MoaA is the radical S-adenosyl-L-methionine GTP 3',8-cyclase that catalyses the first step of molybdenum cofactor biosynthesis in Escherichia coli. It carries two [4Fe-4S] clusters: an N-terminal radical SAM cluster that binds and reductively cleaves SAM to a 5'-deoxyadenosyl radical, and a C-terminal auxiliary cluster that binds the guanine of the GTP substrate. MoaA converts GTP to (8S)-3',8-cyclo-7,8-dihydroguanosine 5'-triphosphate (3',8-cH2GTP), which MoaC then rearranges to cyclic pyranopterin monophosphate (cPMP, precursor Z); cPMP is subsequently converted to molybdopterin and Mo-molybdopterin for nitrate reductase, formate dehydrogenase and other molybdoenzymes. The [4Fe-4S] clusters are delivered by the A-type carriers ErpA and IscA, and moaA mutants (historically chlA) lack the molybdenum cofactor.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | MODIFY | Summary: Root catalytic-activity term from InterPro radical SAM signatures. MoaA has a known specific reaction, GTP 3',8'-cyclase. Reason: Per the RADICAL_SAM treatment rule, the generic catalytic-activity row is replaced by the specific reaction term the gene already carries (GO:0061798, accepted below from IBA, IEA and IGI rows). Proposed replacements: GTP 3',8'-cyclase activity Supporting Evidence: PMID:23627491 Instead, 3′,8-cH2GTP is transferred to MoaC where a complex rearrangement reaction proceeds. |
| GO:0005525 GTP binding | IEA GO_REF:0000104 | KEEP AS NON CORE | Summary: GTP is the substrate of MoaA and binds to the C-terminal [4Fe-4S] cluster and a set of conserved arginines. Reason: Substrate binding is true (shown structurally for the S. aureus ortholog and encoded by the conserved HAMAP binding-site residues of P30745), but it is a component of the cyclase activity rather than a separate function. Kept as non-core, as in PSEPK/moaA. Supporting Evidence: PMID:16632608 MoaA binds 5'-GTP with high affinity and interacts through its C-terminal [4Fe-4S] cluster with the guanine N1 and N2 atoms |
| GO:0006777 Mo-molybdopterin cofactor biosynthetic process | IBA GO_REF:0000033 | ACCEPT | Summary: MoaA catalyses the first committed reaction of Moco biosynthesis, so it is directly involved in Mo-molybdopterin cofactor biosynthesis. Reason: MoaA performs a step of the process (GTP cyclization), passing the participation test. The IBA node covers MoaA, MOCS1A and plant CNX2, all GTP 3',8-cyclases; the target appearing among the sources reflects its own experimental grounding. Supporting Evidence: PMID:14761975 precursor Z purified from Escherichia coli overproducing MoaA and MoaC, two proteins essential for bacterial precursor Z synthesis |
| GO:0006777 Mo-molybdopterin cofactor biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: Same claim as the IBA row; MoaA catalyses the first step of Moco biosynthesis. Reason: Consistent with the IBA and IMP rows and with the biochemistry. Supporting Evidence: file:ECOLI/moaA/moaA-uniprot.txt FUNCTION: Catalyzes, together with MoaC, the conversion of 5'-GTP to cyclic pyranopterin monophosphate (cPMP or molybdopterin precursor Z). |
| GO:0006777 Mo-molybdopterin cofactor biosynthetic process | IMP PMID:1090592 Synthesis of nitrate reductase components in chlorate-resist... | ACCEPT | Summary: Classic chlorate-resistance genetics; chlA mutants have intact nitrate reductase subunits but lack the molybdenum-containing factor. Reason: chlA is a historical synonym of moaA. The chlA locus covers the moa operon, so not every old chlA allele need be in moaA itself, but the curator assigned this to moaA and the claim (moaA is required for and participates in Moco synthesis) is correct. Abstract-only cache; deferring to the curator. Supporting Evidence: PMID:1090592 The chlA and B loci are suggested to function in the synthesis and attachment of a molybdenum-containing factor. |
