3.2 & 7.5 Proteins

1. 3.2 & 7.5 Proteins Pp 50 – 51 & 206 - 210 Pp 15 & 66 - 68

2. Proteins • Proteins are polymers of amino acids • Each has a unique 3D shape • Amino acid sequences vary • Proteins are major component of cell parts • The provide: • support and structural components • storage of amino acids • Several types of proteins are identified: receptor, contractile, defense, enzymes, structural

3. Building Blocks: Amino Acids • There are 20 different types of amino acids • All have this general formular • The R group is a variable group

4. Polar vs. Non-polar Amino Acids • Some amino acids are polar while some are non-polar • This property causes proteins to fold into varied shapes

5. Significance of polar & non-polar amino acids polar amino acids non-polar amino acids • hydrophilic • can make hydrogen bonds • found in hydrophilic channels & parts of proteins projecting from membranes • found on surface of water-soluble proteins • hydrophobic • forms van der Waals (hydrophobic interactions) with other hydrophobic amino acids • found in proteins in interior of membranes • found in interior of water-soluble proteins

6. Peptide Bonds • Proteins are formed by condensation • A peptide bond is formed

7. Four levels of protein structure

8. Primary Structure • Primary structure is a chain of amino acids • number & unique sequence of amino acids determine the properties of primary structure • each position is occupied by one of 20 different amino acids • sequence of amino acids is determined by DNA sequence in genes • linked by peptide bonds

9. Secondary structure • formed by interaction between amino and carboxyl i.e. -NH and -C=O groups • weak hydrogen bonds are formed between – H & = O • there are two types: a-helix and b-sheet • α- helix formed / polypeptide coils up e.g. sheep wool • β- pleated sheet formed e.g. silk in spider web • regular repeated folding of amino acid chain • secondary structure is stabilized by hydrogen bonds

10. Tertiary structure • These are globular proteins with irregular conformation • tertiary structure is the folding up of the polypeptide chain, secondary structure or alpha helix • it gives three dimensional globular shape i.e. shape of active site • the structure is stabilized by disulphide bridges, hydrogen, ionic& hydrophobic bonds • tertiary structure used as enzymes to catalyze biochemical reactions

11. Quaternary structure • made of several polypeptide subunits joined together • they maybe conjugated proteins i.e. proteins which combine with a prosthetic group (non-protein molecules) • prosthetic groups includes: metals e.g. iron in haemoglobin, nucleic acids as in ribosomes , carbohydrates as in glycoprotein or lipids as in glycolipids

12. Denaturation of Proteins • change in protein’s usual regular structure due to: • High temp • Change in pH • Addition of organic solvent (alcohol, acetone) • These factors break the bonds that stabilize the structure

13. Protein Functions Function Examples • Structure • Transport • Enzymes • Movement • Hormones • Antibodies • storage • – collagen/keratin/fibrin • – myoglobin/hemoglobin, bind & transport oxygen • – lysozyme, speeding up metabolic reactions • – actin (and myosin tropomyosin (and troponin) • – insulin, regulate blood glucose • – immunoglobulin • – albumin in egg, casein in milk

14. Difference between Fibrous and Globular Proteins • fibrous proteins are long and narrow strands or sheets whereas globular proteins are rounded, spherical or ball shaped • fibrous protein made of repetitive amino-acid sequences whereas globular proteins are made of irregular amino acid sequences • fibrous proteins are usually insoluble in water whereas globular proteins usually soluble in water • globular protein more sensitive to changes in pH, temperature & salt than fibrous proteins • fibrous proteins have structural or support functions (roles) whereas globular proteins have metabolic functions such as catalysis & transport function • Examples of fibrous proteins: keratin/fibrin/collagen/actin/myosin/silk protein • Examples of globular protein: insulin/immunoglobulin/hemoglobin/amylase

15. Revision Questions • Explain how polar and non-polar amino acids help channel proteins and enzymes carry out their functions. [5] • Distinguish between fibrous and globular proteins with reference to one example of each protein type. [6] • Explain primary structures and tertiary structures of a protein. [3] • Draw the structure of a generalized dipeptide. [4] • Draw the basic structure of an amino acid, and label the groups that are used in peptide bond formation. [4] • List four functions of membrane proteins. [4] • Giving a named example of each, state four functions of proteins. [8]