ACO2 (Aconitase 2, mitochondrial) is an iron-sulfur cluster-containing enzyme that catalyzes the reversible isomerization of citrate to isocitrate in the citric acid (TCA) cycle[@lill1999]. Located in the mitochondrial matrix, ACO2 plays a critical role in cellular energy metabolism through oxidative phosphorylation. The enzyme requires an iron-sulfur [4Fe-4S] cluster for its catalytic activity, making it sensitive to oxidative stress and iron homeostasis disruptions implicated in neurodegenerative diseases.
ACO2 is encoded by the ACO2 gene and is essential for normal mitochondrial function. The enzyme is distinct from the cytosolic aconitase (ACO1), which functions in iron regulatory protein (IRP) post-transcriptional regulation. Mitochondrial ACO2 activity is reduced in several neurodegenerative conditions, including Alzheimer's disease and Parkinson's disease[@wang2020].
| Property |
Value |
| Gene |
ACO2 |
| UniProt |
Q99798 |
| PDB |
5O8T, 6G19, 7B2Y |
| Molecular Weight |
~82 kDa |
| Subcellular Localization |
Mitochondrial matrix |
| Protein Family |
Iron-sulfur isomerase (Aconitase) family |
| Chain Length |
780 amino acids |
| Metal Cofactor |
4Fe-4S cluster |
ACO2 is a 780-amino acid protein with a complex structure enabling its catalytic function[@giachin2016]:
The active site contains a [4Fe-4S] iron-sulfur cluster essential for catalysis:
- Cluster composition: Four iron atoms coordinated by four inorganic sulfides
- Cysteine ligands: Cys358, Cys360, Cys365, and Cys391 coordinate the cluster
- Catalytic role: The cluster acts as a Lewis acid, facilitating citrate isomerization
- Sensitivity: The cluster is highly sensitive to oxidative stress and nitric oxide
¶ Domain Organization
- N-terminal domain: Contains the active site with the 4Fe-4S cluster
- C-terminal domain: Forms a beta-barrel structure supporting the active site
- Alpha-helical insertion: Regulatory region between domains
- Substrate binding induces conformational changes
- The enzyme undergoes "open" and "closed" states during catalysis
- Post-translational modifications can modulate activity
- ACO2 (mitochondrial): Purely enzymatic function in TCA cycle
- ACO1 (cytosolic): Dual function - enzymatic and iron regulatory
- Sequence similarity: ~60% identical at amino acid level
In the mitochondria, ACO2 catalyzes the second step of the TCA cycle[@wang2020]:
- Reaction: Citrate ⇌ Isocitrate
- Mechanism: Through a dehydration/hydration mechanism
- Intermediate: Formed as cis-aconitate (bound to enzyme)
- Rate: High turnover number under physiological conditions
ACO2 is essential for normal TCA cycle function:
- Step 2: Citrate (6C) → Isocitrate (6C)
- Step 3: Isocitrate → α-Ketoglutarate + CO2 (IDH catalyzed)
- Energy production: Generates NADH for oxidative phosphorylation
- Anaplerosis: Provides carbon skeletons for biosynthesis
- Supports oxidative phosphorylation and ATP production
- Provides intermediates for amino acid synthesis
- Generates reducing equivalents (NADPH)
- Supports heme biosynthesis
ACO2 requires proper iron-sulfur cluster assembly:
- ISC machinery: Iron-sulfur cluster (ISC) pathway in mitochondria
- Frataxin: Essential cofactor for ISC assembly (Friedreich's ataxia gene)
- Assembly factors: ISCU, ISCA, NFS1, ferredoxin
Mitochondrial dysfunction is an early hallmark of AD[@chen2021]:
- ACO2 activity is reduced in AD brains and models
- Loss correlates with disease severity
- Precedes visible pathology in some models
- Iron accumulation in AD brains affects 4Fe-4S cluster integrity
- Oxidative stress damages the iron-sulfur cluster
- Reduced aconitase activity contributes to energy deficits