| GO:0006777 Mo-molybdopterin cofactor biosynthetic process | IMP PMID:2947896 Involvement of chlA, E, M, and N loci in Escherichia coli mo... | ACCEPT | Summary: Molybdopterin-deficient mutants mapping to chlA lacked molybdoenzyme activity and form A. Reason: Genetic loss-of-function evidence consistent with the biochemical role of MoaA. Supporting Evidence: PMID:2947896 By P1 mapping, these two mutations mapped to chlA and chlE, loci previously postulated but never definitely shown to be involved in MPT biosynthesis. |
| GO:0006777 Mo-molybdopterin cofactor biosynthetic process | IMP PMID:6307982 Molybdenum cofactor in chlorate-resistant and nitrate reduct... | ACCEPT | Summary: chlA insertion mutants have no detectable molybdenum cofactor. Reason: Genetic loss-of-function evidence consistent with the biochemical role of MoaA. Supporting Evidence: PMID:6307982 The chlA and chlE mutants had no detectable cofactor. |
| GO:0009408 response to heat | IMP PMID:24580753 Inferring gene function from evolutionary change in signatur... | MARK AS OVER ANNOTATED | Summary: From a survival screen of E. coli deletion strains after heat shock, for genes chosen by codon-usage signatures in thermophiles. Reason: Reduced heat-shock survival of a deletion strain is an indirect, pleiotropic phenotype. MoaA does no work in a heat response; its only established activity is GTP cyclization for Moco synthesis, and loss of molybdoenzymes could plausibly lower stress tolerance indirectly. The term is not refuted, but it over-states the gene's role. Supporting Evidence: PMID:24580753 Our experiments indicated a heat shock-specific protective role for 19 of 23 candidate genes |
| GO:0046872 metal ion binding | IEA GO_REF:0000002 | MODIFY | Summary: The iron of MoaA is bound as two [4Fe-4S] clusters; there is no separate metal ion. Reason: Per the RADICAL_SAM treatment rule, a lateral correction to GO:0051539 (the iron is bound as clusters, and GO:0046872 is not an ancestor of the cluster terms). Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: file:ECOLI/moaA/moaA-uniprot.txt Binds 2 [4Fe-4S] clusters. |
| GO:0051536 iron-sulfur cluster binding | IEA GO_REF:0000002 | MODIFY | Summary: Correct but general; both MoaA clusters are [4Fe-4S]. Reason: Per the RADICAL_SAM treatment rule, refined to the descendant GO:0051539, which the gene already carries. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: PMID:15317939 The lateral opening of the incomplete triosephosphate isomerase barrel is covered by the C-terminal part of the protein containing an additional [4Fe-4S] cluster, which is unique to MoaA proteins. |
| GO:0051539 4 iron, 4 sulfur cluster binding | IEA GO_REF:0000120 | ACCEPT | Summary: MoaA binds two [4Fe-4S] clusters, the radical SAM cluster and the substrate-binding C-terminal cluster; both are catalytic cofactors. Reason: Core cofactor binding for an active radical SAM enzyme. In E. coli, ErpA and IscA deliver the [4Fe-4S] clusters to MoaA. Supporting Evidence: PMID:33782054 We show that ErpA and IscA can partially replace each other in their role to provide [4Fe-4S] clusters for MoaA. |
| GO:0051539 4 iron, 4 sulfur cluster binding | ISO PMID:15317939 Crystal structure of the S-adenosylmethionine-dependent enzy... | ACCEPT | Summary: Transfer from the crystal structure of Staphylococcus aureus MoaA, which shows both [4Fe-4S] clusters. Reason: All six cluster-ligating cysteines are conserved in P30745 (UniProt binding sites 24/28/31 and 257/260/274), so the structural evidence transfers. Supporting Evidence: PMID:15317939 The central core of each subunit of the MoaA dimer is an incomplete triosephosphate isomerase barrel formed by the N-terminal part of the protein, which contains the [4Fe-4S] cluster typical for SAM-dependent radical enzymes. |
| GO:0061798 GTP 3',8'-cyclase activity | IBA GO_REF:0000033 | ACCEPT | Summary: The core activity of MoaA, inherited across the MoaA/MOCS1A clade. Reason: GTP 3',8'-cyclase activity is the defining MoaA reaction; the IBA node is well placed and the target appears among its own sources because it has experimental grounding. Supporting Evidence: PMID:23627491 Instead, 3′,8-cH2GTP is transferred to MoaC where a complex rearrangement reaction proceeds. |
