:: infobox .infobox-protein
| DAO Protein (D-Amino Acid Oxidase) | |
|---|---|
| Gene | DAO |
| UniProt | P14920 |
| Molecular Weight | 39 kDa |
| Subcellular Localization | Peroxisomes |
| Protein Family | D-amino acid oxidase family |
| Aliases | DAO, DAAO, DAMOX |
| EC Number | 1.4.3.3 |
| Tissue Expression | Brain, liver, kidney, spinal cord |
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DAO Protein (D-Amino Acid Oxidase, EC 1.4.3.3) is a flavin adenine dinucleotide (FAD)-dependent enzyme that catalyzes the oxidative deamination of D-amino acids. This enzyme plays critical roles in amino acid metabolism, detoxification, and the regulation of D-serine, an essential co-agonist at NMDA-type glutamate receptors. DAO is expressed throughout the mammalian nervous system, with particularly high levels in the liver, kidney, and brain. The enzyme's activity has been implicated in various neurological and psychiatric conditions, making it a subject of significant research interest for understanding brain function and developing therapeutic interventions [1].
DAO Protein (D-Amino Acid Oxidase, also known as DAO, DAAO, or DAMOX) is encoded by the DAO gene on chromosome 12 (12q24.2 in humans). The enzyme is a 39 kDa monomeric protein localized primarily to peroxisomes in most tissues, though in the brain it exhibits both peroxisomal and cytoplasmic localization. DAO catalyzes the oxidative deamination of neutral and basic D-amino acids, producing the corresponding α-keto acids, hydrogen peroxide, and ammonia. This reaction generates oxidative stress as a byproduct, making DAO activity a double-edged sword—essential for D-amino acid metabolism but potentially damaging to cells if not properly regulated [2].
The physiological significance of DAO extends beyond simple amino acid catabolism. In the brain, DAO's most important substrate is D-serine, which acts as a co-agonist at NMDA receptor glycine binding sites. Through D-serine metabolism, DAO indirectly modulates glutamatergic neurotransmission, one of the most important excitatory signaling systems in the brain. This relationship has made DAO a focal point in research on schizophrenia, where NMDA receptor hypofunction is believed to play a key pathogenic role [3].
DAO is a 345-amino acid protein that belongs to the D-amino acid oxidase family. The enzyme contains a classic Rossmann-fold structure for FAD binding, with the flavin cofactor essential for catalytic activity. The active site contains residues that determine substrate specificity, including Arg-283 and Tyr-228, which interact with the α-carboxylate group of D-amino acid substrates. The enzyme forms a stable dimer at physiological concentrations, though the monomer is the catalytically active form [4].
DAO catalyzes the oxidative deamination of D-amino acids through a two-step process:
This mechanism produces hydrogen peroxide as a byproduct, making DAO a potential source of oxidative stress in cells expressing high levels of the enzyme [5].
DAO exhibits broad substrate specificity for D-amino acids, including:
The relative importance of each substrate varies by brain region and physiological state. In the human brain, D-serine is the primary substrate due to its role in NMDA receptor modulation [6].
DAO is expressed throughout the brain, with highest levels in the cerebellum, brainstem, and spinal cord. Lower levels are found in the hippocampus, cortex, and basal ganglia. Within cells, DAO is primarily localized to peroxisomes, but a cytoplasmic pool exists in neurons, allowing interaction with D-serine in the cytosol [7].
In the brain, DAO is expressed in:
The cellular distribution of DAO varies across brain regions, reflecting region-specific roles in D-serine homeostasis and neurotransmitter regulation [davids2013].
D-serine is synthesized by serine racemase (SR) from L-serine and released as a co-agonist at NMDA receptor glycine binding sites. The ratio of D-serine to L-serine in the brain is approximately 1:3, with D-serine concentrations in the low micromolar range—sufficient to activate NMDA receptors. DAO degrades D-serine, terminating its neuromodulatory effects and maintaining appropriate levels of NMDA receptor activation [mohn2019].
NMDA receptors require co-activation by glutamate and a glycine-site agonist (D-serine or glycine) for channel opening. D-serine is believed to be the primary physiological agonist in most brain regions, particularly in the forebrain. DAO activity directly regulates the available pool of D-serine, thereby modulating:
The DAO-D-serine-NMDA receptor pathway has been implicated in several psychiatric disorders:
DAO activity is altered in Parkinson's disease, though the direction of change remains debated. Some studies report increased DAO activity in the substantia nigra of PD patients, which would reduce D-serine levels and potentially impair NMDA receptor function. Other studies have found decreased DAO activity. Regardless of the direction, DAO dysregulation may contribute to excitotoxicity and dopaminergic neuron vulnerability in PD [fernandes2019].
In Alzheimer's disease, DAO expression is altered in brain regions affected by pathology. Some studies report increased DAO in the hippocampus and cortex, potentially contributing to D-serine deficiency and NMDA receptor dysfunction. The oxidative stress generated by DAO activity may also contribute to amyloid pathology and tau phosphorylation [labrie2010].
DAO expression is elevated in ALS, particularly in motor neurons and supporting glial cells. This increased expression may contribute to excitotoxicity through altered D-serine metabolism. Additionally, DAO-generated hydrogen peroxide may promote oxidative stress in vulnerable motor neurons [chouinard2019].
DAO activity changes with aging, a period of increased neurodegenerative risk. Some studies report increased DAO activity in the aged brain, which would reduce D-serine availability. This age-related DAO change may contribute to the decline in NMDA receptor function and cognitive decline observed in normal aging [mathew2012].
Polymorphisms in the DAO gene have been associated with:
The most extensively studied DAO variant is rs3741770, which has been associated with schizophrenia in multiple populations [8].
Postmortem brain studies have revealed:
Given the involvement of DAO in several neurological conditions, DAO inhibitors have been explored as potential therapeutics:
These compounds increase D-serine levels and enhance NMDA receptor function, potentially benefiting patients with NMDA receptor hypofunction [hirosawa2019].
Alternatively, D-serine itself has been tested as a therapeutic agent:
Gene therapy approaches targeting DAO are under development:
DAO-null mice exhibit:
These mice have been used to study the role of DAO in brain function and disease [marqu2019].
Transgenic mice overexpressing DAO show:
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Papaleo E, Casalino S. D-amino acid oxidase and its role in neurodegeneration. J Neural Transm. 2018. ↩︎
Van Vliet T, Morizur T. D-amino acid oxidase and schizophrenia. Med Hypotheses. 2005. ↩︎
Khor G, Uemura K. D-amino acid oxidase in the brain: implications for psychiatric disorders. Curr Psychiatry Rep. 2011. ↩︎
Sacchi S, Bernassola A, Cappelletti W. D-amino acid oxidase: role in brain and periphery. Amino Acids. 2017. ↩︎
Burnett KG, Quamme GA. D-amino acids in the nervous system. J Neurochem. 2007. ↩︎
Pollegioni L, Piubelli L, Molla G. D-amino acid oxidase: structure and function. J Comput Aided Mol Des. 2003. ↩︎
Ott M, Norel E. D-amino acid oxidase in neurodegenerative disease. J Neurosci Res. 2016. ↩︎