Phosphorylation of GTP Cyclohydrolase I and Modulation of Its Activity in Rodent Mast Cells

The signaling functions of dopamine require a finely tuned regulatory network for rapid induction and suppression of output. A key target of regulation is the enzyme tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis, which is activated by phosphorylation and modulated by the availability of its cofactor, tetrahydrobiopterin. The first enzyme in the cofactor synthesis pathway, GTP cyclohydrolase I, is activated by phosphorylation and inhibited by tetrahydrobiopterin. We previously reported that deficits in GTP cyclohydrolase activity in Drosophila heterozygous for mutant alleles of the gene encoding this enzyme led to tightly corresponding diminution of in vivo tyrosine hydroxylase activity that could not be rescued by exogenous cofactor. We also found that the two enzymes could be coimmunoprecipitated from tissue extracts and proposed functional interactions between the enzymes that extended beyond provision of cofactor by one pathway for another. Here, we confirm the physical association of these enzymes, identifying interacting regions in both, and we demonstrate that their association can be regulated by phosphorylation. The functional consequences of the interaction include an increase in GTP cyclohydrolase activity, with concomitant protection from endproduct feedback inhibition. In vivo, this effect would in turn provide sufficient cofactor when demand for catecholamine synthesis is greatest. The activity of tyrosine hydroxylase is also increased by this interaction, in excess of the stimulation resulting from phosphorylation alone. V max is elevated, with no change in K m . These results demonstrate that these enzymes engage in mutual positive regulation.

Section: Discussionmentioning

confidence: 99%

Section: Tyrosine Hydroxylase and Gtp Cyclohydrolase I Physicallymentioning

confidence: 97%

Direct Binding of GTP Cyclohydrolase and Tyrosine Hydroxylase

Bowling

Huang

et al. 2008

“…After 72-h postfertilization, however, GTP cyclohydrolase I activity does not correlate with the onset of rapid production of pteridine pigments. In mammalian cells, various cytokines and growth factors regulate the activity of GTP cyclohydrolase I at the transcriptional or post-transcriptional level (55)(56)(57), through post-translational modification by hyperphosphorylation (58) or by association with a feedback regulatory protein (59). Research is under way into the regulation of GTP cyclohydrolase I activity during the development of the zebrafish and in particular to explain why this activity starts to decrease at around 72-h postfertilization, which coincides with the production of xanthophore pteridine pigments.…”

Section: Discussionmentioning

confidence: 99%

Development of the Pteridine Pathway in the Zebrafish,Danio rerio

Ziegler¹,

McDonald²,

Heßlinger³

et al. 2000

Self Cite

In the zebrafish, the peripheral neurons and the pigment cells are derived from the neural crest and share the pteridine pathway, which leads either to the cofactor tetrahydrobiopterin or to xanthophore pigments. The components of the pteridine pattern were identified as tetrahydrobiopterin, sepiapterin, 7-oxobiopterin, isoxanthopterin, and 2,4,7-trioxopteridine. The expression of GTP cyclohydrolase I activity during the first 24-h postfertilization, followed by 6-pyruvoyl-5,6,7,8-tetrahydropterin synthase and sepiapterin reductase, suggest an early supply of tetrahydrobiopterin for neurotransmitter synthesis in the neurons and for tyrosine supply in the melanophores. At 48-h postfertilization, sepiapterin formation branches off the de novo pathway of tetrahydrobiopterin synthesis. Sepiapterin, via 7,8-dihydrobiopterin and biopterin, serves as a precursor for the formation of 7-oxobiopterin, which may be further catabolized to isoxanthopterin and 2,4,7-trioxopteridine. Neither 7,8-dihydrobiopterin nor biopterin is a substrate for xanthine oxidoreductase. In contrast, both of these compounds are oxidized at C-7 by a xanthine oxidase variant form, which is inactivated by KCN, but is insensitive to allopurinol. The oxidase and the dehydrogenase form of xanthine oxidoreductase as well as the xanthine oxidase variant have specific developmental patterns. It follows that GTP cyclohydrolase I, the formation of sepiapterin, and the xanthine oxidoreductase family control the pteridine pathway in the zebrafish.

“…It has been reported that stimulation of GTPCH activity in mammalian cell culture occurs through phosphorylation and that GTPCH serves as a substrate for both casein kinase II and protein kinase C (32,33). However, specific sites of phosphorylation have not yet been identified in mammalian GTPCH, nor has the effect of phosphorylation been defined enzymatically.…”

mentioning

confidence: 99%

A typical N-terminal Extensions Confer Novel Regulatory Properties on GTP Cyclohydrolase Isoforms in Drosophila melanogaster

Funderburk¹,

Bowling²,

Xu³

et al. 2006

The cofactor tetrahydrobiopterin plays critical roles in the modulation of the signaling molecules dopamine, serotonin, and nitric oxide. Deficits in cofactor synthesis have been associated with several human hereditary diseases. Responsibility for the regulation of cofactor pools resides with the first enzyme in its biosynthetic pathway, GTP cyclohydrolase I. Because organisms must be able to rapidly respond to environmental and developmental cues to adjust output of these signaling molecules, complex regulatory mechanisms are vital for signal modulation. Mammalian GTP cyclohydrolase is subject to end-product inhibition via an associated regulatory protein and to positive regulation via phosphorylation, although target residues are unknown. GTP cyclohydrolase is composed of a highly conserved homodecameric catalytic core and non-conserved N-terminal domains proposed to be regulatory sites. We demonstrate for the first time in any organism that the N-terminal arms of the protein serve regulatory functions. We identify two different modes of regulation of the enzyme mediated through the N-terminal domains. The first is end-product feedback inhibition, catalytically similar to that of the mammalian enzyme, except that feedback inhibition by the cofactor requires sequences in the N-terminal arms rather than a separate regulatory protein. The second is a novel inhibitory interaction between the N-terminal arms and the active sites, which can be alleviated through the phosphorylation of serine residues within the N termini. Both mechanisms allow for acute and highly responsive regulation of cofactor production as required by downstream signaling pathways.