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Title: Proteins

  • Topic 7.5

7.5 Proteins
  • 7.5.1 Explain the four levels of protein
    structure, indicating the significance of each
  • 7.5.2 Outline the difference between fibrous and
    globular proteins, with reference to two examples
    of each protein type.
  • 7.5.3 Explain the significance of polar and
    non-polar amino acids.
  • 7.5.4 State four functions of proteins, giving a
    named example of each.

Four levels of protein structure
  • Primary organization
  • Chain of amino acids held together by polypeptide
  • 20 amino acids may be arranged in any order and
    is determined by DNA
  • Primary structure determines the next three
    levels of protein organization
  • (Changing one amino acid here may completely
    alter the structure and function of a protein, as
    in sickle cell disease)

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  • Secondary organization
  • Created by the formation of hydrogen bonds
    between the oxygen from the carboxyl group of one
    amino acid and the hydrogen from the amino group
    of another
  • Most common configurations
  • a-helix
  • ß-pleated sheet

  • Tertiary organization
  • Polypeptide chain bends and folds over itself
    because of interactions among R-groups and the
    peptide backbone
  • Important in determining the specificity of the
    proteins known as enzymes
  • Interactions
  • Disulfide bonds
  • Hydrogen bonds
  • Hydrophobic/hydrophillic
  • Ionic bonds

  • Quaternary organization
  • Involves multiple polypeptide chains which
    combine to form a single structure
  • Some include prosthetic or non-polypeptide groups
    called conjugated proteins
  • Ex. Hemoglobin contains four polypeptide chains,
    each of which contains a non-polypeptide group
    called a heme (contains an iron atom that binds
    to oxygen)

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Fibrous and globular proteins
  • Fibrous proteins
  • Composed of many polypeptide chains in a long,
    narrow shape
  • Insoluble in water
  • Examples
  • Collagen structure of connective tissue in
  • Actin component of human muscle
  • Globular proteins
  • More 3-D in their shape
  • Water soluble
  • Examples
  • Hemoglobin delivers oxygen to body tissues
  • Insulin regulates blood glucose level in humans

Polar and non-polar amino acidsRefers to
property of R group
  • Polar
  • Hydrophilic
  • Found in regions exposed to water
  • Membrane proteins toward interior and exterior of
  • Create hydrophilic channels in proteins through
    which polar substances can move
  • Non-polar
  • Hydrophobic
  • Found in regions of proteins that are linked to
    the hydrophobic area of the cell membrane

Significance of polar and non-polar amino acids
  • Controls position of proteins in membranes,
    creating hydrophilic channels through membranes
  • Determinacy of specificity of an enzyme
  • Each enzyme has a region called the active site
  • Only specific substrates can combine with a
    particular active site
  • Combination is possible when fitting occurs
    which involves general shape and polar properties
    of the substrate and the amino acids exposed at
    the active site

Function Protein Description
Transport Hemoglobin Contains iron that transports oxygen from the lungs to all parts of the body in vertebrates
Movement Actin and myosin Interact to bring about muscle contraction in animals
Hormone Insulin Secreted by pancreas that aids in maintaining blood glucose level in vertebrates
Defense Immunoglobulins Group that act as antibodies to fight bacteria and viruses