AprA is the alpha (catalytic) subunit of adenylylsulfate (APS) reductase (EC 1.8.99.2), the central enzyme in dissimilatory sulfate reduction. AprA contains the catalytic FAD cofactor and forms an alpha-beta heterodimer with AprB (the beta subunit containing [4Fe-4S] clusters) to constitute the functional AprAB enzyme. AprAB catalyzes the reduction of adenosine 5'-phosphosulfate (APS) to AMP and sulfite, receiving electrons from the membrane-associated QmoABC complex which connects to the quinone pool. This enzyme sits between ATP sulfurylase (Sat) and dissimilatory sulfite reductase (DsrAB) in the dissimilatory sulfate reduction pathway. AprA is a soluble cytoplasmic protein that also associates peripherally with the plasma membrane through the Qmo-Apr complex.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000104 succinate dehydrogenase activity | IEA GO_REF:0000118 | REMOVE | Summary: TreeGrafter propagated succinate dehydrogenase activity with PANTHER node PTN000908678 as provenance. AprA shares the SdhA/FrdA/AprA fold but catalyzes APS reduction to AMP and sulfite, a distinct reaction. The historical graft topology has not been independently reconstructed; the catalytic mismatch is supported without asserting a particular placement error. Reason: AprA has the APS-reductase substrate chemistry, AprA-specific domain assignment and sulfate-reduction pathway context, rather than succinate dehydrogenation. Existing OpenScientist and Falcon reports corroborate this distinction. Their assertion that a covalent FAD-binding histidine is universally necessary for all SDH/FRD catalysis is stronger than needed here and is not used as decisive proof. The incorrect function is established independently of whether the historical PANTHER graft position or ancestral term mapping caused it. Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md APS reductase (EC 1.8.99.2) catalyzes APS + 2e− + H+ → AMP + sulfite (SO3(2−)). The AprA subunit houses the catalytic FAD, while AprB contains iron-sulfur clusters that mediate electron transfer file:DESVH/Q72DT2/Q72DT2-hypotheses/function-hypothesis-go-0000104/openscientist.md Verdict: REFUTED — Within-superfamily mis-placement (Failure mode 3). file:DESVH/Q72DT2/Q72DT2-hypotheses/function-hypothesis-go-0000104/openscientist.md UniProt assigns EC 1.8.99.2 (adenylyl-sulfate reductase). file:DESVH/Q72DT2/Q72DT2-hypotheses/function-hypothesis-go-0000104/falcon.md The AprA alpha subunit of dissimilatory adenosine-5′-phosphosulfate (APS) reductase shares a common fold with the flavoprotein subunit of succinate dehydrogenase (SDH/Complex II), but this structural relationship is explicitly "not reflected in sequence similarity" |
| GO:0005886 plasma membrane | IEA GO_REF:0000118 | ACCEPT | Summary: AprAB has a peripheral plasma-membrane-associated pool in D. vulgaris Hildenborough through its complex with Qmo. Its soluble topology is compatible with this location. Reason: PMID:23842468 reports AprA and AprB together with Qmo subunits in membrane preparations from the exact strain, supported by reciprocal affinity purification. This establishes more than a predicted electron-transfer partnership. GO:0005886 includes the phospholipid bilayer and associated proteins; it does not require a transmembrane helix, exclusive localization, or a measured membrane residence time. The OpenScientist report recovered this evidence but incorrectly treated peripheral association as excluding the broad term. Its suggested extrinsic-membrane term is a descendant through part_of/is_a relations. Retain the original annotation, without changing its electronic evidence code or claiming integral membrane insertion. Supporting Evidence: PMID:23842468 another band contained three subunits of Qmo, as well as subunits of AprA and AprB. file:DESVH/Q72DT2/Q72DT2-hypotheses/function-hypothesis-go-0005886/openscientist.md There is genuine **experimental** evidence for a **peripheral, partially stable membrane-associated pool** of AprAB file:DESVH/Q72DT2/Q72DT2-notes.md GO:0005886 includes associated proteins; GO:0031234 reaches it through part_of/is_a relations. |
