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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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