AprB is the beta subunit of adenylylsulfate (APS) reductase (EC 1.8.99.2) in the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough. The AprAB heterodimer catalyzes the two-electron reduction of adenosine 5'-phosphosulfate (APS) to sulfite and AMP, a central step in dissimilatory sulfate reduction. AprB is a small ferredoxin-like protein containing two [4Fe-4S] clusters that function as an intramolecular electron relay, transferring electrons from the membrane-associated QmoABC complex to the FAD cofactor in the catalytic alpha subunit AprA. AprB is a soluble cytoplasmic protein that transiently interacts with the membrane Qmo complex during electron transfer (KD approximately 90 nM).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0009973 adenylyl-sulfate reductase activity | IEA GO_REF:0000003 | MODIFY | Summary: This annotation is derived from EC number mapping (EC:1.8.99.2). AprB is indeed a subunit of the APS reductase enzyme, but AprB itself is the electron-transfer subunit containing two [4Fe-4S] clusters, not the catalytic subunit. The catalytic activity (APS reduction to sulfite and AMP) resides in AprA which contains the FAD cofactor. AprB's function is to relay electrons to AprA's FAD active site (Meyer & Kuever 2007, Meyer & Kuever 2008). Reason: While AprB is essential for APS reductase function, the catalytic activity per se is mediated by the FAD-containing AprA subunit. AprB should be annotated with electron transfer activity rather than the catalytic adenylyl-sulfate reductase activity. The beta subunit serves as the electron conduit to the active site, which is a distinct molecular function from the catalytic chemistry itself. Proposed replacements: electron transfer activity Supporting Evidence: DOI:10.1099/mic.0.2006/003152-0 reductases are highly conserved among SRP and SOB and form heterodimers with one alpha subunit (75β80 kDa, one FAD) and one beta subunit (18β23 kDa, two [4Feβ4S] centres) which are encoded by the aprBA gene loci DOI:10.1371/journal.pone.0001514 Its beta-subunit can be subdivided in three segments comprising a bacterial ferredoxin-like segment that envelopes both [4Fe-4S] clusters (amino acids B1-B68) file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB's two [4Fe-4S] clusters form the intramolecular electron-transport chain to supply reducing equivalents to the catalytic FAD. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000043 | MODIFY | Summary: This annotation derives from UniProtKB keyword mapping. While technically accurate (AprB participates in an oxidoreductase reaction as part of the AprAB complex), this term is too general and uninformative. AprB's specific role is electron transfer via its [4Fe-4S] clusters, not general oxidoreductase activity. Reason: GO:0016491 (oxidoreductase activity) is a very high-level term that provides little functional insight. AprB's molecular function is more precisely described as electron transfer activity (GO:0009055), as the protein mediates directional electron flow from QmoABC to AprA's FAD cofactor during APS reduction. Proposed replacements: electron transfer activity Supporting Evidence: DOI:10.1371/journal.pone.0001514 The two electrons required for the reduction of APS were postulated to be transferred one by one over 30 AΛ via [4Fe-4S] cluster II at the surface of the protein, cluster I and Trp-B48 to the isoalloxazine ring of the buried FAD [18,23,24] file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB's two [4Fe-4S] clusters form the intramolecular electron-transport chain to supply reducing equivalents to the catalytic FAD. |
| GO:0046872 metal ion binding | IEA GO_REF:0000043 | MODIFY | Summary: This annotation derives from UniProtKB keyword mapping (Metal-binding, Iron). AprB binds iron as part of its two [4Fe-4S] clusters. However, this term is too general - the protein binds iron specifically in the context of iron-sulfur clusters, not as free metal ions. Reason: While AprB does bind iron, this generic term fails to capture the biologically meaningful context. The iron binding is specifically within [4Fe-4S] clusters that are integral to the protein's electron transfer function. The more specific term GO:0051539 (4 iron, 4 sulfur cluster binding) is already annotated and is the appropriate level of specificity. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: DOI:10.1371/journal.pone.0001514 Its beta-subunit can be subdivided in three segments comprising a bacterial ferredoxin-like segment that envelopes both [4Fe-4S] clusters (amino acids B1-B68) file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB is a small ferredoxin-like Fe-S protein with two canonical [4Fe-4S] centers. |
| GO:0051536 iron-sulfur cluster binding | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation derives from UniProtKB keyword mapping (Iron-sulfur). AprB contains two [4Fe-4S] clusters essential for its electron transfer function. This term is accurate but less specific than GO:0051539 which is also annotated. Reason: This annotation is correct - AprB binds iron-sulfur clusters as confirmed by structural and biochemical studies. While GO:0051539 (4 iron, 4 sulfur cluster binding) is more specific and also correctly annotated, this parent term can be retained as it captures the general Fe-S binding capability. The dual annotation at both specificity levels is acceptable for IEA annotations. Supporting Evidence: DOI:10.1371/journal.pone.0001514 Its beta-subunit can be subdivided in three segments comprising a bacterial ferredoxin-like segment that envelopes both [4Fe-4S] clusters (amino acids B1-B68) file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB houses two [4Fe-4S] clusters that shuttle electrons to the catalytic site. |