| GO:0009055 electron transfer activity | IEA GO_REF:0000118 | ACCEPT | Summary: AprA directly transfers reducing equivalents from its catalytic FAD to APS during sulfate respiration. Reason: Live QuickGO rechecked on 2026-09-23 defines directed electron movement between molecular entities and explicitly includes redox transformations in enzymatic metabolic reactions. It does not restrict GO:0009055 to a separate electron-transport carrier. In AprAB, AprB supplies the Fe-S relay while AprA transfers reducing equivalents from catalytic FAD to the APS substrate; this specifies the electron donor/acceptor chemistry, rather than inferring this term merely from an oxidoreductase label. Retain the original electronic annotation under that current broad definition. The exact API response is preserved in apra-membrane-term-check.json and quoted in the notes. Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md The AprA subunit houses the catalytic FAD, while AprB contains iron-sulfur clusters that mediate electron transfer, consistent with a flavoprotein-iron–sulfur oxidoreductase mechanism file:DESVH/Q72DT2/Q72DT2-notes.md The current GO:0009055 definition includes redox transformations during enzymatic reactions; AprA performs the FAD-dependent reduction of APS. |
| GO:0009061 anaerobic respiration | IEA GO_REF:0000118 | MODIFY | Summary: This annotation is accurate but insufficiently specific. AprA functions specifically in dissimilatory sulfate reduction, a type of anaerobic respiration where sulfate serves as the terminal electron acceptor. In DvH's dissimilatory sulfate reduction, sulfate is activated by ATP sulfurylase (Sat) to APS, reduced by AprAB to sulfite, and then by dissimilatory sulfite reductase (DsrAB) to sulfide. Reason: While anaerobic respiration is technically correct, the more specific term GO:0019420 (dissimilatory sulfate reduction) precisely captures AprA's biological role. AprA catalyzes a key step in this pathway where APS is reduced to sulfite. Proposed replacements: dissimilatory sulfate reduction Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md In DvH’s dissimilatory sulfate reduction, sulfate is activated by ATP sulfurylase (Sat) to APS, reduced by AprAB to sulfite, and then by dissimilatory sulfite reductase (DsrAB) to sulfide file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md AprA/AprAB sits between Sat (ATP sulfurylase) and DsrAB in the dissimilatory sulfate reduction chain, central to energy metabolism in DvH |
| GO:0050660 flavin adenine dinucleotide binding | IEA GO_REF:0000118 | ACCEPT | Summary: This annotation is accurate. AprA contains a FAD cofactor that is essential for its catalytic function. UniProt lists FAD as a cofactor (ChEBI:57692), and the deep research confirms that AprA contains the catalytic FAD. The protein has a FAD-binding domain (IPR003953, Pfam:PF00890). Reason: FAD binding is well-established for AprA based on domain architecture (FAD-binding_2 domain) and functional characterization. The FAD cofactor is central to the catalytic mechanism of APS reduction. Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md The AprA subunit houses the catalytic FAD |
| GO:0009973 adenylyl-sulfate reductase activity | IEA GO_REF:0000003 | ACCEPT | Summary: This is the core molecular function of AprA. The annotation is based on EC mapping (EC:1.8.99.2), which is the correct enzyme classification for APS reductase. The deep research extensively supports this: AprA in DvH (DVU_0847; UniProt Q72DT2) encodes the alpha subunit of adenylyl-sulfate (APS) reductase. APS reductase (EC 1.8.99.2) catalyzes APS + 2e- + H+ -> AMP + sulfite. Reason: This is the primary molecular function of AprA, strongly supported by multiple lines of evidence including EC classification, domain architecture (IPR011803 AprA, TIGR02061), UniProt annotation, and extensive literature. Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md AprA is the alpha subunit of APS reductase (AprAB). Together with AprB, it catalyzes reduction of APS to sulfite and AMP; AprA contains the catalytic FAD cofactor file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md In DvH proteomics, AprA is explicitly mapped to DVU_0847, confirming the gene-protein assignment in this organism |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Oxidoreductase activity describes the core APS-reduction chemistry at a broader level than adenylyl-sulfate reductase activity. Reason: AprA catalyzes the reduction of APS through its FAD-containing active site. GO:0009973 provides the more specific reaction, while GO:0016491 remains a correct description of the same core catalytic work. A valid broad parent is not an over-annotation simply because a more specific enzyme term coexists. Supporting Evidence: file:DESVH/Q72DT2/Q72DT2-deep-research-falcon.md APS reductase (EC 1.8.99.2) catalyzes APS + 2e− + H+ → AMP + sulfite (SO3(2−)) |
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