Proteins

1. Proteins What do we need proteins for? What foods are good protein sources? How do we test for the presence of proteins?

2. Primary structure • The sequence of amino acids that forms the protein • Determined by the order of bases in DNA • Position and proportion of amino acids can vary

3. Secondary structure • Formed when the chain of amino acids coils or folds. • Forms either • an alpha helix • beta pleated sheet. • Hydrogen bonds give stability due to quantity.

4. Tertiary structure • 3D shape of a protein • Vital to its function Bonds between R groups that stabilise the 3D shape: • Disulphide bonds • Ionic bonds • Hydrogen bonds • Hydrophilic and hydrophobic interactions Heating a protein causes denaturation

5. Quaternary structure • Some proteins are made up of more than one polypeptide subunit joined together / or a polypeptide and an inorganic component • Examples include haemoglobin (4)and insulin(2) Not all proteins have a quaternary structure

6. Classification of Proteins Globular Fibrous Insoluble Structural Forms fibres Eg Collagen Keratin (mostly β pleating) • Soluble • Metabolically active (involved in reactions) • Rolls up into balls (globules) • Eg • Enzymes • Antibodies • Haemoglobin • Plasma proteins (mostly α helix)

7. Functions of Proteins Actin and Myosin Antibodies (Immunoglobulins) Hormones Enzymes Keratin Haemoglobin Collagen

8. Haemoglobin (globular, water soluble protein) • 4 polypeptide subunits (2 alpha and 2 beta). • Shape vital to function – to carry oxygen from lungs to the tissues • Most of structure is alpha helix • Haem group (prosthetic group), contains Fe2+ and is responsible for the colour • Each molecule binds 4 oxygen molecules 4 O₂

9. Did you know?

10. Collagen (Insoluble fibrous protein) • 3 polypeptide chains, wound around each other. • Each of the three chains are coils, held together with Hydrogen bonds • Each collagen molecule forms strong covalent bonds (cross-links) with other collagen molecules. • Forms collagen fibrils. Many collagen fibrils together form a collagen fibre.

11. Why is Collagen so strong? • Glycine is small and this allows close packing between molecules • Collagen chains form a tight coil….they lie close to each other • Many hydrogen bonds between R groups hold 3 chains together very closely • Strong covalent bonds with adjacent molecules creating a very stable fibril • fibre composed of parallel fibril molecules and the ends of these parallel molecules are staggered whichprevents line of weakness

13. Collagen provides mechanical strength in many areas including.... • Walls of arteries • Tendons • Formation of bones • Cartilage and connective tissue • Used in cosmetic treatments e.g. Lip plumping

14. Making polypeptides and proteins • On ribosome (protein synthesis) • Uses mRNA • According to the code on the mRNA the amino acids are assembled in the right order and then joined by a peptide bond.

16. Breaking down proteins • Enzymes involved... Protease enzymes Looking at two examples: • Hormone regulation • Ageing

18. Answers

19. ci. H bonds ii. • Globular • Made of 2 types of polypeptide • Contains a non-protein/prosthetic/Haem group • Transport of oxygen

20. Roles of Proteins

21. Homework • Revision for Biological molecules test on Monday 9th March