Carboxyl group, amino acid structure

There are 20 common amino acids in the human body. All amino acids contain a carboxylic acid group and an amino group. Amino acids can be linked by amide bonds to form proteins. Each amino acid has a different side chain, which is attached to the centre carbon atom. Figure 2.16, below, shows the structure of an amino acid. The letter G represents the side chain of the amino acid. Examples of side chains include —CH3 (for the amino acid alanine), —CH2CH2CONH2 (for glutamine), and —CH2OH (for serine). 

Amino acids As you saw in Chapter 23, many different functional groups are found in organic compounds. Amino acids, as their name implies, are organic molecules that have both an amino group and an acidic carboxyl group. The general structure of an amino acid is shown below. 

The molecule formed by condensing two amino acids is called a dipeptide. The amino acid with a free a-N+H3 group is known as the amino terminal, or simply the N-terminal amino acid, and the amino acid with a free —COO group is known as the carboxyl, or C-terminal amino acid. Structures of proteins are conventionally written with their N-terminal amino acid on the left. 

Structural formula (A) contains both an amino group (a base) and a carboxyl group (an acid). Proton transfer from the stronger acid (— COOH) to the stronger base (— NH2) gives an internal salt therefore, (B) is the better representation for alanine. Within the field of amino acid chemistry, the internal salt represented by (B) is called a zwitterion (Chapter 18). 

These are the structural units of protein. The term amino indicates the presence of the NHj group-a base-while the term acid indicates the presence of the COOH groups or carboxyl group-an acid. Since all amino acids possess this unique chemical feature of containing both an acid and a base, they are capable of both acid and base reactions in the body. Thus, they are said to be amphoteric. There are 22 amino acids. 

A somewhat different case is that of the a-amino acids, which contain a supplementary carboxylic group in their structure. Two examples are provided, for example, by aspartic and glutamic acids. 

The most important aspect of Table 27.1 is that the 20 anino acids that occur in proteins share the common feature of being a-anino acids, and the differences fflnong them are in their side chains. Peptide bonds linking carboxyl and a-anino groups characterize the structure of proteins, but it is the side chains that are mainly responsible for theh properties. The side chains of the 20 commonly occuning amino acids encompass both large and small differences. The major differences between amino acid side chains concern ... 

The structures and abbreviations for the 20 amino acids commonly found in proteins are shown in Figure 4.3. All the amino acids except proline have both free a-amino and free a-carboxyl groups (Figure 4.1). There are several ways to classify the common amino acids. The most useful of these classifications is based on the polarity of the side chains. Thus, the structures shown in Figure 4.3 are grouped into the following categories (I) nonpolar or hydrophobic... 

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