Aromatic L-amino acid decarboxylase
|PDB structures||RCSB PDB PDBe PDBsum|
|Gene Ontology||AmiGO / EGO|
|DOPA decarboxylase (aromatic L-amino acid decarboxylase)|
|Locus||Chr. 7 p11|
- L-DOPA to dopamine - a neurotransmitter
- 5-HTP to serotonin (5-HT) - a neurotransmitter
- L-histidine to histamine - a neurotransmitter
- phenylalanine to phenethylamine - trace amine neuromodulator
- L-tyrosine to tyramine - trace amine neuromodulator
- tryptophan to tryptamine - trace amine neuromodulator
As a rate-limiting step
In normal dopamine and serotonin (5-HT) neurotransmitter synthesis, AAAD is not the rate-limiting step in either reaction. However, AAAD becomes the rate-limiting step of dopamine synthesis in patients treated with L-DOPA (such as in Parkinson's Disease), and the rate-limiting step of serotonin synthesis in people treated with 5-HTP (such as in mild depression or dysthymia). AAAD is inhibited by Carbidopa outside of the blood brain barrier to inhibit the premature conversion of L-DOPA to Dopamine in the treatment of Parkinson's.
In humans, AAAD is also the rate-limiting enzyme in the formation of trace amines. Deficiency of AAAD is associated with various symptoms as severe developmental delay, oculogyric crises and autonomic dysfunction. The molecular and clinical spectrum of AAAC deficiency is heterogeneous. The first case of AADC deficiency was described in twin brothers 1990. Patients can be treated with dopamine agonists, MAO inhibitors, and pyridoxine (vitamin B6). Clinical phenotype and response to treatment is variable and the long-term and functional outcome is unknown. To provide a basis for improving the understanding of the epidemiology, genotype/phenotype correlation and outcome of these diseases their impact on the quality of life of patients, and for evaluating diagnostic and therapeutic strategies a patient registry was established by the noncommercial International Working Group on Neurotransmitter Related Disorders (iNTD).
The gene encoding the enzyme is referred to as DDC and located on chromosome 7 in humans. Single nucleotide polymorphisms and other gene variations have been investigated in relation to neuropsychiatric disorders, e.g., a one-base pair deletion at –601 and a four-base pair deletion at 722–725 in exon 1 in relation to bipolar disorder and autism. No direct correlation between gene variation and autism was found.
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The highest level of brain CYP2D activity was found in the substantia nigra (Bromek et al., 2010). The in vitro and in vivo studies have shown the contribution of the alternative CYP2D-mediated dopamine synthesis to the concentration of this neurotransmitter although the classic biosynthetic route to dopamine from tyrosine is active. CYP2D6 protein level is approximately 40% lower in the frontal cortex, cerebellum, and hippocampus in PD patients, even when controlling for CYP2D6 genotype (Mann et al., 2012). ... Tyramine levels are especially high in the basal ganglia and limbic system, which are thought to be related to individual behavior and emotion (Yu et al., 2003c). Studies have demonstrated that dopamine is formed from p-tyramine as well as m-tyramine via tyramine 3-hydroxylation or 4-hydroxylation by rat CYP2D2, 2D4, and 2D18 as well as human CYP2D6. ... Both rat CYP2D and human CYP2D6 have a higher affinity for m-tyramine compared with p-tyramine for the generation of dopamine. Rat CYP2D isoforms (2D2/2D4/2D18) are less efficient than human CYP2D6 for the generation of dopamine from p-tyramine. The Km values of the CYP2D isoforms are as follows: CYP2D6 (87–121 μm) ≈ CYP2D2 ≈ CYP2D18 > CYP2D4 (256 μm) for m-tyramine and CYP2D4 (433 μm) > CYP2D2 ≈ CYP2D6 > CYP2D18 (688 μm) for p-tyramine (Bromek et al., 2010; Thompson et al., 2000).
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