Gene: moaC (ordered locus PP_1292)
Protein: Cyclic pyranopterin monophosphate synthase (EC 4.6.1.17)
Organism: Pseudomonas putida (strain ATCC 47054 / DSM 6125 / KT2440)
UniProt: Q88NC0 · Family: MoaC (HAMAP MF_01224) · Length: 156 aa
moaC encodes cyclic pyranopterin monophosphate (cPMP) synthase, a cytoplasmic
lyase that catalyzes the second half of the first, committed step of molybdenum
cofactor (Moco) biosynthesis. Working immediately downstream of the radical-SAM
enzyme MoaA, MoaC converts the cyclic nucleotide (8S)-3′,8-cyclo-7,8-dihydroguanosine
5′-triphosphate (3′,8-cH₂GTP) into cyclic pyranopterin monophosphate (cPMP, "precursor Z"),
releasing diphosphate (EC 4.6.1.17). It is this reaction that builds the characteristic
pyranopterin ring and the terminal cyclic phosphate that ultimately coordinate
molybdenum in every molybdoenzyme. The enzyme functions as a homohexamer (trimer of
dimers) in the cytoplasm, with composite active sites at subunit interfaces.
The gene identity is unambiguous: the UniProt catalytic annotation, the MoaC/HAMAP family
assignment, the conserved active-site motif, and the InterPro domain complement all agree,
and the P. putida protein is a canonical, full-length MoaC ortholog of the extensively
characterized E. coli enzyme.
| Attribute | Finding |
|---|---|
| Protein name | Cyclic pyranopterin monophosphate synthase (UniProt recommended name) |
| EC number | 4.6.1.17 (a carbon–oxygen lyase acting on phosphates) |
| Family / rule | MoaC family; HAMAP MF_01224 |
| InterPro domains | MoaC (IPR023045), MoaC_bact/euk (IPR047594), MoaC_sf (IPR036522), MoaA/MoaC (IPR050105), Molybdenum-cofactor-biosynthesis-C domain (IPR002820) |
| Length | 156 aa (typical for bacterial MoaC, ~17 kDa) |
| Active-site motif | Conserved cyclohydrolase-type "…LCHPLM…" segment present in the sequence |
| Catalytic reaction (UniProt) | 3′,8-cH₂GTP = cyclic pyranopterin phosphate + diphosphate |
| Quaternary structure (UniProt) | Homohexamer, trimer of dimers |
Conclusion: Q88NC0 is a bona fide MoaC. All literature cited below concerns the same
MoaC / cPMP-synthase protein family (bacterial orthologs and the human homolog MOCS1B),
so there is no gene-symbol ambiguity for this target.
Quantitative homology evidence (this work): A pairwise alignment of Q88NC0 against the
experimentally and structurally characterized E. coli MoaC (P0A738) shows 72.7% amino-acid
identity (112/154 aligned positions), and the catalytically essential E. coli Asp128
aligns with an aspartate in the P. putida sequence. KEGG independently assigns PP_1292 to
ortholog K03637 (cyclic pyranopterin monophosphate synthase, EC 4.6.1.17), Pfam MoaC,
and module M00880 ("Molybdenum cofactor biosynthesis, GTP => molybdenum cofactor"). This
high identity plus conservation of the catalytic residue provides strong homology-based support
for transferring the E. coli function to this specific P. putida protein.
Genomic context (this work): PP_1292 (moaC, 1,478,937–1,479,407) lies in a contiguous,
same-strand moaC–moaD–moaE cluster: PP_1293 = moaD (molybdopterin synthase small subunit)
and PP_1294 = moaE (molybdopterin synthase large subunit). moaC and moaD overlap by ~4 bp
and moaD/moaE by 3 bp, indicating translational coupling / operonic organization. Thus
the gene for pathway step 1 (MoaC → cPMP) is co-located and co-transcribed with the genes for
step 2 (MoaD/MoaE, cPMP → molybdopterin) — genomic evidence that MoaC's product is directly
handed to the downstream enzymes of Moco biosynthesis.
The first step of Moco biosynthesis is a remarkable transformation of GTP into cPMP that
is carried out by two enzymes acting in sequence:
Historically MoaC was thought to play only an accessory role (pyrophosphate release / cyclic
phosphate formation). The isolation of the MoaA product 3′,8-cH₂GTP and its efficient
conversion to cPMP by MoaC (and by the human homolog MOCS1B) redefined MoaC as the enzyme
that performs the majority of the pyranopterin-forming chemistry — "in contrast to previous
proposals, MoaC plays a major role in the complex rearrangement to generate the pyranopterin
ring" [Hover 2013, PMID 23627491].
