NaAO2_candidate_AO_1 is the second current NICAT L-aspartate oxidase candidate for the duplicated pyridine branch that feeds nicotine biosynthesis. Like AO_0, it is a bona fide chloroplast-localized NadB-family enzyme, but current public evidence still does not distinguish it as the specialized pathway paralog.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0008734
L-aspartate oxidase activity
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: This specific catalytic activity fits AO_1 well.
Reason: AO_1 is clearly an L-aspartate oxidase family member rather than only a generic oxidoreductase.
Supporting Evidence:
file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
|
|
GO:0009435
NAD+ biosynthetic process
|
IEA
GO_REF:0000120 |
ACCEPT |
Summary: AO_1 belongs in NAD biosynthesis.
Reason: AO family chemistry feeds the de novo NAD branch from which the nicotine pyridine branch evolved.
Supporting Evidence:
file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
|
|
GO:0009507
chloroplast
|
IEA
GO_REF:0000044 |
KEEP AS NON CORE |
Summary: Chloroplast localization is supported but not central to the paralog-resolution question.
Reason: Retain the location as plausible contextual information while keeping the main review emphasis on pathway assignment.
Supporting Evidence:
file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-uniprot.txt
CC -!- SUBCELLULAR LOCATION: Plastid, chloroplast
|
|
GO:0016491
oxidoreductase activity
|
IEA
GO_REF:0000002 |
MARK AS OVER ANNOTATED |
Summary: This generic parent term adds little beyond the specific catalytic annotation.
Reason: GO:0008734 already captures the informative chemistry for this candidate.
Supporting Evidence:
file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
|
Q: Is AO_1 transcription or protein abundance more tightly coupled to nicotine induction than AO_0?
Q: Do AO_1 and AO_0 make redundant or distinct contributions to the pyridine precursor pool in roots?
Experiment: Measure AO_1 and AO_0 expression and coexpression with established nicotine genes after topping, jasmonate, and herbivory treatments.
Hypothesis: One of the two AO paralogs is more tightly integrated into the nicotine pathway regulatory program.
Type: expression profiling
Experiment: Compare the metabolic impact of AO_1 and AO_0 perturbation on quinolinate-, nicotinate-, and nicotine-related metabolites.
Hypothesis: Only one AO paralog has a major effect on nicotine precursor flux.
Type: genetics plus metabolite profiling
The gene NaAO2_candidate_AO_1 in Nicotiana attenuata encodes the enzyme L-aspartate oxidase (LASPO), a pivotal component in the biosynthesis of nicotinamide adenine dinucleotide (NAD⁺) and nicotine alkaloids. This enzyme catalyzes the oxidation of L-aspartate to iminosuccinate, a reaction integral to both NAD⁺ biosynthesis and the production of nicotine, a key secondary metabolite in N. attenuata.
Enzymatic Function and Catalytic Activity
L-aspartate oxidase (EC 1.4.3.16) facilitates the conversion of L-aspartate and molecular oxygen into iminosuccinate and hydrogen peroxide:
[ \text{L-aspartate} + O_2 \rightarrow \text{iminosuccinate} + H_2O_2 ]
This reaction represents the initial step in the de novo NAD⁺ biosynthetic pathway. In Escherichia coli, iminosuccinate is further processed by quinolinate synthase to produce quinolinic acid, a precursor for NAD⁺ synthesis (en.wikipedia.org). While the specific downstream enzymes in N. attenuata remain to be fully elucidated, it is likely that a similar pathway exists, given the conservation of NAD⁺ biosynthesis across species.
Structural Characteristics and Cofactor Requirements
LASPO belongs to the FAD-dependent oxidoreductase 2 family, characterized by the presence of several conserved domains:
FAD-binding domain (IPR003953): Essential for the binding of flavin adenine dinucleotide (FAD), a cofactor necessary for the enzyme's oxidoreductase activity.
FAD/NAD-binding domain superfamily (IPR036188): Facilitates interactions with nucleotide cofactors, crucial for electron transfer during the catalytic process.
Fumarate reductase/succinate dehydrogenase flavoprotein-like C-terminal domain (IPR015939): Shares structural similarities with other flavoproteins involved in redox reactions.
