Trytophan FAQs

What is tryptophan?
What does the name "tryptophan" mean?
When was tryptophan identified?
What organisms can make tryptophan?
How is tryptophan biosynthesized?
How is tryptophan metabolized?

Q.  What is tryptophan?
A.  Tryptophan is one of the twenty-one amino acids commonly found in proteins which are encoded in the genetic code.  As with other amino acids, the configuration found in proteins is the L or S stereoisomer.

Q.  What does the name "tryptophan" mean?
A.  Tryptophan means, from Greek, "illumination by fragmentation".

Q.  When was tryptophan identified?
A.  The structure of tryptophan was determined by Hopkins and Cole in 1903 by synthesis.

Q.  What organisms can make tryptophan?
A.  Bacteria, archea, and plants can synthesize tryptophan.  All animals require it in the diet.

Q.  How is tryptophan biosynthesized?
A.  Tryptophan biosynthesis is linked to the biosynthesis of the other aromatic amino acids, all of which are derived from the shikimic acid pathway.  Tryptophan biosynthesis branches with anthranilate synthase eventually leading to indoleglycerol phosphate, which reacts with L-serine catalyzed by tryptophan synthase to give L-tryptophan.

Q.  How is tryptophan metabolized?
A.  Some bacteria can degrade L-tryptophan to indole and ammonium pyruvate with tryptophanase (tryptophan indole-lyase).  Eucaryotes and a few bacteria (e.g., Pseudomonas fluorescens) cleave the pyrrole ring with a dioxygenase, followed by hydrolysis to give L-kynurenine.  In eucaryotes, kynureninase cleaves L-3-hydroxykynurenine to give L-alanine and 3-hydroxyanthranilic acid, which subsequently is cleaved by a dioxygenase to give quinolinic acid.  Quinolinic acid can be used for synthesis of NAD+, but if it is produce in excess can be neurotoxic.