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Substitutions:
As Tyrosine is an aromatic, partially hydrophobic,
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amino
acid, it prefers substitution with other amino acids of the same
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type
(see above). It particularly prefers to exchange with
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Phenylalanine,
which differs only in that it lacks the hydroxyl group in
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the
ortho position on the benzene ring.
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Role
in function: Unlike the very similar Phenylalanine, Tyrosine
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contains
a reactive hydroxyl group, thus making it much more likely to
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be
involved in interactions with non protein atoms. Like other
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aromatic
amino acids, Tyrosine can be involved in interactions with
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non-protein
ligands that themselves contain aromatic groups via
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stacking
interactions.
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A
common role for Tyrosines (and Serines and Threonines) within
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intracellular
proteins is phosphorylation. Protein kinases frequently
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attach
phosphates to Tyrosines in order to fascilitate the signal
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transduction
process. Note that in this context, Tyrosine will rarely
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substitute
for Serine or Threonine, since the enzymes that catalyse the
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reactions
(i.e. the protein kinases) are highly specific (i.e. Tyrosine
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kinases
generally do not work on Serines/Threonines and vice versa)
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