Functional Annotation Report: MobA (Q88HA3 / PP_3457) in Pseudomonas putida KT2440

Gene: mobA (OrderedLocusName PP_3457)
UniProt: Q88HA3
Organism: Pseudomonas putida (strain ATCC 47054 / DSM 6125 / NCIMB 11950 / KT2440), taxon code PSEPK
EC: 2.7.7.77 · KEGG Orthology: K03752 · Protein family: MobA family (HAMAP MF_00316)
Domains: MobA-like_NTP_Trfase (IPR025877); Molybde_CF_guanTrfase (IPR013482); Nucleotide-diphospho-sugar transferase fold (IPR029044); NTP_transf_3 (PF12804)
Protein-existence level: PE = 3 (inferred from homology)

Summary

MobA (Q88HA3, locus PP_3457) of Pseudomonas putida KT2440 is a molybdenum cofactor (Moco) guanylyltransferase (EC 2.7.7.77; also GTP:molybdopterin guanylyltransferase / Mo-MPT guanylyltransferase / MGD synthase). Its primary biochemical function is to catalyze the Mg²⁺-dependent transfer of a GMP moiety from GTP onto the terminal phosphate of molybdopterin (Mo-MPT), producing molybdopterin guanine dinucleotide (MGD) and ultimately the bis-MGD form of the cofactor, with release of pyrophosphate. This is the fourth, bacteria/archaea-specific step of molybdenum cofactor biosynthesis, converting the base Mo-MPT into the dinucleotide form.

The bis-MGD product built by MobA is the obligatory cofactor of the DMSO reductase family of molybdoenzymes — nitrate reductases, DMSO/TMAO reductases, formate dehydrogenases, and related molybdopterin oxidoreductases — which drive anaerobic respiration and nitrogen/carbon/sulfur cycling. MobA therefore gates the maturation and catalytic activation of a broad set of respiratory enzymes. In P. putida KT2440 the plausible downstream "client" enzymes encoded in the genome are a molybdopterin oxidoreductase (PP_4676) and two formate dehydrogenase catalytic subunits (PP_0489, PP_2185). MobA acts in the cytoplasm, assembling bis-MGD on its own surface via a bis-Mo-MPT intermediate and then delivering the finished cofactor to folded apo-molybdoenzymes.

The functional assignment for the P. putida protein is made by strong orthology and conserved-motif analysis rather than by direct experiment: no P. putida-specific biochemical study of this protein exists (hence PE = 3). Q88HA3 shares 29.6 % sequence identity / 52.4 % similarity with the extensively characterized E. coli MobA (P32173, PE = 1), and retains the diagnostic glycine-rich N-terminal nucleotidyltransferase (GTP-binding) motif, the MPT-binding region, and the conserved C-terminal motif. Genomic-context analysis adds organism-specific nuance: unlike E. coli, P. putida KT2440 encodes mobA as a stand-alone (monocistronic) gene and lacks a MobB accessory homolog, while retaining upstream Moco machinery and two MocA-family cytidylyltransferase paralogs. The conclusions below rest on E. coli / R. sphaeroides / M. tuberculosis biochemistry combined with bioinformatic verification that Q88HA3 is a bona fide MobA. The gene symbol mobA unambiguously matches the protein description, family and domain content — there is no gene-identity ambiguity.

Key Findings

F001 — MobA is a molybdenum cofactor guanylyltransferase (EC 2.7.7.77)

The core function of MobA is to link a guanosine monophosphate (GMP) group, donated by GTP, onto the terminal phosphate of molybdopterin (MPT), forming molybdopterin guanine dinucleotide (MGD). This was established in E. coli, whose MobA is the direct ortholog of P. putida Q88HA3/PP_3457. In a defined in vitro system, purified MobA "links a guanosine 5′-phosphate to MPT forming molybdopterin guanine dinucleotide. This reaction requires GTP, MgCl₂, and the MPT form of the cofactor and can efficiently reconstitute Rhodobacter sphaeroides apo-DMSOR" (PMID: 10978347). Near-atomic-resolution structural work defined the catalytic mechanism as a "nucleophilic attack by MPT on the GMP donor, most likely GTP, to produce MGD and pyrophosphate" (PMID: 11080634).

