Why are there only 20 Amino Acids Found in Nature?

[joeyjo100]

We all know an amino acid consists of a carbon atom with an amine group, a carboxylic acid group, and a hydrogen atom. Also attached to this carbon is an 'R' group, which varies from amino acid to amino acid.

My question regards the 'R' group. If the R group can be anything from a hydrogen atom to a benzene ring or anythig in between, does this mean that there can be theoretically be an infinite number of amino acids?

why are there only 20 amino acids found in nature? why not more?

 

[morrobay]

There are more than 20 amino acids found in nature. Its that proteins are sequences
of 20 amino acids. Why not more than 20 ?
With a protein of 100 amino acids, made up of 20 different amino acids, there are
20¹⁰⁰ different protein chains that can be formed. So maby its not necessary to have more amino acids

 

[Ygggdrasil]

My question regards the 'R' group. If the R group can be anything from a hydrogen atom to a benzene ring or anythig in between, does this mean that there can be theoretically be an infinite number of amino acids?

Yes. Some chemists and biochemists have, in fact, been able to create proteins that incorporate amino acids not found in nature (for example, introducing functional groups like ketones, azides, alkenes, alkynes, and other chemical moieties) for various research and biotechnological applications. See for example the research done by Peter Schultz's lab at Scripps or David Tirrell's lab at Caltech.

why are there only 20 amino acids found in nature? why not more?

As morrobay mentioned, 20 amino acids probably gives proteins enough chemical variety to be able to evolve proteins that can function. Drastically expanding the genetic code to incorporate many more amino acids would probably tax the biochemistry of the cell as it would need some way to acquire or synthesize all of the different amino acids. Reducing the number of amino acids would probably make it harder for proteins to perform certain functions or fold into correct shapes. 20 amino acids likely sits in some happy medium between these two extremes. Is it the optimal number of amino acids? Probably not, but it works well enough, so nature has just stuck with it.

Also, as an aside, while most cells incorporate only 20 different types of amino acids into proteins, some specialized organisms can incorporate non-standard amino acids into proteins like selenocysteine or pyrrolysine. So, there is evidence that organisms can evolve ways to expand their genetic code (I'm not aware of any examples of organisms evolving to get rid of an amino acid).