Function of Tryptophan

For many organisms (including humans), tryptophan is an <a href="http://en.wikipedia.org/wiki/Essential_amino_acid" title="Essential amino acid" >essential amino acid</a>. This means that it cannot be synthesized by the organism and therefore must be part of its diet. Amino acids, including tryptophan, act as building blocks in <a href="http://en.wikipedia.org/wiki/Protein_biosynthesis" title="Protein biosynthesis" >protein biosynthesis</a>. In addition, tryptophan functions as a biochemical <a href="http://en.wikipedia.org/wiki/Precursor_(chemistry)" title="Precursor (chemistry)" >precursor</a> for the following compounds (see also figure to the right):

<ul >

<li ><a href="http://en.wikipedia.org/wiki/Serotonin" title="Serotonin" >Serotonin</a> (a <a href="http://en.wikipedia.org/wiki/Neurotransmitter" title="Neurotransmitter" >neurotransmitter</a>), synthesized via <a href="http://en.wikipedia.org/wiki/Tryptophan_hydroxylase" title="Tryptophan hydroxylase" >tryptophan hydroxylase</a>.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid6132421-7" >[8]</a><a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid1704290-8" >[9]</a> Serotonin, in turn, can be converted to <a href="http://en.wikipedia.org/wiki/Melatonin" title="Melatonin" >melatonin</a> (a <a href="http://en.wikipedia.org/wiki/Neurohormone" title="Neurohormone" >neurohormone</a>), via <a href="http://en.wikipedia.org/wiki/N-Acetyltransferase" title="N-Acetyltransferase" class="mw-redirect" >N-acetyltransferase</a> and <a href="http://en.wikipedia.org/wiki/5-hydroxyindole-O-methyltransferase" title="5-hydroxyindole-O-methyltransferase" class="mw-redirect" >5-hydroxyindole-O-methyltransferase</a> activities.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid4391290-9" >[10]</a></li>

</ul>

<ul >

<li ><a href="http://en.wikipedia.org/wiki/Niacin" title="Niacin" >Niacin</a> is synthesized from tryptophan via <a href="http://en.wikipedia.org/wiki/Kynurenine" title="Kynurenine" >kynurenine</a> and<a href="http://en.wikipedia.org/wiki/Quinolinic_acid" title="Quinolinic acid" >quinolinic acids</a> as key biosynthetic intermediates.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid14284754-10" >[11]</a></li>

</ul>

<ul >

<li ><a href="http://en.wikipedia.org/wiki/Auxin" title="Auxin" >Auxin</a> (a <a href="http://en.wikipedia.org/wiki/Phytohormone" title="Phytohormone" class="mw-redirect" >phytohormone</a>) when <a href="http://en.wikipedia.org/wiki/Sieve_tube_elements" title="Sieve tube elements" class="mw-redirect" >sieve tube elements</a> undergo <a href="http://en.wikipedia.org/wiki/Apoptosis" title="Apoptosis" >apoptosis</a> tryptophan is converted to auxins.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid18394986-11" >[12]</a></li>

</ul>

The disorders <a href="http://en.wikipedia.org/wiki/Fructose_malabsorption" title="Fructose malabsorption" >fructose malabsorption</a> and <a href="http://en.wikipedia.org/wiki/Lactose_intolerance" title="Lactose intolerance" >lactose intolerance</a> cause improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-12" >[13]</a> and depression.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-13" >[14]</a>

In bacteria that synthesize tryptophan, high cellular levels of this amino acid activate a <a href="http://en.wikipedia.org/wiki/Repressor" title="Repressor" >repressor</a> protein, which binds to

the <a href="http://en.wikipedia.org/wiki/Trp_operon" title="Trp operon" >trp operon</a>.<a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-pmid16285852-14" >[15]</a> Binding of this repressor to the tryptophan operon prevents transcription of downstream DNA that codes for the enzymes involved in the biosynthesis of tryptophan.

So high levels of tryptophan prevent tryptophan synthesis through a negative feedback loop and, when the cell's tryptophan levels are reduced, transcription from the trp operon resumes. The genetic organisation of the trp operon thus permits tightly regulated and rapid responses to changes in the cell's internal and external tryptophan levels.