionary time rather than assembled at once.

Evidence Base

Additional P. putida-focused papers reviewed (e.g., on CbrA/B carbon/nitrogen regulation, Crc catabolite control, and c-di-GMP/biofilm signaling — PMIDs 25031426, 20553554, 19526543, 27489550, 35001503) confirmed that histidine metabolism in P. putida is embedded in the organism's broader carbon/nitrogen regulatory networks (histidine is a usable carbon/nitrogen source), but none of them implicate the biosynthetic PRA-CH (HisI) in any moonlighting or signaling role. This absence is itself informative: it supports the conclusion that HisI's function is confined to biosynthetic catalysis.

Limitations and Knowledge Gaps

1. No direct experimental characterization of the P. putida protein. All catalytic, metal-binding, and structural conclusions rest on orthologous enzymes — the M. thermoautotrophicum crystal structure, the monofunctional archaeal metalloenzyme biochemistry, and the bifunctional HisIE/HISN2 kinetics. The functional transfer to Q88D15 is a well-supported inference (family membership, domain conservation, and the conserved Cys78/Cys95/Cys102 zinc cluster all align), but no purified P. putida HisI has been kinetically or structurally characterized in the literature reviewed.

2. Residue numbering for metal ligands is inferred. The Zn (Cys78/95/102) and Mg (Asp77/79/81) positions are derived from the P. putida sequence and the conserved PRA-CH motif, consistent with the structural ortholog, but have not been experimentally mutated/verified in Q88D15 itself.

3. Mechanistic detail is extrapolated from the bifunctional enzyme. The "ring-opening then rate-limiting proton transfer" mechanism (PMID: 37288093) was established on a bifunctional HisIE. While the HisI-like domain is homologous, subtle kinetic differences between fused and standalone enzymes cannot be excluded.

4. Regulation and expression in P. putida are not fully resolved. The hisIE mini-operon is transcribed from an independent promoter, but the specific regulatory inputs (e.g., histidine-mediated feedback, growth-condition dependence) on PP_5014 expression were not directly established here.

5. No structure of the P. putida enzyme (experimental or predicted) was analyzed. An AlphaFold model of Q88D15 exists in principle but was not evaluated; doing so could confirm the dimer interface and metal-site geometry computationally.

Proposed Follow-up Experiments / Actions

1. Recombinant expression and enzyme assay. Clone PP_5014, express and purify the His-tagged protein, and measure PRA-CH activity (PR-AMP → ProFAR) spectrophotometrically. Determine kcat, Km for PR-AMP, and the pH-rate profile to confirm the predicted pKa ≈ 7.3 metal-water titration.

2. Metal-dependence and stoichiometry. Quantify bound Zn²⁺ (ICP-MS / atomic absorption) and test activity loss upon chelation (1,10-phenanthroline), replicating the ≤10⁻⁹ M Zn²⁺ affinity result for the P. putida ortholog. Titrate Mg²⁺ to verify the low-affinity requirement.

3. Site-directed mutagenesis of the predicted ligands. Mutate Cys78, Cys95, Cys102 (Zn) and Asp77/79/81 (Mg) individually to confirm their roles in catalysis and metal binding, directly testing the inferred residue assignments.

4. Quaternary-structure confirmation. Use size-exclusion chromatography with multi-angle light scattering (SEC-MALS) or analytical ultracentrifugation to verify the obligate homodimer, and, if feasible, solve the crystal or cryo-EM structure of P. putida HisI to confirm the inter-subunit active site.

5. Genetic essentiality/complementation. Construct a PP_5014 deletion in P. putida KT2440 and confirm histidine auxotrophy (growth rescued by histidine supplementation), and complement an E. coli hisI mutant with PP_5014 to demonstrate functional PRA-CH activity in vivo.

6. Computational validation. Analyze the AlphaFold model of Q88D15 (pLDDT, dimer prediction via AlphaFold-Multimer) to corroborate the metal-site geometry and dimer interface, and dock PR-AMP to test the proposed catalytic arrangement.

7. Expression/regulation profiling. Use RT-qPCR or RNA-seq under histidine-replete vs. limited conditions to characterize regulation of the hisIE mini-operon in P. putida.

Conclusion

The evidence — spanning UniProt/HAMAP annotation, InterPro/Pfam/CATH/SCOP domain and fold classification, a high-resolution crystal structure of an archaeal ortholog, detailed metalloenzyme biochemistry, pre-steady-state mechanistic kinetics of the homologous bifunctional enzyme, complementation genetics, and comparative genomics — converges on a single, high-confidence conclusion. HisI (PP_5014, Q88D15) is a soluble cytoplasmic, Zn²⁺/Mg²⁺-dependent, obligate homodimeric phosphoribosyl-AMP cyclohydrolase (PRA-CH, EC 3.5.4.19) that catalyzes the third step of de novo L-histidine biosynthesis — the hydrolytic opening of the adenine ring (N1–C6 bond cleavage) of PR-AMP to form ProFAR. In P. putida KT2440 it is a standalone monofunctional enzyme encoded separately from the adjacent hisE (PRA-PH), reflecting the piecewise evolutionary assembly of the proteobacterial histidine genes. Its function is squarely biochemical and biosynthetic; no signaling or moonlighting role is indicated.

Artifacts

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• OpenScientist final report

Citations

1. PMID:16042384
2. PMID:9931020
3. PMID:9733547
4. PMID:15871048
5. PMID:8760919
6. PMID:37288093