MoaC is highly specific for the cyclic nucleotide substrate: 3′,8-cH₂GTP is converted to cPMP
with Km < 0.060 μM for bacterial MoaC (0.79 ± 0.24 μM for human MOCS1B) [Hover 2013,
PMID 23627491]. Mechanistic probing with an uncleavable substrate analogue (3′,8-cH₂GMP[CH₂]PP)
showed that the early catalytic steps proceed before cyclic-phosphate formation, and that
partially active MoaC variants generate a defined reaction intermediate — providing direct
evidence on the ordering of bond-forming events in the pyranopterin rearrangement
[Hover 2015, PMID 26575208].
The E. coli MoaC crystal structure identified a conserved active-site pocket at the
dimer interface; Asp128 is catalytically essential (Asp128→Ala nearly abolishes activity),
and a substitution in the human ortholog (Thr182→Pro) that causes Moco deficiency maps to the
same region [Wuebbens 2000, PMID 10903949]. GTP-bound MoaC structures (e.g., from Thermus
thermophilus) further map the substrate-binding residues [Kanaujia 2010, PMID 20606263].
MoaC catalyzes step 1 of the four-step bacterial Moco pathway [Mendel & Leimkühler 2015,
PMID 24980677; Schwarz & Mendel 2006, PMID 16669776]:
Moco is the essential catalytic cofactor of >50 molybdoenzymes, in which a pyranopterin
coordinates the molybdenum atom at the active site [Magalon & Mendel 2015, PMID 26435257].
These enzymes carry out key oxygen-atom-transfer and redox reactions in global carbon, nitrogen
and sulfur metabolism — e.g., nitrate reductase, DMSO/TMAO reductase, sulfite oxidase,
xanthine dehydrogenase/oxidase, and formate dehydrogenase. In a metabolically versatile soil
bacterium such as P. putida KT2440, Moco-dependent enzymes support alternative/anaerobic
respiration and nitrogen and sulfur metabolism; MoaC is therefore the gatekeeping biosynthetic
step that activates all of this molybdenum-dependent chemistry. Because cPMP synthesis is the
entry point of the pathway, loss of MoaC function would abolish Moco production and inactivate
the entire downstream molybdoenzyme repertoire. In humans, defects in the orthologous first step
cause fatal Molybdenum Cofactor Deficiency, underscoring the step's essentiality
[Wuebbens 2000, PMID 10903949].
Pathway completeness in P. putida KT2440 (this work): KEGG ortholog mapping confirms the
organism encodes the entire Moco biosynthetic machinery — moaA (three paralogs: PP_1969,
PP_2482, PP_4597), moaC (single copy, PP_1292), moaD (PP_1293), moaE (PP_1294), moeA
(PP_2123, Mo insertase) and mogA (PP_3457) — i.e., every step needed to convert GTP into mature
Moco (KEGG module M00880). Notably, MoaC is present as a single, non-redundant copy, so
PP_1292 is the sole cyclic-pyranopterin-monophosphate synthase in the cell and an obligatory
node of the pathway; the three MoaA paralogs contrast with this single MoaC. The presence of all
downstream enzymes confirms that MoaC's product (cPMP) is channeled through a complete, functional
pathway to activated molybdenum cofactor.
| Hypothesis | Status | Basis |
|---|---|---|
| Q88NC0 is a canonical MoaC / cPMP synthase | Supported | UniProt catalytic annotation, HAMAP MF_01224, InterPro domains, conserved CHPL motif, 156-aa length |
| Substrate is the cyclic nucleotide 3′,8-cH₂GTP (product of MoaA), not free GTP | Supported | Hover 2013 (PMID 23627491) |
| MoaC performs the pyranopterin ring-forming rearrangement (not just PPi release) | Supported (revised historical view) | Hover 2013/2015 |
| MoaC functions as an oligomer with interfacial active sites | Supported | Wuebbens 2000 (PMID 10903949); UniProt homohexamer |
| The reaction occurs in the cytoplasm as step 1 of Moco biosynthesis | Supported | Mendel & Leimkühler 2015; Schwarz & Mendel 2006 |
| MoaC has a molybdenum-independent "moonlighting" primary role | Not supported | No evidence; all data point to Moco biosynthesis as the sole primary function |
| PP_1292 is functionally equivalent to E. coli MoaC | Supported | 72.7% identity, conserved catalytic Asp, KEGG K03637/M00880 (this work) |
| PP_1292 is organized in a moa operon in P. putida | Supported | Contiguous, overlapping moaC–moaD–moaE cluster (this work) |