Proteins

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  • Proteins
    • Amino Acid
      • All amino acids contain:
        • Nitrogen
        • Hydrogen
        • Oxygen
        • Carbon
        • Many also contain sulphur or phosphorous
      • Structure
        • All amino acids have the same general structure
        • The only difference between each amino acid is the nature of the R group
          • There are 20 different R groups giving 20 different proteins
            • The R group represents a side chain from the central 'alpha' carbon and can be anything from a simple hydrogen atom to a more complex ring structure
    • Diverse class of biological molecules
    • Polypeptides (polymers) built of long chains of monomers called amino acids
      • Amino Acid
        • All amino acids contain:
          • Nitrogen
          • Hydrogen
          • Oxygen
          • Carbon
          • Many also contain sulphur or phosphorous
        • Structure
          • All amino acids have the same general structure
          • The only difference between each amino acid is the nature of the R group
            • There are 20 different R groups giving 20 different proteins
              • The R group represents a side chain from the central 'alpha' carbon and can be anything from a simple hydrogen atom to a more complex ring structure
    • Dipeptide
      • Two amino acids bond to form a dipeptide
      • A peptide bond forms between the carboxylic group of one amino acid and the amino group of the other amino acid
        • This is a condensation reaction so a mole of water is formed.
    • Polypeptides
      • Three or more amino acids joined by peptide bonds
      • There are up to 4 levels of structure in a protein, each playing an important role in the overall structure and function of the protein:
        • Secondary Structure
          • The polypeptide chains can twist to form either:
            • A beta pleated sheet
              • Hydrogen bonds between the peptide bonds maintain the shape of the structure
                • An alpha helix
            • An alpha helix
        • Primary Structure
          • Determined by the type, number and sequence of amino acids in the chain
            • If the sequence changes (due to mutation in the DNA) then the protein will be non functional, even though the type and number of amino acids stays the same
        • Tertiary
          • Forms when the secondary structured polypeptide folds to into a specific 3 dimensional shape
            • The structure is held in place by the following bonds:
              • Ionic Bonds between R groups containing oppositely charged ions
              • Hydrogen Bonds between R groups containing O- and H+
              • Untitled
        • Quarternary
          • Formed when two or more polypeptides with a tertiary structure combine
          • Held together by Hydrogen bonds, Disulphide bonds and Ionic bonds
          • E.g. Haemoglobin
            • Made up of 4 tertiary structured polypeptides bonded together (Two alpha helix and two beta pleated sheet)
    • Protein Classification
      • Fibrous Proteins
        • Secondary Structure
        • Long, parallel polypeptide chains, joined by peptide bonds, twisted into an alpha helix or beta pleated sheet and held in position by hydrogen bonds
        • Several polypeptide chains twist together to form long, rope like fibres
        • Insoluble, so cannot dissolve
        • Structural Function E.g.
          • Collagen
            • Main component of connective tissue found in ligaments, tendons and cartilage
          • Keratin
            • The main component of hard structures such as hair, nails, hooves and claws
      • Globular Proteins
        • Tertiary or quarternary structure
        • Long polypeptide chains tightly fold into a specific 3D shape held in position by Hydrogen bonds, Disulphide bonds and Ionic bonds between R groups of amino acids
        • Soluble in water so can dissolve and diffuse into water
        • Biochemical Function
          • Enzymes
            • Tertiary structured globular proteins that catalyse chemical reactions e.g. lipase
          • Transport Proteins
            • Such as haemoglobin, myoglobin and those embedded in membranes
          • Hormones
            • E.g. Oestrogen and Insulin
          • Antibodies
    • Denaturing Proteins
      • If the bonds that maintain a proteins shape are broken, the protein will stop working properly and is denatured
        • Changes in pH, temperature or salt concentration can all denature a protein, although the specific conditions will vary from protein to protein
          • Fibrous proteins lose their structural strength when denatured, whereas globular proteins become insoluble and inactive
    • Testing for proteins
      • 1. Add biuret agent (Copper sulphate solution and sodium hydroxide)
        • A purple/lilac colour shows protein is present (if it stays blue-negative result)

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