WJEC Level 3 Certificate in Food Science and Nutrition

57 The letter ‘ R ’ is called a side chain , and it varies in different amino acids. For example, in the simplest amino acid, glycine , the side chain is one hydrogen (H) atom. This is called the carboxyl group and is often written as COOH. NH 2 is called the amino group . Amino acids are joined to other amino acids by peptide bonds . These are formed when the amino group (NH 2 ) of one amino acid reacts with the carboxyl group (COOH) of another amino acid, with the loss of one molecule of water. This is called a condensation reaction . When many amino acids are joined together in chains in this way, they are known as polypeptides (‘poly’ means many). Most protein molecules are very large and contain many amino acids. The polypeptide chains of amino acids are not straight, but are folded in different ways. This helps to make the large protein molecules become compact. To prevent the compact protein molecules from coming apart, the polypeptide chains are held together by bonds such as hydrogen bonds and sulphur-to-sulphur (disulphide) ‘bridges’. There are four stages of folding in protein molecules: Stage 1 Primary structure: polypeptide chain This is composed of lots of different amino acids. Stage 2 Secondary structure The polypeptide chain either twists like a corkscrew into a helix shape or folds into a concertina-shaped sheet. This is helped by the bonds that form between the polypeptide chains. Some of the amino acids are hydrophilic (they are attracted to water) and some are hydrophobic (they are repelled by water) and this also influences how the chains fold or twist. δ + C NH 2 H OH O R C C NH 2 H OH O R C C NH 2 OH R C + N N + C NH 2 H H O O R C C N H H H OH O R C water peptide bond C NH 2 H OH O R δ + C C NH 2 H OH O R C C NH 2 H OH O R C C NH 2 H OH O R C C NH 2 H OH O R C + C NH 2 H OH O R C C NH 2 H OH O R C + + C NH 2 H H O O R C C N H H H OH O R C water peptide bond + C NH 2 H H O O R C C N H H H OH O R C water peptide bond Condensation reaction between two amino acids Amino acids – organic compounds that contain a carboxyl (—COOH) and an amino (—NH 2 ) group, and join together to form proteins Condensation reaction – when two molecules bond (join) together with the loss of a molecule of water Hydrophilic – a substance that has an affinity (is attracted to) water Hydrophobic – a substance that repels water and will not mix with it Peptide bond – the bond that forms when the amino group of one amino acid reacts with the carboxyl group of another amino acid, with the loss of one molecule of water Polymers – the general name for a substance that is made up of many similar units bonded together Polypeptides – many amino acids joined together in long chains Key terms The chemical structure of the macronutrients