- Loss of ACO2 function leads to impaired TCA cycle activity
- Reduced ATP production in neurons
- Compensatory shifts to glycolysis (Warburg effect)
- Iron chelation therapy may protect aconitase
- Antioxidants can preserve 4Fe-4S clusters
- Mitochondrial-targeted compounds in development
PD is associated with mitochondrial complex I deficiency[@liu2020]:
- Dopaminergic neurons are particularly vulnerable to energy deficits
- ACO2 activity reduced in PD substantia nigra
- Mutations in ACO2-related genes cause parkinsonism
- Iron accumulation in PD substantia nigra
- Disrupted iron homeostasis impacts ACO2 function
- Interaction with α-synuclein aggregation
- Mitochondrial quality control pathways affected
- Damaged ACO2 not properly cleared
- Autophagy-lysosome system impairment
- Frataxin deficiency affects iron-sulfur cluster assembly[@martelli2012]
- ACO2 activity reduced due to impaired cluster insertion
- Similar mechanism to Aconitase 1 (ACO1) deficiency
- Neurodegeneration with brain iron accumulation (NBIA)
- Disrupted iron homeostasis impacts ACO2 function
- Common pathway to neuronal death
- Mitochondrial dysfunction in HD
- ACO2 activity altered in HD models
- Energy deficits contribute to progression
- Coenzyme Q10: Supports electron transport chain function
- Alpha-lipoic acid: Antioxidant that supports mitochondrial function
- Creatine: Energy buffer for mitochondrial dysfunction
- Iron chelators: Deferoxamine, deferasirox
- Iron regulation: Modulators of ferritin expression
- Mitochondria-targeted antioxidants: MitoQ, SkQ1
- N-acetylcysteine: Precursor to glutathione
- AAV-mediated ACO2 delivery
- Enhanced expression of iron-sulfur cluster assembly factors
- Mitochondrial targeting sequences
- Allosteric activators of aconitase
- Stabilizers of the 4Fe-4S cluster
- Compounds protecting against oxidative damage
- Ketogenic diet for alternative energy sources
- Pyruvate supplementation
- Dichloroacetate for PDH activation
- Lill R, et al, The iron-sulfur cluster proteins aconitase and iron regulatory protein 1 (1999)
- Giachin G, et al, Structure of human mitochondrial aconitase (2016)
- Wang T, et al, Mitochondrial dysfunction and therapeutic targets in Alzheimer's disease (2020)
- Keeney PM, et al, Alzheimer's disease brain mitochondrial dysfunction and melancholia (2013)
- Martelli A, et al, Iron metabolism and mitochondrial disorders (2012)
- Rouault TA, et al, Mitochondrial iron metabolism and disease (2012)
- Chen X, et al, Mitochondrial dysfunction in Alzheimer's disease (2021)
- Liu Y, et al, Aconitase activity and expression in Parkinson's disease (2020)
- Crofts AR, et al, Cytochrome bc1 complex and mitochondrial iron homeostasis (2013)
- Fenton WA, et al, Iron-sulfur cluster biogenesis and human disease (2015)
- Lill R, et al, The iron-sulfur cluster proteins aconitase and iron regulatory protein 1 (1999)
- Giachin G, et al, Structure of human mitochondrial aconitase (2016)
- Wang T, et al, Mitochondrial dysfunction and therapeutic targets in Alzheimer's disease (2020)
- Keeney PM, et al, Alzheimer's disease brain mitochondrial dysfunction and melancholia (2013)
- Martelli A, et al, Iron metabolism and mitochondrial disorders (2012)
- Rouault TA, et al, Mitochondrial iron metabolism and disease (2012)
- Chen X, et al, Mitochondrial dysfunction in Alzheimer's disease: molecular mechanisms and therapeutic strategies (2021)
- Liu Y, et al, Aconitase activity and expression in Parkinson's disease (2020)
- Crofts AR, et al, Cytochrome bc1 complex and mitochondrial iron homeostasis (2013)
- Fenton WA, et al, Iron-sulfur cluster biogenesis and human disease (2015)