TDO (Tryptophan 2,3-dioxygenase, AGAP002721) is the sole enzyme initiating L-tryptophan degradation via the kynurenine pathway in Anopheles gambiae. It is a heme-dependent dioxygenase that catalyzes the oxidative cleavage of the indole ring of L-tryptophan to produce N-formylkynurenine, the first and rate-limiting step of tryptophan catabolism. TDO is a homolog of the Drosophila vermilion gene and is essential for ommochrome eye pigment biosynthesis in insects. The gene maps to chromosome 2R (inside inversion 2Rb), encodes a 392 amino acid protein that assembles as a homotetramer, and is ubiquitously expressed across tissues with developmental regulation. Its enzymatic product feeds into the kynurenine pathway, which in mosquitoes is critical for processing tryptophan from blood meals, producing xanthurenic acid (a trigger of Plasmodium gametogenesis in the midgut), and maintaining midgut homeostasis. The A. gambiae TDO cDNA functionally complements Drosophila vermilion eye color mutants, confirming conserved function. The gene also serves as a visible phenotypic marker for mosquito germline transformation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004833 L-tryptophan 2,3-dioxygenase activity | IBA GO_REF:0000033 | ACCEPT | Summary: This IBA annotation is derived from phylogenetic inference, with supporting evidence from Drosophila vermilion (FBgn0003965) and other TDO orthologs. The A. gambiae protein shares 74% identity with Drosophila TDO and functional complementation has been demonstrated experimentally. Reason: TDO activity is the core molecular function of this protein. The phylogenetic inference is strongly supported by sequence conservation (74% identity to Drosophila), functional complementation of Drosophila vermilion mutants (PMID:9443379), and direct biochemical characterization of recombinant A. gambiae TDO showing Michaelis-Menten kinetics (PMID:18687401). Supporting Evidence: PMID:18687401 In the malaria vector Anopheles gambiae, tryptophan 2,3-dioxygenase (TDO) is the only enzyme able to initiate l-tryptophan degradation through the kynurenine pathway PMID:9443379 The successful genetic complementation by this construct demonstrated both the proper function of the tryptophan oxygenase product file:ANOGA/TDO/TDO-deep-research-bioreason-sft.md the active-site geometry and redox environment enable GO:0004833 tryptophan 2,3-dioxygenase activity |
| GO:0020037 heme binding | IBA GO_REF:0000033 | ACCEPT | Summary: This IBA annotation for heme binding is inferred from phylogenetic analysis and supported by the conserved heme-binding residue (His-313 axial ligand) and direct spectral analysis of the recombinant protein. Reason: Heme binding is integral to TDO catalysis. Paglino et al. (2008) purified recombinant A. gambiae TDO in its heme-ferric form and performed spectral analyses confirming heme incorporation. The active-site histidine serving as the proximal axial ligand for the heme iron is conserved. Supporting Evidence: PMID:18687401 Spectral analyses showed that the enzyme was purified in its heme-ferric form |
| GO:0004833 L-tryptophan 2,3-dioxygenase activity | IEA GO_REF:0000120 | ACCEPT | Summary: Redundant IEA annotation for TDO activity derived from combined automated methods (ARBA, InterPro, RHEA, EC number, UniRule). Correct but redundant with the IBA annotation above. Reason: The annotation is correct. TDO activity is experimentally validated by biochemical characterization (PMID:18687401) and functional complementation (PMID:9443379). Redundancy with IBA is expected and acceptable. Supporting Evidence: PMID:18687401 TDO converts l-tryptophan to N-formylkynurenine by catalyzing the heme-dependent oxidative opening of the substrate indole ring |
| GO:0006727 ommochrome biosynthetic process | IEA GO_REF:0000120 | ACCEPT | Summary: This IEA annotation places TDO in the ommochrome biosynthetic pathway, derived from UniRule mapping. TDO catalyzes the first committed step of ommochrome biosynthesis by diverting tryptophan into the kynurenine pathway. Reason: TDO is the first enzyme in the ommochrome biosynthetic pathway in insects. The Drosophila vermilion mutant (TDO loss-of-function) lacks ommochrome eye pigments. Functional complementation of vermilion by A. gambiae TDO confirms its role in ommochrome biosynthesis (PMID:9443379). Multiple studies on insect ommochrome pathways confirm TDO as the initiating step. Supporting Evidence: PMID:8969464 Since this gene is known to control adult eye color in other flies, its cloning from A. gambiae provides the basis for a dominant phenotypic marker for germline transformation PMID:9443379 The chimeric construct, expressed transiently in vermilion (tryptophan oxygenase) mutants of Drosophila melanogaster, partially rescued adult eye color |