| GO:0061798 GTP 3',8'-cyclase activity | IEA GO_REF:0000120 | ACCEPT | Summary: HAMAP/EC mapping to the GTP 3',8-cyclase reaction (RHEA:49576). Reason: Consistent with the IBA and IGI rows and the biochemistry. Supporting Evidence: file:ECOLI/moaA/moaA-uniprot.txt FUNCTION: Catalyzes the cyclization of GTP to (8S)-3',8-cyclo-7,8- dihydroguanosine 5'-triphosphate. |
| GO:0061798 GTP 3',8'-cyclase activity | IGI PMID:14761975 The tetrahydropyranopterin structure of the sulfur-free and ... | ACCEPT | Summary: Co-overproduction of MoaA with MoaC (P0A738) in E. coli yields precursor Z (cPMP), whose structure was solved. Reason: The cyclase step is the MoaA half of the MoaA-MoaC conversion of GTP to cPMP; the genetic interaction with moaC is how the curator captured that partnership. Abstract-only cache; deferring to the curator, and the claim is independently supported by PMID:23627491. Supporting Evidence: PMID:14761975 precursor Z purified from Escherichia coli overproducing MoaA and MoaC, two proteins essential for bacterial precursor Z synthesis |
| GO:0061799 cyclic pyranopterin monophosphate synthase activity | IBA GO_REF:0000033 | REMOVE | Summary: cPMP synthase activity is the MoaC reaction, not the MoaA reaction. Reason: The donors are human MOCS1 and mouse Mocs1, bifunctional fusion proteins whose C-terminal MOCS1B domain is a MoaC homolog that carries the cPMP synthase activity. The IBA was placed at the MoaA/MOCS1A node (PTN000539172), so the MOCS1B activity propagates to single-domain MoaA proteins that lack any MoaC domain. E. coli MoaA (329 aa) has only the radical SAM core and twitch domains; the rearrangement to cPMP is carried out by the separate MoaC protein, and MoaA cannot perform it. Same action as PSEPK/moaA. Propagation Review Root cause: PROPAGATION BAD Failure modes: ROLE CONFLATION Sources checked: PANTHER:PTN000539172 · PTN000539172 SUPPORTS SOURCE BUT NOT TARGET Node seeded by MOCS1 fusion proteins; their cPMP synthase activity resides in the MOCS1B (MoaC-homologous) domain, absent from single-domain MoaA descendants. UniProtKB:Q9NZB8 · MOCS1 SUPPORTS SOURCE BUT NOT TARGET Human MOCS1 is bifunctional (MOCS1A cyclase plus MOCS1B cPMP synthase); only the MOCS1A activity is homologous to MoaA. Supporting Evidence: PMID:23627491 In contrast to GTP cyclohydrolases I, MoaA is incapable of catalyzing the rearrangement reactions, presumably due to the lack of appropriate amino acid residues in the active-site. PMID:23627491 Instead, 3′,8-cH2GTP is transferred to MoaC where a complex rearrangement reaction proceeds. |
| GO:1904047 S-adenosyl-L-methionine binding | IEA GO_REF:0000104 | ACCEPT | Summary: SAM binds the unique iron of the N-terminal radical SAM cluster and is cleaved to the 5'-deoxyadenosyl radical that initiates GTP cyclization. Reason: Cosubstrate binding, true of every active radical SAM enzyme (RADICAL_SAM treatment rule). The SAM-binding residues seen in the S. aureus structure are conserved in P30745. Supporting Evidence: PMID:15317939 SAM is the fourth ligand to the cluster and binds to its unique Fe as an N/O chelate. |
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Download this section (compressed HTML)Q: Which physiological reductant (flavodoxin FldA or another electron donor) reduces the radical SAM cluster of E. coli MoaA in vivo, and is MoaA physically channelled to MoaC to protect the labile 3',8-cH2GTP intermediate?
Experiment: Test MoaA-MoaC interaction by anaerobic pull-down or crosslinking, and compare cPMP yield from coupled versus separately incubated enzymes with E. coli flavodoxin/flavodoxin reductase as reductant.
Hypothesis: MoaA and MoaC form a transient complex in E. coli that channels 3',8-cH2GTP.
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