These domains collectively contribute to the enzyme's ability to catalyze the oxidation of L-aspartate, underscoring its role in redox biology.
Biological Role in NAD⁺ Biosynthesis
In plants, LASPO initiates the de novo synthesis of NAD⁺ by converting L-aspartate into iminosuccinate. Subsequent enzymatic steps lead to the formation of quinolinic acid and ultimately NAD⁺. This pathway is vital for maintaining cellular redox balance and energy metabolism. Studies in Arabidopsis thaliana have demonstrated that LASPO activity is crucial for NAD⁺ homeostasis, with the enzyme localized in chloroplasts, indicating its integration into photosynthetic metabolism (pubmed.ncbi.nlm.nih.gov).
Involvement in Nicotine Biosynthesis
N. attenuata is renowned for its production of nicotine, a secondary metabolite that serves as a defense mechanism against herbivores. The biosynthesis of nicotine involves two primary pathways:
Pyridine Nucleotide Cycle: Generates nicotinic acid from L-aspartate via the action of LASPO, quinolinate synthase, and quinolinic acid phosphoribosyltransferase.
Methylpyrrole Pathway: Produces putrescine, which is methylated to form N-methylputrescine, a precursor for the pyrrolidine ring of nicotine.
The integration of these pathways leads to the synthesis of nicotine, with LASPO playing a foundational role by providing nicotinic acid (frontiersin.org).
Subcellular Localization
While direct experimental evidence for LASPO localization in N. attenuata is limited, studies in related species suggest a chloroplastic localization. In A. thaliana, LASPO is localized in chloroplasts, aligning with its role in NAD⁺ biosynthesis and linking it to photosynthetic processes (pubmed.ncbi.nlm.nih.gov). Given the conservation of metabolic pathways, it is plausible that LASPO in N. attenuata shares a similar subcellular distribution.
Pathway Integration and Functional Implications
The activity of LASPO in N. attenuata is integral to both primary and secondary metabolism:
NAD⁺ Biosynthesis: Essential for numerous metabolic processes, including glycolysis, the tricarboxylic acid cycle, and oxidative phosphorylation.
Nicotine Production: Provides nicotinic acid, a precursor for nicotine biosynthesis, contributing to the plant's defense mechanisms.
The dual role of LASPO underscores its importance in the metabolic network of N. attenuata, influencing both energy metabolism and ecological interactions through secondary metabolite production.
Conclusion
The gene NaAO2_candidate_AO_1 encodes L-aspartate oxidase, a key enzyme in N. attenuata that catalyzes the oxidation of L-aspartate to iminosuccinate. This reaction is crucial for the biosynthesis of NAD⁺ and serves as a foundational step in nicotine production. While direct studies on LASPO in N. attenuata are limited, insights from related species provide a framework for understanding its function, localization, and role in metabolic pathways. Further research is needed to elucidate the specific regulatory mechanisms and interactions of LASPO within the unique metabolic context of N. attenuata.
id: A0A314LIN0
gene_symbol: NaAO2_candidate_AO_1
product_type: PROTEIN
status: DRAFT
aliases:
- AO_1
- NaAO2
taxon:
id: NCBITaxon:49451
label: Nicotiana attenuata
description: >-
NaAO2_candidate_AO_1 is the second current NICAT L-aspartate oxidase candidate
for the duplicated pyridine branch that feeds nicotine biosynthesis. Like AO_0,
it is a bona fide chloroplast-localized NadB-family enzyme, but current public
evidence still does not distinguish it as the specialized pathway paralog.
references:
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with GO terms
findings: []
- id: GO_REF:0000044
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot Subcellular Location vocabulary mapping, accompanied by conservative changes to GO terms applied by UniProt
findings: []
- id: GO_REF:0000120
title: Combined Automated Annotation using Multiple IEA Methods
findings: []
- id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-uniprot.txt
title: UniProt entry A0A314LIN0 for Nicotiana attenuata AO_1
findings:
- statement: AO_1 is an L-aspartate oxidase in the NadB family
supporting_text: 'DE RecName: Full=L-aspartate oxidase'
reference_section_type: DATABASE_ENTRY
- statement: UniProt places AO_1 in plastid/chloroplast
supporting_text: 'CC -!- SUBCELLULAR LOCATION: Plastid, chloroplast'
reference_section_type: DATABASE_ENTRY
- id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
title: NaAO2 AO_1 candidate notes
findings:
- statement: AO_1 is a credible biochemical candidate for the upstream pyridine branch
supporting_text: UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
reference_section_type: LITERATURE_REVIEW
- statement: AO_1 remains unresolved relative to AO_0 for nicotine-pathway specialization
supporting_text: AO_1 remains a plausible NaAO2 candidate, but the present public annotations do not justify treating both AO_0 and AO_1 as equally core without further paralog-resolution evidence.
