Mechanism of Inhibition of Novel Tryptophan Hydroxylase Inhibitors Revealed by Co-crystal Structures and Kinetic Analysis

Giovanni Cianchetta*, 1, Terry Stouch1, Wangsheng Yu2, Zhi-Cai Shi1, Leslie W. Tari3, Ronald V. Swanson3, Michael J Hunter3, Isaac D. Hoffman3, Qingyun Liu*, 2
1 Department of Medicinal Chemistry, Lexicon Pharmaceuticals, Inc., 350 Carter Rd., Princeton, New Jersey, USA
2 Department of Pharmaceutical Discovery, Lexicon Pharmaceuticals, Inc., 8800 Technology Forest Pl., The Woodlands, Texas, USA
3 Activesight, Inc., San Diego, California, USA

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© Cianchetta et al.; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

* Address correspondence to these authors at the Department of Medicinal Chemistry, Lexicon Pharmaceuticals, Inc., 350 Carter Rd., Princeton, NJ, USA; Tel: (609)466-6055; Fax: (609)466-3562; E-mail: gcianchetta@lexpharma.comCorrespondence: Brown Foundation Institute of Molecular Medicine, 1825 Pressler St., Suite 330H, Houston, TX, USA; Tel: (713)500-3808; Fax: (713)500-0319; E-mail:


Trytophan Hydroxylase Type I (TPH1), most abundantly expressed in the gastrointestinal tract, initiates the synthesis of serotonin by catalyzing hydroxylation of tryptophan in the presence of biopterin and oxygen. We have previously described three series of novel, periphery-specific TPH1 inhibitors that selectively deplete serotonin in the gastrointestinal tract. We have now determined co-crystal structures of TPH1 with three of these inhibitors at high resolution. Analysis of the structural data showed that each of the three inhibitors fills the tryptophan binding pocket of TPH1 without reaching into the binding site of the cofactor pterin, and induces major conformational changes of the enzyme. The enzyme-inhibitor complexes assume a compact conformation that is similar to the one in tryptophan complex. Kinetic analysis showed that all three inhibitors are competitive versus the substrate tryptophan, consistent with the structural data that the compounds occupy the tryptophan binding site. On the other hand, all three inhibitors appear to be uncompetitive versus the cofactor 6-methyltetrahydropterin, which is not only consistent with the structural data but also indicate that the hydroxylation reaction follows an ordered binding mechanism in which a productive complex is formed only if tryptophan binds only after pterin, similar to the kinetic mechanisms of tyrosine and phenylalanine hydroxylase.

Keywords: Serotonin, structure, kinetics, gastrointestinal disorder, carcinoid, monooxygenase..