Flavoprotein (catalytic) subunit of succinate dehydrogenase (respiratory Complex II) in Pseudomonas putida KT2440. SdhA carries a covalently bound FAD cofactor and the succinate/fumarate active site, catalysing the oxidation of succinate to fumarate with transfer of electrons into the membrane quinone pool (EC 1.3.5.1). This reaction couples the tricarboxylic acid (TCA) cycle to the aerobic respiratory electron transport chain. SdhA forms the soluble catalytic head of the four-subunit enzyme together with the iron-sulfur subunit SdhB (PP_4190) and the membrane anchor subunits SdhD (PP_4192) and SdhC (PP_4193), to which it is peripherally attached on the cytoplasmic face of the inner (plasma) membrane. FAD incorporation (flavinylation) depends on the accessory assembly factor SdhE.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000104 succinate dehydrogenase activity | IEA GO_REF:0000118 | ACCEPT | Summary: SdhA is the flavoprotein catalytic subunit of succinate dehydrogenase (Complex II); succinate dehydrogenase activity is its core molecular function. Reason: Strongly supported by family/domain assignment (TIGR01816 sdhA_forward, Pfam FAD_binding_2, FRD/SDH subfamily), conserved active-site and FAD-binding residues, and UniProt EC 1.3.5.1. Consistent across all lines of evidence. |
| GO:0005886 plasma membrane | IEA GO_REF:0000120 | ACCEPT | Summary: SdhA is a peripheral membrane protein attached to the cytoplasmic (inner) face of the inner/plasma membrane as part of the membrane-bound Complex II. Reason: Matches UniProt subcellular location (Cell inner membrane; peripheral membrane protein; cytoplasmic side). In Gram-negative bacteria the inner membrane is the GO plasma membrane. |
| GO:0006099 tricarboxylic acid cycle | IEA GO_REF:0000120 | ACCEPT | Summary: Succinate dehydrogenase catalyses the succinate-to-fumarate step of the TCA cycle; this is a core biological process for SdhA. Reason: Supported by UniProt pathway annotation (tricarboxylic acid cycle; fumarate from succinate, step 1/1) and conserved enzyme function. |
| GO:0008177 succinate dehydrogenase (quinone) activity | IEA GO_REF:0000120 | MARK AS OVER ANNOTATED | Summary: This is the precise quinone-coupled reaction (succinate + quinone = fumarate + quinol, RHEA:40523, EC 1.3.5.1) catalysed by the holo-enzyme to which SdhA contributes the catalytic flavoprotein head. Reason: SdhA performs the FAD-dependent succinate-oxidation half-reaction, but quinone reduction requires the SdhB electron-transfer chain and SdhC/D membrane domain. The term is correct for the assembled SdhABCD complex and should be represented as a contribution for this subunit. |
| GO:0009055 electron transfer activity | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: SdhA participates in electron transfer, passing electrons abstracted from succinate via FAD toward the iron-sulfur clusters of SdhB and the quinone pool. Reason: Biologically true but more generic than the subunit-level succinate dehydrogenase activity that captures SdhA's core function. Retain as supporting/non-core rather than primary. |
| GO:0009061 anaerobic respiration | IEA GO_REF:0000118 | MARK AS OVER ANNOTATED | Summary: This subunit is the forward/aerobic succinate dehydrogenase flavoprotein (TIGR01816 sdhA_forward), not the fumarate reductase used in anaerobic respiration. Reason: UniProt notes that two distinct FAD enzymes interconvert fumarate and succinate, with fumarate reductase (FrdA) used in anaerobic growth and succinate dehydrogenase used in aerobic growth. The forward SdhA is assigned to aerobic respiration; this TreeGrafter-propagated anaerobic respiration term reflects the broader SdhA/FrdA family and over-annotates the aerobic SdhA. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Generic parent of the specific subunit-level succinate dehydrogenase activity already annotated. Reason: Correct but uninformative high-level term; subsumed by GO:0000104 succinate dehydrogenase activity for SdhA. Retain as non-core. |
| GO:0016627 oxidoreductase activity, acting on the CH-CH group of donors | IEA GO_REF:0000120 | KEEP AS NON CORE | Summary: Accurate intermediate-level description of the chemistry (oxidation of the succinate CH-CH bond to the fumarate C=C bond), but less specific than SdhA's succinate dehydrogenase activity. Reason: Correct grouping term but subsumed by the specific MF term; keep as non-core supporting annotation. |
| GO:0022900 electron transport chain | IEA GO_REF:0000120 | ACCEPT | Summary: Complex II feeds electrons from succinate into the respiratory quinone pool, contributing to the aerobic electron transport chain. Reason: Supported by UniProt (electron transport keyword) and the established role of Complex II linking the TCA cycle to respiration. |
| GO:0050660 flavin adenine dinucleotide binding | IEA GO_REF:0000120 | ACCEPT | Summary: SdhA binds a (covalently attached) FAD cofactor essential for succinate oxidation. Reason: Supported by UniProt COFACTOR (FAD), multiple conserved FAD-binding residues, the Tele-8alpha-FAD histidine modified residue, and SdhE-dependent flavinylation. |
| GO:0160308 succinate dehydrogenase (FAD) activity | IEA GO_REF:0000002 | ACCEPT | Summary: Describes the FAD-dependent succinate->fumarate half-reaction occurring at the SdhA flavin site, prior to electron transfer to quinone. Reason: Accurately captures the FAD-coupled catalytic step intrinsic to the SdhA subunit and complements GO:0000104. SdhA contributes to, but does not independently enable, the holo-enzyme quinone-coupled activity GO:0008177. |
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Download this section (compressed HTML)Q: Has the SdhA-SdhBCD Complex II of P. putida KT2440 been biochemically characterised (kinetics, FAD content, quinone specificity), and does it show any fumarate reductase activity in vitro?
Experiment: Purify the P. putida KT2440 SdhABCD complex and measure succinate:quinone oxidoreductase kinetics and FAD flavinylation status, with and without SdhE, to confirm the conserved mechanism in this organism.
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