Amino acids and proteins

The essentials

Proteins are polymers of α-amino acids, which contain an amino group and a carboxyl group on the same carbon. Amino acids are joined by peptide bonds (–CO–NH–), formed when the –COOH of one reacts with the –NH₂ of another, losing water. In aqueous solution an amino acid exists as a zwitterion, H₃N⁺–CHR–COO⁻, so amino acids behave like salts: they are crystalline, water-soluble and have high melting points, and they are amphoteric.

Key points

• About twenty amino acids occur in proteins. Those the body cannot make and must be obtained from food are essential amino acids (e.g. valine, leucine, lysine); others are non-essential (e.g. glycine, alanine).
• Except glycine (R = H), all naturally occurring α-amino acids are optically active and most have the L-configuration.
• Amino acids are classed as neutral, acidic or basic according to the relative number of amino and carboxyl groups.
• A chain of n amino acids has n – 1 peptide bonds. Chains with more than about a hundred residues are usually called proteins.
• Fibrous proteins (keratin in hair and wool, myosin in muscle) have parallel chains held by hydrogen and disulphide bonds; insoluble in water. Globular proteins (insulin, albumins) are folded into spheres; usually water-soluble.
• Primary structure: sequence of amino acids. Secondary: α-helix or β-pleated sheet, held by hydrogen bonds between –CO– and –NH– groups. Tertiary: overall folding, held by hydrogen bonds, disulphide links, van der Waals and electrostatic forces. Quaternary: arrangement of two or more polypeptide subunits.
• Denaturation: heat or pH change disrupts the secondary and tertiary structure, so the protein loses its biological activity; the primary structure stays the same. Examples: coagulation of egg white on boiling, curdling of milk.

Common mistakes

• Thinking denaturation breaks peptide bonds; it does not change the sequence.