reference_section_type: LITERATURE_REVIEW
existing_annotations:
- term:
id: GO:0008734
label: L-aspartate oxidase activity
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: This specific catalytic activity fits AO_1 well.
action: ACCEPT
reason: >-
AO_1 is clearly an L-aspartate oxidase family member rather than only a
generic oxidoreductase.
supported_by:
- reference_id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
supporting_text: UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
reference_section_type: LITERATURE_REVIEW
- term:
id: GO:0009435
label: NAD+ biosynthetic process
evidence_type: IEA
original_reference_id: GO_REF:0000120
review:
summary: AO_1 belongs in NAD biosynthesis.
action: ACCEPT
reason: >-
AO family chemistry feeds the de novo NAD branch from which the nicotine
pyridine branch evolved.
supported_by:
- reference_id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
supporting_text: UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
reference_section_type: LITERATURE_REVIEW
- term:
id: GO:0009507
label: chloroplast
evidence_type: IEA
original_reference_id: GO_REF:0000044
review:
summary: Chloroplast localization is supported but not central to the paralog-resolution question.
action: KEEP_AS_NON_CORE
reason: >-
Retain the location as plausible contextual information while keeping the
main review emphasis on pathway assignment.
supported_by:
- reference_id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-uniprot.txt
supporting_text: 'CC -!- SUBCELLULAR LOCATION: Plastid, chloroplast'
reference_section_type: DATABASE_ENTRY
- term:
id: GO:0016491
label: oxidoreductase activity
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: This generic parent term adds little beyond the specific catalytic annotation.
action: MARK_AS_OVER_ANNOTATED
reason: >-
GO:0008734 already captures the informative chemistry for this candidate.
supported_by:
- reference_id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
supporting_text: UniProt curates A0A314LIN0 as an L-aspartate oxidase/NadB-family flavoprotein that catalyzes oxidation of L-aspartate to iminoaspartate, placing it in the same upstream NAD-derived chemistry that can feed pyridine alkaloid biosynthesis.
reference_section_type: LITERATURE_REVIEW
core_functions:
- molecular_function:
id: GO:0008734
label: L-aspartate oxidase activity
directly_involved_in:
- id: GO:0009435
label: NAD+ biosynthetic process
description: >-
AO_1 is an L-aspartate oxidase candidate within the duplicated upstream
pyridine-source pathway that may contribute to nicotine precursor supply.
supported_by:
- reference_id: file:NICAT/NaAO2_candidate_AO_1/NaAO2_candidate_AO_1-notes.md
supporting_text: AO_1 remains a plausible NaAO2 candidate, but the present public annotations do not justify treating both AO_0 and AO_1 as equally core without further paralog-resolution evidence.
reference_section_type: LITERATURE_REVIEW
proposed_new_terms: []
suggested_questions:
- question: Is AO_1 transcription or protein abundance more tightly coupled to nicotine induction than AO_0?
- question: Do AO_1 and AO_0 make redundant or distinct contributions to the pyridine precursor pool in roots?
suggested_experiments:
- description: Measure AO_1 and AO_0 expression and coexpression with established nicotine genes after topping, jasmonate, and herbivory treatments.
experiment_type: expression profiling
hypothesis: One of the two AO paralogs is more tightly integrated into the nicotine pathway regulatory program.
- description: Compare the metabolic impact of AO_1 and AO_0 perturbation on quinolinate-, nicotinate-, and nicotine-related metabolites.
experiment_type: genetics plus metabolite profiling
hypothesis: Only one AO paralog has a major effect on nicotine precursor flux.