| GO:0020037 heme binding | IEA GO_REF:0000120 | ACCEPT | Summary: Redundant IEA annotation for heme binding from combined automated methods. Correct but redundant with the IBA annotation. Reason: Correct. Heme binding is experimentally confirmed by spectral analysis of recombinant A. gambiae TDO (PMID:18687401). Supporting Evidence: PMID:18687401 Spectral analyses showed that the enzyme was purified in its heme-ferric form |
| GO:0046872 metal ion binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: This IEA annotation for metal ion binding is derived from InterPro domain mapping (IPR037217, Tryptophan/Indoleamine 2,3-dioxygenase-like superfamily). TDO binds iron as part of its heme prosthetic group. Reason: Technically correct because TDO binds heme iron, but the term is overly general. The more specific GO:0020037 (heme binding) already captures this function accurately. Metal ion binding is not independently informative for this protein. Supporting Evidence: PMID:18687401 Spectral analyses showed that the enzyme was purified in its heme-ferric form |
| GO:0004833 L-tryptophan 2,3-dioxygenase activity | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation for TDO activity based on manual transfer from Q17P71 (Aedes aegypti TDO) by curator judgment. Redundant with IBA and IEA annotations but independently validated. Reason: Correct and well-supported. The ISS reference organism (Aedes aegypti) is a closely related mosquito. Direct biochemical evidence exists for A. gambiae TDO (PMID:18687401). Supporting Evidence: PMID:18687401 TDO converts l-tryptophan to N-formylkynurenine by catalyzing the heme-dependent oxidative opening of the substrate indole ring |
| GO:0006569 L-tryptophan catabolic process | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation for L-tryptophan catabolic process transferred from Aedes aegypti TDO. TDO catalyzes the first step of tryptophan catabolism. Reason: TDO is the sole enzyme initiating tryptophan catabolism in A. gambiae (PMID:18687401). The annotation is appropriate as TDO commits tryptophan irreversibly to catabolism. The term GO:0006569 is suitably general for what TDO does (initiates catabolism) without over-specifying the downstream fate of the carbon. Supporting Evidence: PMID:18687401 In the malaria vector Anopheles gambiae, tryptophan 2,3-dioxygenase (TDO) is the only enzyme able to initiate l-tryptophan degradation through the kynurenine pathway |
| GO:0020037 heme binding | ISS GO_REF:0000024 | ACCEPT | Summary: ISS annotation for heme binding transferred from Aedes aegypti TDO. Redundant with IBA and IEA annotations but correct. Reason: Correct. Heme binding is experimentally confirmed by spectral analysis of recombinant A. gambiae TDO (PMID:18687401). Supporting Evidence: PMID:18687401 Spectral analyses showed that the enzyme was purified in its heme-ferric form |
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Download this section (compressed HTML)Q: Does A. gambiae TDO expression change in response to blood feeding, and if so, what is the tissue-specific and temporal pattern of induction?
Q: What are the kinetic parameters (Km, kcat) of A. gambiae TDO for L-tryptophan under physiologically relevant reductant conditions?
Q: Does TDO disruption in A. gambiae affect Plasmodium transmission by altering xanthurenic acid levels in the midgut?
Experiment: CRISPR knockout of TDO in A. gambiae to assess effects on eye pigmentation, blood meal processing, survival, fecundity, and Plasmodium transmission.
Hypothesis: TDO knockout will produce white/vermilion-like eyes, impair tryptophan homeostasis after blood feeding, and alter Plasmodium development in the midgut through changes in xanthurenic acid production.
Experiment: Tissue-specific and blood meal-responsive transcriptomic profiling of TDO and downstream kynurenine pathway genes in A. gambiae.
Hypothesis: TDO expression is induced in the midgut following blood feeding to process the tryptophan load from hemoglobin digestion.
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