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Protein with all Amino Acids
Name: Mehraj
Status: educator
Grade: n/a
Location: Outside U.S.
Country: India
Date: Fall 2012
Question:
Is there any protein which is made up of all 20 amino acids?
Replies:
Mehraj,
To answer your question honestly, I'm not sure if there is a single
protein that contains every one of the 20 canonical amino acids. It's
quite possible that some of the larger structural proteins do, but I
don't know.
However, your question raises an interesting point. We like to think
of all proteins as being composed of the same 20 basic building
blocks. However, that is not actually true. There are a number of
other amino acids used sparingly in protein production. Notably,
selenocysteine is the '21st' amino acid and is used in a few proteins
in the place of cysteine (with a selenium in the place of a sulfur
atom). Another amino acid not contained in the classical 20 is
pyrrolysine, which is a lysine with a pyrrole ring attached; it is
used in some microorganisms.
If you expand your search a bit further, you'll find even more
variants on amino acids. Pyrrolysine and selenocysteine are inserted
directly into the protein during the translation process from mRNA,
but some amino acids are modified after translation into different
chemical structures. Notably, hydroxyproline is a chemically modified
form of proline that is a very common posttranslational modification
in humans; it forms nearly a third of the amino acids in most
collagens. A number of other amino acids are similarly modified after
translation to have different structure and properties than the simple
20 we learn in school.
Expanding our thinking even further, and there are additional
posttranslational modifications that affect the chemical structure of
an amino acid. There are a variety of enzymes that can modify amino
acids in a protein to add chemical groups, sugar chains, and lipids to
change the structure and properties of the protein.
In this context, it becomes clear that there's such a large diversity
of different functional amino acids - including the wide variety of
posttranslational modifications - that finding a protein containing
all of the 20 canonical amino acids is largely meaningless. The cell
is capable of providing so much more flexibility and diversity in its
amino acids and their modifications that it's impossible for a single
protein to contain every single possibility.
S. Unterman Ph.D.
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Update: November 2011
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