All proteins are linear polymers of amino acids, large sequences of which constitute a peptide chain. Our focus is on globular proteins, whose peptide chain has a folded structure. In general, they are soluble in water and in other polar solvents. Although their structure is complex, we will see in this chapter that they often assume similar forms and that their shapes, sequence, and conformation can be understood by considering some fundamental aspects of their structure.

Amino acids and primary structure

The fundamental unit (monomer) of the protein molecule is the α amino acid. It consists of an acidic carboxyl group and an amino group attached to a single carbon atom, referred to as the α-carbon, and a hydrogen atom. This is illustrated in Fig. 2.1. A side-chain of molecules, designated as “R,” is also attached to the amino acid. This side-chain is specific to each amino acid and is what differentiates them from each other. The side-chains can vary in complexity; examples are simple hydrogen atoms, an extra amino group, an extra carboxylic group, a sulphydryl group, a hydroxyl group, or a simple hydro-chain or hydro-carbon ring. There are 20 biologically important amino acids listed in Table 2.1. The table also lists some proteins and their amino acid composition.