| GO:0051539 4 iron, 4 sulfur cluster binding | IEA GO_REF:0000043 | ACCEPT | Summary: This annotation derives from UniProtKB keyword mapping (4Fe-4S). AprB specifically contains two [4Fe-4S] ferredoxin-type clusters (domains at positions 1-35 and 38-67 per UniProt). This is well-supported by domain analysis (PROSITE PS51379, IPR017896, IPR017900) and homology modeling studies (Meyer & Kuever 2008). Reason: This is the core molecular function annotation for AprB. The two [4Fe-4S] clusters are essential for the protein's electron transfer role, relaying electrons from membrane electron donors to the AprA catalytic subunit. This annotation is at the appropriate level of specificity. Supporting Evidence: DOI:10.1371/journal.pone.0001514 Its beta-subunit can be subdivided in three segments comprising a bacterial ferredoxin-like segment that envelopes both [4Fe-4S] clusters (amino acids B1-B68) DOI:10.1099/mic.0.2006/003152-0 reductases are highly conserved among SRP and SOB and form heterodimers with one alpha subunit (75β80 kDa, one FAD) and one beta subunit (18β23 kDa, two [4Feβ4S] centres) which are encoded by the aprBA gene loci file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB is a small ferredoxin-like Fe-S protein with two canonical [4Fe-4S] centers. |
| GO:0009055 electron transfer activity | ISS DOI:10.1371/journal.pone.0001514 | NEW | Summary: AprB functions as the electron transfer subunit of APS reductase, relaying electrons via its two [4Fe-4S] clusters from membrane-associated QmoABC to the FAD cofactor in AprA. This is the core molecular function of AprB. Reason: This annotation is not currently present but represents the primary molecular function of AprB. The protein's role as an electron relay between QmoABC and AprA is well-established through biochemical and structural studies. Supporting Evidence: DOI:10.1371/journal.pone.0001514 The two electrons required for the reduction of APS were postulated to be transferred one by one over 30 AΛ via [4Fe-4S] cluster II at the surface of the protein, cluster I and Trp-B48 to the isoalloxazine ring of the buried FAD [18,23,24] DOI:10.3389/fmicb.2012.00137 This showed that the QmoABCβAprAB complex has a strong steady-state afο¬nity (K D = 90 Β± 3 nM), but has a transient character due to a fast dissociation rate. file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprB's two [4Fe-4S] clusters form the intramolecular electron-transport chain to supply reducing equivalents to the catalytic FAD. |
| GO:0019420 dissimilatory sulfate reduction | ISS DOI:10.1099/mic.0.2006/003152-0 | NEW | Summary: AprB is an essential component of the dissimilatory sulfate reduction pathway in sulfate-reducing bacteria. The AprAB complex catalyzes the reduction of APS to sulfite, a central step in this energy-conserving pathway where sulfate serves as terminal electron acceptor. Reason: This biological process annotation would appropriately capture the pathway context in which AprB functions. Dissimilatory sulfate reduction is the core metabolic pathway of D. vulgaris and AprB is essential for this process. Supporting Evidence: DOI:10.1099/mic.0.2006/003152-0 After the activation of the chemically inert sulfate to adenosine-5 9-phosphosulfate (APS) by ATP sulfurylase (Sat), the second enzyme, APS reductase (Apr), converts APS to AMP and sulfite, which is finally reduced to sulfide by the activity of the sulfite reductase (Dsr) file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md The reaction is central and conserved among sulfate-reducing prokaryotes (SRP). |
| GO:0005737 cytoplasm | ISS DOI:10.3389/fmicb.2012.00137 | NEW | Summary: AprAB is a soluble cytoplasmic enzyme that transiently associates with the membrane-bound QmoABC complex during electron transfer. The enzyme is not membrane-integral. Reason: This cellular component annotation accurately describes the localization of AprB. Ramos et al. (2012) establish that AprAB is soluble and not membrane integral; Meyer & Kuever (2008) place the dissimilatory APS reductase in the cytoplasm or at the cytoplasmic face of the inner membrane, with QmoABC acting as the electron carrier between the quinone pool and the cytoplasmic enzyme. The protein is therefore cytoplasmic/soluble but functionally interacts with membrane complexes. Supporting Evidence: DOI:10.3389/fmicb.2012.00137 The two terminal reductases, APS reductase (AprAB) and dissimilatory sulο¬te reductase (DsrAB), are soluble and thus not directly involved in membrane-linked electron transport. file:DESVH/Q72DT3/Q72DT3-deep-research-falcon.md AprAB is a soluble cytoplasmic enzyme (not membrane integral). |
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Download this section (compressed HTML)Q: Has the electron transfer pathway through AprB's two [4Fe-4S] clusters been characterized spectroscopically (e.g., EPR) in D. vulgaris specifically?
Q: What are the redox potentials of the individual [4Fe-4S] clusters in AprB, and how do they compare to homologous proteins?
Q: What is the stoichiometry of the AprAB heterodimer - is it strictly 1:1 alpha:beta?
Experiment: Site-directed mutagenesis of conserved cysteine residues coordinating the [4Fe-4S] clusters to confirm their essential role in electron transfer
Hypothesis: Mutation of cysteine residues coordinating the [4Fe-4S] clusters will abolish electron transfer activity and APS reductase function
Experiment: EPR spectroscopy to characterize the redox states and properties of the individual iron-sulfur clusters
Hypothesis: The two [4Fe-4S] clusters will show distinct redox potentials consistent with their role in directional electron transfer
Experiment: In vivo complementation studies with aprB deletion mutants to confirm essentiality for sulfate reduction
Hypothesis: Deletion of aprB will abolish sulfate reduction capability, which can be restored by complementation with wild-type aprB
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