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1.2 CARBOHYDRATES

1.2 CARBOHYDRATES. 1.1 WATER . 1.3 LIPIDS . MOLECULES OF LIFE. 1.5 NUCLEIC ACIDS . 1.4 PROTEINS . 1.4 PROTEINS. 1.4 Proteins (2 hours). Objectives : Describe the basic structure of amino acids. Describe the classes of amino acids and explain how they are grouped.

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1.2 CARBOHYDRATES

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  1. 1.2 CARBOHYDRATES 1.1 WATER 1.3 LIPIDS MOLECULES OF LIFE 1.5 NUCLEIC ACIDS 1.4 PROTEINS

  2. 1.4 PROTEINS

  3. 1.4 Proteins (2 hours) Objectives : • Describe the basic structure of amino acids. • Describe the classes of amino acids and explain how they are grouped. • Explain primary, secondary, tertiary & quaternary levels of protein structure and the types of bonds involved. • Explain the effect of pH & temperature on the structure of protein. • Classify proteins according to their structures.

  4. Structure of amino acid fibrous, globular, conjugated Classes of amino acids Classification according to structure PROTEINS polar, non-polar, acidic, basic Formation & breakdown of dipeptide Effect of pH & temperature Levels of protein structure 1o, 2o, 3o, 4o levels

  5. PROTEINS • Polymers called polypeptides • A protein consists of 1 or more polypeptides in specific conformations • Always composed of C, H, O & nitrogen; & sometimes sulphur • Monomers: amino acids

  6. AMINO ACID • Basic unit of a polypeptide/protein • The components of amino acid: • a basic amino group (-NH2) • an acidic carboxyl group (-COOH) • a variable R group (or side chain) • a hydrogen atom 6

  7. AMINO ACID O H N C H OH R H C AMINO Group CARBOXYL Group R Group Structure of amino acid 7

  8. CLASSES OF AMINO ACIDS • There are 20 common amino acids for proteins • All have the same basic structure but differ in the side chain ( R group) • Based on properties of the side chains, amino acids are grouped as: i. Non-polar ii. Polar iii. Acidic iv. Basic 8

  9. Classification of AMINO ACIDS Based on Side chain (R group) iii. Acidic (eg: aspartic acid) • Non-polar • (eg: glycine) ii. Polar (eg: serine) iv. Basic (eg: lysine) 9

  10. Non-polar amino acids have hydrophobic non-polar side chains

  11. Polar amino acids have polar side chains (making them hydrophilic)

  12. Acidic amino acids have –ve charged side chains Basic amino acids have +ve charged side chains

  13. FORMATION & BREAKDOWN OF DIPEPTIDE DIPEPTIDE : basic unit of protein H H R R N C C N C C H OH H OH H H O O H2O • Dipeptide: • consists of 2 amino acids • linked by peptide bond (a covalent bond) • formed through condensation reaction

  14. Peptide bond H H R R N C C N C C H OH H H O O H2O Formation of dipeptide 14

  15. Peptide bond Formation of dipeptide

  16. FORMATION & BREAKDOWN OF DIPEPTIDE DIPEPTIDE : basic unit of protein • A protein/polypeptide usually contains hundreds of amino acids linked by peptide bonds • Breakdown of dipeptide occurs due to hydrolysis (catalysed by protease)

  17. A functional protein consists of 1 or more polypeptide chains which may be twisted, folded & coiled Each protein has a specific 3-D conformation 4 levels of protein structure: primary (1O), secondary (2O), tertiary (3O) & quaternary (4O) LEVELS OF PROTEIN STRUCTURE

  18. Primary structure CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE • Describes the unique sequence of amino acids joined by peptide bonds in a linear polypeptide chain • The 20 common amino acids can be arranged in different ways (determined by genetic information) • Eg: glucagon consists of a sequence of 29 amino acids 18

  19. Primary structure CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE Glucagon consists of 29 amino acid units 19

  20. Primary structure of lysozyme Lysozyme: causes lysis of the bacterial cell wall 20

  21. Secondary structure Once a linear chain of amino acids is formed, it spontaneously … coils to form the alphahelix or folds to form the beta pleated sheet CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE

  22. The primary structure will spontaneously coil or fold, forming the secondary structure Alpha helix (coiled structure) Beta pleated sheet (folded structure) 22

  23. Secondary structure Hydrogen bonds holds the secondary structure together H bondsare formed between C=O& -NH groupsfrom the peptide bond regions maintain the stable structure of α-helix& β-pleated sheets CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE

  24. Hydrogen bonds hold helix in shape (a)

  25. Hydrogen bonds hold neighboring strands of sheet together (b)

  26. Secondary structure Shown by fibrous proteins (structural proteins) such as.. keratin (α-helix) found in hair, nails, horn silk protein (β-pleated sheet) produced by many insects & spiders CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE

  27. silk protein fibroin ~ produced by many insects & spiders 27

  28. Secondary structure 28

  29. Tertiary structure A polypeptide may be further coiled into a globular shape which is maintained by bonds & interactions among side chains Disulfide bonds: strong, covalent bonds between Rs with sulfhydryl groups Ionic bonds: strong bonds between +ve & -ve charged Rs H bonds: weak bonds between polar Rs Hydrophobic & van der Waals interactions: weak interactions between non-polar Rs LEVELS OF PROTEIN STRUCTURE

  30. Hydrogen bond Ionic bond Hydrophobic interaction Disulfide bond

  31. Tertiary structure (between non-polar side chains) (between polar side chains) (covalent bond between side chains with sulfhydryl groups) (weak bond between +ve & -ve charged side chains) 31

  32. Formation of disulphide bridge 32

  33. Tertiary structure : globular proteins Eg : myoglobin,enzymes, insulin

  34. Quaternary structure consists of 2ormorepolypeptide chainsjoined toform a single functional molecule CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE 34

  35. Quaternary structure CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE 35

  36. (a) Hemoglobin (b) Collagen Beta chain ( -globin) Alpha chain ( -globin) a b Heme Alpha chain ( -globin) Beta chain ( -globin) b a

  37. eg: haemoglobin consists of 4 globular polypeptide chains ( 2 α&2 β chains ) CLASSIFICATION of PROTEIN LEVELS OF PROTEIN STRUCTURE ..Quaternary structure 37

  38. CLASSIFICATION of PROTEIN eg:Collagen fibrous protein has 3 helical subunits intertwined to become a strong fibre LEVELS OF PROTEIN STRUCTURE …Quaternary structure 38

  39. 4 levels of protein structure Primary structure Secondary structure Tertiary structure Quaternary structure The linear amino acids sequence Results from hydrogen bonding from polypeptide backbone Can either form α-helix or β-pleated sheet eg.: fibrous proteins: keratin, silk protein of spider • Depends on interactions among side chains • Hydrogen bond • Hydrophobic & van der Waals interactions • Disulfide bridge • Ionic bond • eg.: globular proteins: enzymes, hormones Results from interactions among polypeptides eg.: Hb, collagen 39

  40. Levels of protein structure 40

  41. LEVELS OF PROTEIN STRUCTURE

  42. EFFECT OF pH & TEMPERATURE • Change in pH & temperature may cause denaturation of protein • where protein lose its natural specific conformation • due to disruption of H, ionic, disulfide bonds & hydrophobic interactions • causes the protein to lose its ability to function

  43. How denaturation occurs at levels of protein structure Quaternary structure (40) Dissociation of protein sub-units Disruption of the arrangement of the subunits Tertiary structure (30) Disruption of: Disulfide bridges Hydrogen bonds Ionic bonds van der Waals & hydrophobic interactions

  44. How denaturation occurs at levels of protein structure Secondary structure(20) proteins lose all regular patterns (alpha-helixes & beta-pleated sheets) because of the disruption of hydrogen bonds Primary structure (10) not disrupted by denaturation remain as sequence of amino acids held together by covalent peptide bonds

  45. Denaturationis sometimesreversible; ~anunfolded protein can berestoredtoits correct folding & regains its biological activity RENATURATION • If the denatured protein remains in aqueous environment & the denaturing agent is removed, it may renaturewhen chemical & physical aspects of its environment revert back to normal • Eg: keratin in rebonding technique

  46. 46

  47. CLASSIFICATION OF PROTEINS ACCORDING TO STRUCTURE 1. Fibrous proteins polypeptide chains organized as strands / sheets stable structures; won’t dissolve in water role in mechanical & structural functions eg: collagen, keratin

  48. 2. Globular proteins polypeptides folded into spherical shape relatively unstable structure; may form colloids in water generally for metabolic & chemical processes eg: enzymes, haemoglobin, myoglobin

  49. 3. Conjugated proteins proteins with non-protein material (prosthetic group) within their structure eg: - glycoprotein - lipoprotein - haemoglobin - nucleoprotein - flavoprotein - mucin

  50. [fibrous protein] [conjugated globular protein]

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