Eduqas Biology for A Level Book 1: 2nd Edition

Exam tip Make sure you refer to a ‘peptide’ bond and not a ‘dipeptide’ bond. Study point Make sure you remember the elements in macromolecules: carbohydrates – C, H, O lipids – C, H, a little O proteins – C, H, O, N Study point The amino group and the carboxyl group give amino acids their name. Key term Peptide bond: The chemical bond formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of another. Knowledge check State the types of bonds that are formed as a result of condensation reactions between: A. Two glucose molecules. B. Fatty acids and glycerol. C. Two amino acids. 1.6 Proteins Proteins differ from carbohydrates and lipids in that, in addition to carbon, hydrogen and oxygen, they always contain nitrogen. Many proteins also contain sulphur and some contain phosphorus. Proteins are polymers made of monomers called amino acids. The chains of amino acids are called polypeptides. About 20 different amino acids are used to make up proteins. There are thousands of different proteins and their shape is determined by the specific sequence of amino acids in the chain. All amino acids have the same basic structure. Attached to a central carbon atom are: ▪ An amino group, –NH 2 , at one end of the molecule, called the N-terminal. ▪ A carboxyl, group –COOH, at the other end of the molecule, called the C-terminal. ▪ A hydrogen atom. ▪ The R group, which is different in each amino acid. Generalised structure of an amino acid An amino acid as a zwitterion The amino group is basic. At pH 7, the pH of the cell, it gains an H and becomes positively charged. The carboxyl group is acidic and at pH 7, it loses an H, becoming negatively charged. So at pH 7, an amino acid has both a positive and a negative charge. Such an ion is a ‘zwitterion’. Formation of a peptide bond Proteins are linear sequences of amino acids. The amino group of one amino acid reacts with the carboxyl group of another, with the elimination of water. The bond that is formed from this condensation reaction is a peptide bond , and the resulting compound is a dipeptide. Formation of a dipeptide The dipeptide can be written as NH 2 . CHR 1 . CO-NH . CHR 2 . COOH. But if the amino acids bonded the other way, they would form the dipeptide NH 2 . CHR 2 . CO-NH . CHR 1 . COOH. This is a different dipeptide with different properties. Fig 09 H H N R H C C OH O Fig 09 H H H N R H C C O O + – Fig 10 N C C H R 1 O H H OH amino acid amino acid N C C H H OH H R 2 O N C C H R 1 O H H dipeptide peptide bond water N C C + H O H OH H R 2 O 25  1.1 Chemical elements and biological compounds