The reaction can be summarized as:

   GTP  +  Mo-MPT   --(MobA, Mg2+)-->   MGD  +  PPi
   (repeated to build the bis-MGD cofactor)

The strict Mg²⁺ and GTP requirement, plus the specific use of the MPT (not the fully mature dinucleotide) substrate, defines the enzyme's substrate specificity. This is the reaction that EC 2.7.7.77 encodes, and it is the primary function assigned to Q88HA3.

F002 — The MobA product (bis-MGD) is required by the DMSO reductase family of molybdoenzymes

The physiological importance of MobA lies in the enzymes that depend on its product. The MGD/bis-MGD modification "is required for the functioning of many bacterial molybdoenzymes, including the nitrate reductases, dimethylsulfoxide (DMSO) and trimethylamine-N-oxide (TMAO) reductases, and formate dehydrogenases" (PMID: 10978347). Genetic evidence from Mycobacterium tuberculosis confirms the causal link: MobA "converts MoCo to bis-molybdopterin guanine dinucleotide (bis-MGD), a form of the cofactor that is required by the dimethylsulfoxide (DMSO) reductase family of enzymes, which includes the nitrate reductase NarGHI," and a mobA deletion mutant lost both assimilatory and respiratory nitrate reductase activity (PMID: 25404027). MobA is therefore a maturation factor whose loss silences a whole class of downstream respiratory enzymes — through the precise removal of an essential cofactor modification, not a broad pleiotropic effect.

F003 — MobA is a two-domain Rossmann-fold enzyme with a division of labor between its domains

MobA (~191–194 aa, ~22 kDa) has an α/β architecture in which "the N-terminal half of the molecule adopts a Rossman fold. The structure of MobA has striking similarity to Bacillus subtilis SpsA," a nucleotide-diphospho-sugar transferase (PMID: 10978347). This homology is the structural basis of the InterPro/Pfam domain annotations for Q88HA3 (MobA-like_NTP_Trfase IPR025877; nucleotide-diphospho-sugar transferase fold IPR029044; PF12804 NTP_transf_3). A MobA:GTP cocrystal localizes the GTP-binding site to the N-terminal domain via three signature sequence motifs, with the MPT-binding site adjacent.

Domain-swap and site-directed mutagenesis experiments in E. coli dissected the two domains' roles: "Exchange of the complete N-terminal domain of each protein resulted in the total inversion of nucleotide specificity activity, showing that the N-terminal domain determines nucleotide recognition and binding. Analysis of protein-protein interactions showed that the C-terminal domain of either MocA or MobA determines the specific binding to the respective acceptor protein" (PMID: 21081498). Thus MobA reads GTP with its N-terminus and reads the destination molybdoenzyme with its C-terminus — a modular design that explains how closely related transferases (MobA/GMP vs MocA/CMP) achieve distinct specificities.

F004 — bis-MGD is assembled on MobA via a bis-Mo-MPT intermediate, then transferred to apoenzymes

MobA does more than a single guanylyl transfer: it acts as an assembly platform. In vitro reconstitution with purified components identified a novel bis-Mo-MPT intermediate bound on MobA prior to nucleotide attachment. "The addition of Mg-GTP to MobA loaded with bis-Mo-MPT resulted in formation and release of the final bis-MGD product. This cofactor was fully functional and reconstituted the catalytic activity of apo-TMAO reductase (TorA)" (PMID: 24003231). The proposed sequence is: (1) formation of bis-Mo-MPT on MobA, (2) addition of two GMP units to form bis-MGD, and (3) release and transfer of the completed cofactor to the target molybdoenzyme. This establishes MobA's mechanistic role as both catalyst and delivery scaffold.

F005 — MobA works in the cytoplasm as the fourth step of Moco biosynthesis; MobB is an accessory modulator

Moco biosynthesis proceeds in four conserved steps, the fourth (present only in bacteria and archaea) being that "an additional modification of Moco is possible with the attachment of a nucleotide (CMP or GMP) to the phosphate group of MPT, forming the dinucleotide variants of Moco" (PMID: 32239579). Moco is inserted into apoenzymes after folding: "these enzymes require complex molybdenum-containing cofactors, which are inserted into the apoenzymes after folding. For almost all the bacterial molybdoenzymes, molybdenum cofactor insertion requires the involvement of specific chaperones" (PMID: 26468212), locating MobA's activity firmly in the cytoplasm. In E. coli, the accessory protein MobB is co-transcribed with MobA and, although not essential, "the dimeric MobB increases the activation of molybdoenzymes, incorporating this cofactor by a mechanism that is not understood," and a MobA:MobB complex is structurally feasible (PMID: 12682065). This context is important for interpreting the P. putida genome, which — as shown below — lacks MobB.

F006 — Sequence analysis confirms Q88HA3 is a bona fide MobA ortholog

A global BLOSUM62 (Needleman–Wunsch) alignment of P. putida MobA (Q88HA3, 191 aa) against the experimentally characterized E. coli MobA (P32173, 194 aa) gives 29.6 % identity and 52.4 % similarity over 189 aligned positions (alignment score 199). Critically, the diagnostic N-terminal glycine-rich nucleotidyltransferase motif is strictly conserved:

The conservation of the GTP-binding, MPT-binding, and C-terminal motifs across a moderately divergent sequence (typical for orthologous bacterial Moco enzymes) confirms that Q88HA3 possesses the intact catalytic apparatus of a functional MoCo guanylyltransferase. Q88HA3 is annotated PE = 3 (inferred from homology), while E. coli MobA is PE = 1 (evidence at protein level) — underscoring that the P. putida assignment is a well-supported homology inference rather than direct measurement.

F007 — In P. putida KT2440, mobA is monocistronic and the genome lacks MobB

KEGG genomic context places PP_3457 (mobA, chromosome 3,919,409–3,919,984, + strand) between the multidrug efflux gene mexB/PP_3456 and acs/PP_3458, with no adjacent Moco-biosynthesis genes — i.e., it is not organized in a mob operon as in E. coli. A KEGG orthology search of the KT2440 genome for MobB (K03753) returns no hits, whereas the core upstream Moco machinery is present: moaA (K03750, PP_2123), a moeA-family gene (PP_1294), and mobA itself (K03752, PP_3457, EC 2.7.7.77). This is an organism-specific finding: P. putida MobA must function without the MobB accessory protein that modulates cofactor insertion in E. coli — suggesting either that MobB is dispensable in this organism or that another factor substitutes.

F008 — Candidate bis-MGD client molybdoenzymes are encoded in P. putida KT2440

KEGG orthology mapping identifies the plausible downstream enzymes that would consume MobA's bis-MGD product: a molybdopterin oxidoreductase (K07147, PP_4676) and formate dehydrogenase catalytic subunits (K00123, PP_0489 and PP_2185) — all members of the DMSO-reductase / Moco-dependent family (formate dehydrogenase assembly and catalysis reviewed in PMID: 25514355). The genome also encodes two MocA-family paralogs (K07141, PP_2483 and PP_4230), the CTP:molybdopterin cytidylyltransferases that make the alternative MCD cofactor for a distinct set of molybdoenzymes — consistent with the general pattern that MobA (GMP) and MocA (CMP) provide parallel dinucleotide-cofactor branches. Notably, no respiratory nitrate reductase narG (K00370) was detected; the gene PP_2092 annotated "nitrate reductase" is in fact an NNP-family nitrate/nitrite MFS transporter (K02575), so the P. putida client set differs from the classic E. coli NarGHI paradigm.

Mechanistic Model / Interpretation

Integrating the findings yields a coherent picture of where MobA sits in P. putida metabolism and how it works.

Pathway position

 Step 1        Step 2        Step 3            Step 4 (bacteria/archaea)         Client enzymes
 GTP ─► cPMP ─► MPT  ─► Mo-MPT (Moco) ─────► MobA: + GMP (from GTP) ──► bis-MGD ─► apo-molybdoenzyme
(MoaA…) (MPT synth.) (MoeA/MogA)      ↑ Mg2+, via bis-Mo-MPT              (holoenzyme, active)
                              │ N-domain binds GTP                 in P. putida KT2440:
                              │ C-domain binds MPT + target        • Mo-oxidoreductase PP_4676
                              └───────────────────────────────►    • FDH PP_0489 / PP_2185

MobA catalyzes the terminal maturation step that converts base Moco (Mo-MPT) into the dinucleotide form. Mechanistically (F003, F004): the N-terminal Rossmann-fold domain binds Mg-GTP and enforces guanine specificity; the enzyme assembles a bis-Mo-MPT intermediate on its surface; two successive guanylyl transfers add GMP groups (each releasing pyrophosphate) to form bis-MGD; and the C-terminal domain recognizes the specific apo-molybdoenzyme to which the finished cofactor is handed off. This makes MobA simultaneously a catalyst and a chaperone-like delivery platform, operating in the cytoplasm on already-folded apoenzymes (F005).

Functional consequence

Because bis-MGD is obligatory for the DMSO reductase family (F002), MobA gates the activation of P. putida's formate dehydrogenases and molybdopterin oxidoreductase (F008). In organisms where this has been tested genetically, mobA loss abolishes downstream molybdoenzyme activity (nitrate reductase in M. tuberculosis, F002). MobA's role is therefore precise and upstream: it does not itself catalyze respiration but is a prerequisite for it.

Organism-specific tailoring

The P. putida system is streamlined relative to E. coli: mobA is monocistronic and there is no MobB (F007). Combined with the substitution of NarGHI (absent) by formate dehydrogenases and a generic molybdopterin oxidoreductase as the client set (F008), this indicates that P. putida uses its single MobA to feed a smaller, distinct panel of bis-MGD enzymes, alongside a parallel MocA/MCD branch for other molybdoenzymes.

Comparison table: E. coli (reference) vs P. putida KT2440

Localization

MobA is a soluble cytoplasmic enzyme. Moco and its dinucleotide variants are synthesized in the cytoplasm, and the mature cofactor is inserted into apoenzymes after they fold, with insertion "for almost all the bacterial molybdoenzymes" requiring specific chaperones (PMID: 26468212). MobA therefore operates in the cytoplasm together with system-specific chaperones (and, in organisms that have it, MobB) to mature apo-molybdoenzymes. Periplasmic molybdoenzymes receive their fully assembled, cofactor-loaded holoenzyme via Tat-dependent export after cytoplasmic maturation, consistent with MobA acting entirely on the cytoplasmic side.

Evidence Base

Supporting reviews on regulation and sulfur delivery (PMID: 31517366, PMID: 38631442, PMID: 31655739, PMID: 28098827) place MobA within the broader Moco / Fe-S / sulfur-transfer network but do not directly characterize the P. putida protein.

How the evidence maps onto Q88HA3: all mechanistic and structural evidence derives from orthologs (E. coli, R. sphaeroides, M. tuberculosis). The bridge to the P. putida protein is the sequence/motif conservation in F006 and the genomic context in F007–F008. This is a well-supported homology inference, not a direct measurement.

Supported and Refuted Hypotheses

Limitations and Knowledge Gaps

1. No P. putida-specific experimental data. Q88HA3 is annotated PE = 3 (inferred from homology). Its catalytic activity, kinetics, substrate specificity, and structure have not been measured directly; all functional claims are transferred from orthologs. Given the near-universal conservation of MobA and the HAMAP MF_00316 rule, the inference is strong but not experimentally confirmed for PP_3457.

2. Moderate sequence identity to the reference. The 29.6 % identity to E. coli MobA, while typical for orthologous Moco enzymes and accompanied by strictly conserved catalytic motifs, leaves residual uncertainty about fine details (e.g., exact kinetic parameters, acceptor-protein specificity).

