NATURE OF AMINO ACIDS
An amino acid is any molecule with at least one carboxyl (COOH) group and one amino group (–NH2). The amino acids found in proteins are called alpha (α) amino acids because the amino group is attached to the first or α-carbon atom adjacent to the carboxyl group.
The general formula of α-amino acids is shown below.
The general formula of α-amino acids is shown below.
The variable R group commonly referred to as the side chain can contain alkyl groups, aromatic groups, –NH2 , –COOH , –OH, and S-containing groups.
Amino acids are divided into four groups based on the characteristics of the amino acid side chains. This classification has been chosen to emphasize the importance of the side chains in amino acid and protein structure.
Amino acids are classified as basic, neutral, or acidic, depending on whether the ratio of –NH2 to –COOH groups in the molecules is greater than 1:1, equal to 1:1, or less than 1:1, respectively. This ratio differs from 1:1 only if the amino acid side chain (R–) contains an additional amino or carboxyl group.
The table below shows the 20 common amino acids in proteins:
Amino acids are divided into four groups based on the characteristics of the amino acid side chains. This classification has been chosen to emphasize the importance of the side chains in amino acid and protein structure.
- Nonpolar amino acids: The side chains are either aliphatic or aromatic and are hydrophobic.
- Polar, uncharged amino acids: The side chains contain functional groups with polar bonds such as alcohols or amides and are hydrophilic.
- Acidic amino acids: The side chain contains a carboxyl group. Also called “negatively charged” amino acids due to their form a physiological pH.
- Basic amino acids: The side chain contains a nitrogen that can act as a base. Also called “positively charged” amino acids due to their form a physiological pH.
Amino acids are classified as basic, neutral, or acidic, depending on whether the ratio of –NH2 to –COOH groups in the molecules is greater than 1:1, equal to 1:1, or less than 1:1, respectively. This ratio differs from 1:1 only if the amino acid side chain (R–) contains an additional amino or carboxyl group.
The table below shows the 20 common amino acids in proteins: