Structural mechanism for tyrosine hydroxylase inhibition by dopamine

In our brain, there are billions of nerve cells that talk to each other using signaling substances called neurotransmitters. One of these neurotransmitters is dopamine, known to many as one of the happiness hormones. In addition to giving us a sense of reward and satisfaction, dopamine is also important for motor control, concentration, learning and attention.

The importance of dopamine becomes very clear in patients with Parkinson's disease. In the brains of these patients, cells that synthesize and secrete dopamine die. These cells are called dopaminergic neurons and act as small dopamine factories. The mental and physical symptoms in Parkinson's patients only occur when about half of these cells are gone. This means that the disease is not discovered until a lot of damage has occurred. One of the goals of research on the brain's dopamine system is to find out how Parkinson's and related diseases can be detected at an earlier stage.

"In this way, you can find drugs that can prevent the disease from developing further", says the researcher.

Enzymes are proteins that act as catalysts for chemical reactions in the body. The three-dimensional structure of the enzymes is absolutely essential for their activity and regulation. Inside the dopamine-producing cells, there are large amounts of an enzyme called tyrosine hydroxylase (TH).

"Such 3D structures can tell us how the cellular processes take place at the atomic level and thus also how we can find targeted treatment to correct errors in enzymes, which, for example, occur by disease mutations. For TH, such mutations cause TH deficiency, a neurological disease classified as a subgroup of parkinsonism", says the senior author.

The dedicated task of TH is to convert the amino acid tyrosine to L-dopa, which is further converted to dopamine by another enzyme. It is well known that TH is the key enzyme in the regulation of dopamine synthesis, but it has not been understood in what way this happens at the level of structural details. It has long been known that dopamine can regulate its own production. Dopamine can bind to the TH enzyme and inactivate it. Such regulatory mechanisms are called negative feedback and ensure that dopamine synthesis is turned off when the cell has sufficient dopamine.

TH presents a tetrameric structure with dimerized regulatory domains that are separated 15 Å from the catalytic domains. Upon DA binding, a 20-residue α-helix in the flexible N-terminal tail of the regulatory domain is fixed in the active site, blocking it, while S40-phosphorylation forces its egress.

The structures reveal the molecular basis of the inhibitory and stabilizing effects of DA and its counteraction by S40-phosphorylation, key regulatory mechanisms for homeostasis of DA and TH.

"When the dopamine level drops again, signaling pathways will be activated in the cell. This leads to TH being modified, by so-called phosphorylation, which causes dopamine to be released and the enzyme to be reactivated", says another researcher in the study.

It is the interplay between these regulatory mechanisms that researchers now believe they can understand at the level of detail.

"This knowledge gives us new opportunities to develop medicines for neuropsychiatric and neurodegenerative diseases", says the senior author.

These research results are a product of long-term and world-leading research activity on TH at using sophisticated cryoelectron microscopes with high resolution to determine the three-dimensional structure of proteins.