Structure and properties of proteins

Each species has proteins specific to it; even proteins with the same function (for example haemoglobin) differ slightly in different animals. The diversity of proteins is explained by three factors: the amino acid composition, their number and their order in the chain. The primary structure is the sequence of amino acids joined by peptide bonds. In the secondary structure the chain coils into a helix (or pleated sheet), and hydrogen bonds form between NH and CO groups on neighbouring turns; keratin and collagen are examples. In the tertiary structure the helix folds in its own way into a globule (for example myoglobin), held by hydrogen, ionic, disulphide and hydrophobic bonds. The quaternary structure forms when several polypeptide chains join: haemoglobin has four chains and four iron-containing haem groups. Even the replacement of a single amino acid can change a protein’s shape and function.