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Updated: Aug 26, 2025

PCR Mutagenesis, Cloning, Expression, Fast Protein Purification Protocols and Crystallization of the Wild Type and Mutant Forms of Tryptophan Synthase
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Systemic tryptophan homeostasis.

Simon Klaessens1,2, Vincent Stroobant1,2, Etienne De Plaen1,2

  • 1Ludwig Institute for Cancer Research, Brussels, Belgium.

Frontiers in Molecular Biosciences
|October 3, 2022
PubMed
Summary

Tryptophan homeostasis is crucial for health, regulated by protein TDO stability. TDO degrades excess dietary tryptophan or is degraded itself when tryptophan is scarce, preventing immune and neurological issues.

Keywords:
IDO1 (indoleamine 2,3-dioxygenase 1)SLC16A10SLC6A19TDO (tryptophan 2,3-dioxygenase)hartnup diseasepellagratryptophantumor

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Area of Science:

  • Biochemistry
  • Immunology
  • Metabolic pathways

Background:

  • Tryptophan is an essential amino acid vital for protein synthesis, co-enzyme production (NAD/NADP(H)), and neurotransmitter synthesis (serotonin, melatonin).
  • Tryptophan's role in immune homeostasis is critical, as its catabolism impacts T-lymphocyte function.
  • Maintaining stable plasma tryptophan levels is essential despite dietary fluctuations.

Purpose of the Study:

  • To review the checkpoints governing tryptophan homeostasis, including absorption, transport, metabolism, and elimination.
  • To discuss the pathophysiology of disorders arising from dysfunctions in tryptophan homeostasis.
  • To highlight the regulatory role of tryptophan 2,3-dioxygenase (TDO) protein stability in controlling plasma tryptophan levels.

Main Methods:

  • Review of existing literature on tryptophan metabolism and homeostasis.
  • Analysis of the kynurenine pathway, focusing on indoleamine 2,3-dioxygenase 1 (IDO1) and tryptophan 2,3-dioxygenase (TDO).
  • Examination of TDO protein regulation by tryptophan levels and its impact on tryptophan catabolism.

Main Results:

  • Tryptophan is primarily catabolized via the kynurenine pathway by IDO1 and TDO.
  • TDO, predominantly in the liver, accounts for 90% of tryptophan catabolism and its stability is tightly regulated by plasma tryptophan.
  • TDO protein is stabilized and rapidly degrades tryptophan when levels are high, but is degraded via the proteasome when levels are low, preventing excessive catabolism.

Conclusions:

  • Tryptophan homeostasis is maintained through intricate regulatory mechanisms, including TDO protein stability.
  • Dysfunction in TDO or hepatic transporters like SLC16A10 can lead to severe hypertryptophanemia.
  • Hypertryptophanemia poses risks to immune and neurological homeostasis, underscoring the importance of tryptophan balance.