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Article Addendum

Enzymatic features of serotonin biosynthetic enzymes and serotonin biosynthesis in plants

Kiyoon Kang, Sei Kang, Kyungjin Lee, Munyoung Park and Kyoungwhan Back

volume 3 | issue 6

june 2008
Pages: 389 - 390

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Serotonin, a pineal hormone in mammals, is found in a wide range of plant species at detection levels from a few nanograms to a few milligrams, and has been implicated in several physiological roles, such as flowering, morphogenesis, and adaptation to environmental changes. Serotonin synthesis requires two enzymes, tryptophan decarboxylase (TDC) and tryptamine 5-hydroxylase (T5H), with TDC serving as a rate-limiting step because of its high K_m in relation to the substrate tryptophan (690 µM) and its undetectable expression level in control plants. However, T5H and downstream enzymes, such as serotonin N-hydroxycinnamoyl transferase (SHT), have low K_m values with corresponding substrates. This suggests that the biosynthesis of serotonin or serotonin-derived secondary metabolites is restricted to cellular stages when high tryptophan levels are present.