amino acid | Learn Science at Scitable

Amino
acids are small molecules that are the building blocks of proteins. Proteins
serve as structural support inside the cell and they perform many vital
chemical reactions. Each protein is a molecule made up of different
combinations of 20 types of smaller, simpler amino acids. Protein molecules are
long chains of amino acids that are folded into a three-dimensional shape.

Chemically,
an amino acid is a molecule that has a carboxylic acid group and an amine group
that are each attached to a carbon atom called the α carbon. Each of the 20
amino acids has a specific side chain, known as an R group, that is also
attached to the α carbon. The R groups have a variety of shapes, sizes,
charges, and reactivities. This allows amino acids to be grouped according to
the chemical properties of their side chains. For example, some amino acids
have polar side chains that are soluble in water; examples include serine,
threonine, and asparagine. Other amino acids avoid water and are called
hydrophobic, such as isoleucine, phenylalanine, and valine. The amino acid
cysteine has a chemically reactive side chain that can form bonds with another
cysteine. Amino acids can also be basic, like lysine, or acidic, like glutamic
acid. The sequence and interactions between the side chains of these different
amino acids allow each protein to fold into a specific three-dimensional shape
and perform biological functions.