Amino acids are the building blocks of proteins
Amino acids are the building blocks of proteins. A protein is hydrolyzed into its constituent
amino acids when incubated in 6 M HCl at 120˚ C for 24 h. Proteins are in fact linear (i.e.,
unbranched) polymers of amino acids. Similarly, DNA is a linear polymer of nucleic acids, and
complex carbohydrates are either linear or branched polymers of simple sugars (monosaccharides).
The protein ‘alphabet’ consists of 20 amino acids, each of which is coded for by a specific triplet of
bases in DNA (although there is some degeneracy in this code). The protein alphabet is over two
million years old is the same for viruses, bacteria, plants, and animals. Glycine was the first amino
acid to be discovered in 1820, while the last was threonine in 1935.
Basic structure of an amino acid
An amino acid is a carboxylic acid with an amino group. All of the naturally occurring amino
acids are a-amino acids. They all contain a single hydrogen atom (H), a carboxyl (COOH) group
and an amine (NH2) group attached to a central a-carbon atom. The fourth position on the tetrahedral
a-carbon is filled by a sidechain (denoted “R”) which varies and is in fact what distinguishes one
amino acid from another. At physiological pH, the carboxyl and a-amino groups are both ionized.
The a-carbon atom of each amino acid (except glycine) is chiral because it has four different
substituents. Hence, there are two different stereoisomers which are mirror images of one another.
If you look down the aC–H bond of an amino acid with the H atom closest to you, the L-stereoisomer spells “CORN” when the substituents are read in a clockwise direction, while the mirror-image
D-stereoisomer spells “CORN” when read in an anticlockwise direction. All of the twenty naturally
occurring amino acids are of the L-configuration (except glycine, which is achiral).
