Tryptophan
<p><b >Tryptophan</b> (IUPAC-IUBMB abbreviation: <b >Trp</b> or <b >W</b>; IUPAC abbreviation: L-Trp or D-Trp; sold for medical use as <b >Tryptan</b>)<sup id="cite_ref-1" class="reference" ><a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-1" >[2]</a></sup> is one of the 20 <a href="http://en.wikipedia.org/wiki/List_of_standard_amino_acids" title="List of standard amino acids" class="mw-redirect" >standard amino acids</a>, as well as an <a href="http://en.wikipedia.org/wiki/Essential_amino_acid" title="Essential amino acid" >essential amino acid</a> in the <a href="http://en.wikipedia.org/wiki/Human" title="Human" >human</a> diet. </p>
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It is encoded in the standard <a href="http://en.wikipedia.org/wiki/Genetic_code" title="Genetic code" >genetic code</a> as the <a href="http://en.wikipedia.org/wiki/Codon" title="Codon" class="mw-redirect" >codon</a> <i >UGG</i>. Only the L-<a href="http://en.wikipedia.org/wiki/Stereoisomer" title="Stereoisomer" class="mw-redirect" >stereoisomer</a> of tryptophan is used in <a href="http://en.wikipedia.org/wiki/Fibrous_protein" title="Fibrous protein" class="mw-redirect" >structural</a> or <a href="http://en.wikipedia.org/wiki/Enzyme" title="Enzyme" >enzyme</a> proteins, but the D-<a href="http://en.wikipedia.org/wiki/Stereoisomer" title="Stereoisomer" class="mw-redirect" >stereoisomer</a> is occasionally found in naturally produced <a href="http://en.wikipedia.org/wiki/Peptide" title="Peptide" >peptides</a>(for example, the marine venom <a href="http://en.wikipedia.org/wiki/Peptide" title="Peptide" >peptide</a> <a href="http://en.wikipedia.org/wiki/Contryphan" title="Contryphan" >contryphan</a>).<sup id="cite_ref-Pallaghy_1999_2-0" class="reference" ><a href="http://en.wikipedia.org/wiki/Tryptophan#cite_note-Pallaghy_1999-2" >[3]</a></sup> The distinguishing structural characteristic of tryptophan is that it contains an <a href="http://en.wikipedia.org/wiki/Indole" title="Indole" >indole</a> functional group. It is an essential amino acid as demonstrated by its growth effects on <a href="http://en.wikipedia.org/wiki/Animal_testing_on_rodents" title="Animal testing on rodents" >rats</a>.</p>