WJEC Level 3 Certificate in Food Science and Nutrition

Haemoglobin molecule 58 Stage 3 Tertiary (third) structure The polypeptide chains fold more and more so that the protein molecule starts to become compact and more bonds are formed to hold it together. Stage 4 Quaternary (fourth) structure In the final stage of folding, several folded polypeptide chains join and fold together to form compact bundles. This is what happens, for example, in the protein called haemoglobin in red blood cells, as the model of a haemoglobin molecule demonstrates. Denaturation of protein molecules Heat, mechanical action (e.g. whisking) and pH (acidity /alkalinity) can disturb and break the bonds that hold protein molecules together and cause them to unravel. This is called denaturation . Digestion of proteins When food has been eaten and swallowed, it is then digested. Digestion means breaking something down into its component parts. During digestion, the long chains of polypeptides in protein molecules are broken down into individual amino acids by substances called enzymes in the stomach and small intestine. Enzymes speed up chemical reactions. They break the peptide bonds between the amino acids by putting back a molecule of water. This is called a hydrolytic reaction or hydrolysis ( hydro = water, lysis = set free /release), and is the reverse of the condensation reaction: 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 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 Hydrolytic reaction between two amino acids Denaturation – the breaking of bonds that hold protein molecules together and cause them to unravel Digestion – breaking something down into its component parts Enzymes – chemical substances (usually proteins) that act as catalysts to speed up chemical reactions Hydrolysis – breaking the bond between two molecules that was formed by a condensation reaction by adding a molecule of water Key term bonds Section 2: Nutrition (LO2) Chapter 5: The structure of nutrients