Kuhn D M, Arthur R, States J C
Department of Psychiatry and Behavioral Neurosciences, Wayne State University School of Medicine, Detroit, Michigan, U.S.A.
J Neurochem. 1997 May;68(5):2220-3. doi: 10.1046/j.1471-4159.1997.68052220.x.
Tryptophan hydroxylase, the initial and rate-limiting enzyme in the biosynthesis of the neurotransmitter serotonin, is activated by protein kinase A and calcium/calmodulin-dependent protein kinase. One important aspect of the regulation of any enzyme by a phosphorylation-dephosphorylation cascade, and one that is lacking for tryptophan hydroxylase, lies in the identification of its site of phosphorylation by protein kinases. Recombinant forms of brain tryptophan hydroxylase were expressed as glutathione S-transferase fusion proteins and exposed to protein kinase A. This protein kinase phosphorylates and activates full-length tryptophan hydroxylase. The inactive regulatory domain of the enzyme (corresponding to amino acids 1-98) was also phosphorylated by protein kinase A. The catalytic core of the hydroxylase (amino acids 99-444), which expresses high levels of enzyme activity, was neither phosphorylated nor activated by protein kinase A. Conversion of serine-58 to arginine resulted in the expression of a full-length tryptophan hydroxylase mutant that, although remaining catalytically active, was neither phosphorylated nor activated by protein kinase A. These results indicate that the activation of tryptophan hydroxylase by protein kinase A is mediated by the phosphorylation of serine-58 within the regulatory domain of the enzyme.
色氨酸羟化酶是神经递质血清素生物合成过程中的起始限速酶，可被蛋白激酶A以及钙/钙调蛋白依赖性蛋白激酶激活。通过磷酸化-去磷酸化级联反应对任何一种酶进行调节的一个重要方面，也是色氨酸羟化酶所缺乏的一个方面，在于确定蛋白激酶对其磷酸化的位点。脑源性色氨酸羟化酶的重组形式被表达为谷胱甘肽S-转移酶融合蛋白，并与蛋白激酶A接触。这种蛋白激酶可使全长色氨酸羟化酶磷酸化并激活。该酶的无活性调节结构域（对应于氨基酸1-98）也被蛋白激酶A磷酸化。羟化酶的催化核心（氨基酸99-444），其具有高水平的酶活性，既未被蛋白激酶A磷酸化，也未被其激活。将丝氨酸58突变为精氨酸导致全长色氨酸羟化酶突变体的表达，该突变体虽然仍具有催化活性，但既未被蛋白激酶A磷酸化，也未被其激活。这些结果表明，蛋白激酶A对色氨酸羟化酶的激活是由该酶调节结构域内丝氨酸58的磷酸化介导的。