3. Client enzyme assignments are predictions. PP_4676, PP_0489, and PP_2185 are candidate bis-MGD-dependent enzymes identified by KEGG orthology, not by demonstrated MobA→enzyme cofactor transfer in P. putida.

4. Absence of MobB is inferred from KEGG. The lack of a K03753 hit strongly suggests no MobB, but the functional consequence — how P. putida achieves efficient molybdoenzyme activation without MobB — is unknown.

5. MobA vs MocA branch partitioning. P. putida encodes two MocA paralogs; which molybdoenzymes receive MGD (from MobA) vs MCD (from MocA) has not been experimentally delineated in this organism.

6. Physiological/regulatory conditions. The conditions under which P. putida upregulates mobA and its client molybdoenzymes (e.g., anaerobiosis, formate metabolism) were not investigated here; regulation by FNR/Fur/ArcA is documented only in E. coli (PMID: 31517366).

Proposed Follow-up Experiments / Actions

1. Direct biochemical assay. Recombinantly express and purify Q88HA3; measure MoCo guanylyltransferase activity (GTP + Mo-MPT → MGD + PPi) in vitro, and determine kinetic constants and Mg²⁺/GTP dependence — the single most valuable experiment to lift the protein from PE = 3 to PE = 1.

2. Cofactor reconstitution. Test whether purified Q88HA3-generated bis-MGD reconstitutes the activity of purified P. putida apo-formate dehydrogenase (PP_0489/PP_2185) and the molybdopterin oxidoreductase (PP_4676), confirming the predicted client relationships.

3. Gene knockout phenotyping. Construct a ΔPP_3457 mutant and assay formate dehydrogenase and molybdopterin-oxidoreductase activities; loss of activity would establish MobA's in vivo requirement, paralleling the M. tuberculosis nitrate-reductase result.

4. Structural determination. Solve the crystal or cryo-EM structure of Q88HA3 (± GTP, ± Mo-MPT), or validate an AlphaFold model against the E. coli MobA structures (PDB 1FR9/1E5K family), to confirm the two-domain Rossmann-fold architecture and the three GTP-binding motifs plus the acceptor-binding C-terminal surface.

5. MobB-independence test. Determine whether P. putida molybdoenzyme maturation proceeds efficiently without MobB, and search for a functional substitute; complement with heterologous E. coli MobB to see if activation improves.

6. MobA/MocA branch mapping. Use targeted mutants (ΔPP_3457, ΔPP_2483, ΔPP_4230) plus cofactor extraction/LC-MS (MGD vs MCD) to assign which molybdoenzymes depend on the guanylyl (MobA) vs cytidylyl (MocA) branch.

7. Expression/regulation profiling. Measure PP_3457 transcription under aerobic vs anaerobic and formate-rich conditions to connect MobA cofactor supply with client-enzyme demand.

Conclusion

MobA (Q88HA3 / PP_3457) is the Pseudomonas putida KT2440 molybdenum cofactor guanylyltransferase that performs the fourth, bacteria-specific step of Moco biosynthesis — transferring GMP from GTP onto molybdopterin to build the bis-MGD cofactor in the cytoplasm and deliver it to DMSO-reductase-family molybdoenzymes (in this organism, formate dehydrogenases and a molybdopterin oxidoreductase). The assignment is a robust homology inference anchored on the biochemically and structurally characterized E. coli MobA, reinforced by conserved catalytic motifs and organism-specific genomic evidence showing a streamlined, MobB-less, monocistronic arrangement. Direct biochemical and genetic validation in P. putida remains the key outstanding work.

Artifacts

• OpenScientist final report
• OpenScientist final report

Citations

1. PMID:10978347
2. PMID:11080634
3. PMID:25404027
4. PMID:21081498
5. PMID:24003231
6. PMID:32239579
7. PMID:26468212
8. PMID:12682065
9. PMID:25514355
10. PMID:31517366
11. PMID:38631442
12. PMID:31655739
13. PMID:28098827