The amino acid leucine, is used in proteins more than others. Leucine with 9.1 percent (its average in more than 1.150 different proteins) is used most and tryptophan with 1.4 percent is used less than all other amino acids.

Why is leucine used more than tryptophan?

  • $\begingroup$ Are you talking in humans, animals, plants, single celled critters? My comment under @WYSIWYG 's answer depends on that as plants can synthesize Trp de novo and we can't $\endgroup$ – AliceD Jan 13 '15 at 11:35

This is a guess.

Tryptophan is a bulky amino acid. Having a lot of if would make the protein bulky. It is anyways hydrophobic and cannot be present on protein exteriors; high tryptophan may distort hydrophobic core possibly because of its bulkiness and ability to form pi-stacking interactions.
However, tryptophan is an essential amino acid and cannot be made de novo in humans. Indeed, it has only one codon perhaps because of its limited usage.
Nonetheless, tryptophan, despite its low abundance, is important in stabilization of protein structures such as β-hairpin [1] and it is also required for synthesizing serotonin.

Leucine is present in the hydrophobic core and β-regions. It also surrounds active sites which causes exclusion of water; this in turn facilitates catalysis by allowing polar residues to bind to the substrates [2].

I don't have any comment on why not isoleucine or valine instead of leucine.

  • $\begingroup$ @ChrisStronks yes.. and it also has only one codon perhaps because its usage is less. Trp is also essential for making serotonin. $\endgroup$ – WYSIWYG Jan 13 '15 at 11:34
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    $\begingroup$ @ChrisStronks it looks better but I don't think it is the availability that restricts its usage. Even other species do not use Trp much. Leu is hydrophobic. $\endgroup$ – WYSIWYG Jan 13 '15 at 11:47
  • $\begingroup$ maybe you're right and we cannot make it because we don't need it.... feel free to ignore my additions. Interesting thoughts! $\endgroup$ – AliceD Jan 13 '15 at 12:04

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