Tyrosine Hydroxylase (TH) Laboratory
Contact: Phil Dickson
Our work is focused on the enzyme tyrosine hydroxylase (TH). TH is the rate limiting enzyme in the biosynthesis of the catecholamines dopamine, noradrenaline and adrenaline. As the rate limiting enzyme TH forms the primary control mechanism for the production of catecholamines in cells. TH hydroxylates tyrosine to L-DOPA. L-DOPA is then converted to dopamine by the aromatic amino acid decarboxylase. In noradrenergic and adrenergic cells the dopamine is further converted to noradrenaline and adrenaline respectively. When the catecholamines are secreted from vesicles, the lost catecholamines must be replaced. In order to achieve this TH is activated by phosphorylation. When TH is activated there is a change in the level of phosphorylation of three sites in TH. These sites are Ser19, Ser31 and Ser40. The primary mechanism for the short term regulation of TH is inhibition by catecholamine and relief of that inhibition by phosphorylation. Catecholamines bind to TH with very high affinity and the binding of catecholamine to TH directly reduces the activity of the enzyme. Phosphorylation of Ser40 abolishes this high affinity site such that the catecholamines no longer inhibit TH activity.
We have recently identified a number of novel features about the regulation of TH activity. We have found that in addition to the high affinity catecholamine binding site there is a low affinity binding site which operates independently of phosphorylation. This site acts as an automatic sensor of the cytosolic catecholamine levels and will adjust TH activity to maintain a constant level of cytosolic catecholamine. The low affinity site plays an important role in protecting cells from a build up of catecholamines in the cytosol. This is particularly important in dopaminergic neurons as it is known that dopamine can break down into a number of different toxic compounds. Our current work is aimed at defining the molecular nature of the low affinity catecholamine binding site in TH and examining the role that this site plays the control of catecholamine synthesis in intact cells.
Structure of the TH catalytic domain with bound dopamine
We have also found that phosphorylation of the Ser19 and Ser31 residues will increase the rate of phosphorylation of Ser40 and therefore increase TH activity. We term this effect hierarchical phosphorylation. Hierarchical phosphorylation in TH has important implications for the regulation of catecholamine synthesis in humans. This is because humans are unique in that they contain four different isoforms of TH whereas higher primates only have two isoforms and all other mammals only contain one isoform. Hierarchical phosphorylation is the only regulatory mechanism identified that is different between the different human TH isoforms. Our current work is focused on the analysis of the distribution of the TH isoforms in human brain. In particular our work is focused on the dopaminergic neurons that are susceptible or resistant to degeneration in Parkinson's disease. Our hypothesis is that the differential distribution of human TH isoforms may lead to different levels of synthesis of the potentially toxic dopamine molecule in different dopaminergic neurons. This may be the key to why the dopaminergic neurons of the substantia nigra are particularly susceptible to degeneration whereas other dopaminergic neurons show very little degeneration in Parkinson's disease.
Immunohistochemistry of rat substantia nigra
(green-TH low Ser40 phosphorylation, Orange-TH high Ser40 phosphorylation)