How many amino acids are there inside the cell?
Sterioisomerism in Amino Acids
Before answering how many amino acids are there inside the cell, I would like to define what amino acids are. Amino acids are those carboxylic acids that contain an amino group at the α-carbon. Amino acids are chiral compounds (except for the glycine which is achiral). They contain two main functional groups; an amino group and a carboxylic acid attached to the same α-carbon.
Amino acids are the building blocks of proteins and polypeptides that are essential to the life. Because of these two functional groups, they are considered as amphoteric compounds. An amphoteric compound that has both acidic and basic nature.
Amino acids do exist in zwitterionic form with positive and negative ions in the same molecule. Because of their differences in the side chain, they have two or more ionizable groups. Each ionizable groups contribute to its own pKa value and having two or more pK values make them existing in more than one ionic form.
Zwitterioinic form of Amino Acids
Classification of Amino Acids
There are 20 standard and few non-standard amino acids that can be categorized in various ways. Some of them are essential amino acids that we need to take through the diet while others are non-essential. Non-essential in the sense that they can be synthesized in our body from the essential amino acids.
Amino acids can also be categorized based on their chemical properties; acidic amino acids, basic amino acids, neutral polar amino acids, aromatic amino acids, amide amino acids, alcohol amino acids, non-polar amino acids. For example, tryptophan, phenylalanine, and tyrosine are aromatic amino acids. In the same way, aspartate, glutamate are acidic amino acids while arginine, lysine, and histidine are basic amino acids. Asparagine, glutamine, glycine, serine, threonine, cysteine are polar amino acids while, alanine, valine, leucine, isoleucine, proline, methionine, are non-polar amino acids.
Now, come to the point; how many amino acids are there inside the cell? There are two main types of amino acids; standard amino acids and non-standard amino acids. Standard amino acids are those that are naturally present in all proteins and enzymes. Amino acids that are not present in naturally occurring proteins are called as non-standard amino acids.
All of the naturally occurring standard amino acids are in the form of L-configuration and are called as L-amino acids. In contrast to that, non-standard amino acids can either be L- or D- but the most common form of non-standard amino acids are of the D-form. Aforementioned amino acids such as glutamate, aspartate, glutamine, asparagines, etc. are standard amino acids while β-alanine, D-alanine, ornithine, and citrulline are some of the non-standard amino acids.
Some of the non-standard amino acids
Non-standard amino acids have some special purposes as mentioned below;
- 4-Hydroxypyroline; a derivative of proline that can be found as a component of collagen.
- 5-Hydroxylysine; a derivative of lysine found as a component of collagen.
- 6-N-Methyllysine; a derivative of lysine that is found in muscles protein collagen.
- γ-carboxyglutamate; a derivative of glutamate.
- selenocysteine; a derivative of cysteine that can be found in some selenoproteins such as thioredoxin reductase and glutathione-S-transferase.
- ornithine; a derivative of lysine that is present in the urea cycle.
- citrulline; a derivative of arginine that is found in the urea cycle.
- Β-alanine and D-alanine that are present in the bacterial cells for the protection from external harsh conditions.
© 2015 Shrawan Kumar Upadhyay