PROTEINS

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PROTEINS

• Proteins are the most complex and most diverse
  group of biological compounds.
• If you weigh about 70 kgAbout 50 of your 70 kg
  is water.Many and various chemicals make up the
  remaining 20 kg.About half of that, or 10 kg, is
  protein.

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• Proteins have an astonishing range of different
  functions,
• Structure e.g. collagen (bone, cartilage,
  tendon), keratin (hair), actin (muscle)
• Enzymes e.g. amylase, pepsin, catalase, etc
  (gt10,000 others)
• Transport e.g. hemoglobin (oxygen)
• Pumps e.g. NaK pump in cell membranes
• Hormones e.g. insulin, glucagon

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• Motors e.g. myosin (muscle), kinesin (cilia)
• Receptors e.g. rhodopsin (light receptor in
  retina)
• Antibodies e.g. immunoglobulins
• Blood clotting e.g. thrombin, fibrin

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• Proteins are made of amino acids.
• Amino acids are made of four elements
• C H O and Nitrogen.
• General structure of amino acid molecules
• a central carbon atom (called the "alpha
  carbon"), with four different chemical groups
  attached to it
• -a hydrogen atom
• -a basic amino group
• -an acidic carboxyl group
• -a variable "R" group (or side chain)

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• There are 20 different R groups, and therefore 20
  different amino acids.
• Since each R group is different, each amino acid
  has different properties some are hydrophobic,
  some are hydrophilic, and some are ionic.
• The side chains interact with each other in a
  wide variety of ways
• This in turn means that proteins can have a wide
  range of properties

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Peptide Bonds

• Amino acids are joined together by peptide bonds.
• The reaction involves the formation of a
  molecule of water in a dehydration synthesis
  reaction
• Two amino acids joined together dipeptide.
• Many amino acids a polypeptide.

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Amino acid end
Carboxyl end
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The Rules of Protein Structure

• The function of a protein is determined by its
  shape.
• The shape of a protein is determined by its
  primary structure(sequence of amino acids).
• The sequence of amino acids in a protein is
  determined by the sequence of nucleotides in the
  gene (DNA) encoding it.

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Protein Structure

• Primary structure
• the sequence of amino acids. (This is dictated
  by genes).

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• Secondary structure
• This is the most basic level of protein folding.
• The secondary structure is held together by
  hydrogen bonds between the carboxyl groups and
  the amino groups in the polypeptide backbone.
• Because it is formed by backbone interactions it
  is largely independent of primary sequence
• The two most common secondary structures are the
  a-helix and the b- pleated sheet.

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• The a-helix.
• The polypeptide chain is wound round to form a
  helix.
• It is held together by hydrogen bonds running
  parallel with the long helical axis. There are so
  many hydrogen bonds that this is a very stable
  and strong structure

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• The b-sheet.
• The polypeptide chain zig-zags back and forward
  forming a sheet of antiparallel strands. Once
  again it is held together by hydrogen bonds.

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• Tertiary Structure
• This is the compact globular structure formed by
  the folding up of a whole polypeptide chain.
• Every protein has a unique tertiary structure,
  which is responsible for its properties and
  function.
• For example the shape of the active site in an
  enzyme is due to its tertiary structure.

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• The tertiary structure is held together by bonds
  between the R groups of the amino acids in the
  protein, and so depends on what the sequence of
  amino acids is. There are three kinds of bonds
  involved
• hydrogen bonds, which are weak.
• ionic bonds between R-groups with positive or
  negative charges, which are quite strong.
• sulphur bridges - covalent S-S bonds

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The tertiary structure is due to side chain
interactions and thus depends on the amino acid
sequence.
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The final three-dimensional shape of a protein
can be classified as globular or fibrous.
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Globular Proteins

• Globular proteins are relatively spherical in
  shape.
• Common globular proteins include egg albumin,
  insulin, and many enzymes
• They are somewhat soluble in water (depending on
  the sequence of amino acids)
• They are easily denatured

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FIBROUS PROTEINS

• Fibrous proteins form long protein filaments with
  rodlike shapes.
• They are usually structural or storage proteins.
• They are generally water-insoluble and not easily
  denatured
• Fibrous proteins are usually used to construct
  connective tissues tendons, bone matrix and
  muscle fiber.

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• Quaternary Structure
• This structure is found in proteins containing
  more than one polypeptide chain, and simply means
  how the different polypeptide chains are arranged
  together.
• Hemoglobin, the oxygen-carrying protein in red
  blood cells, consists of four globular subunits
  arranged in a tetrahedral structure.

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• Immunoglobulins, the proteins that make
  antibodies, comprise four polypeptide chains
  arranged in a Y-shape. The chains are held
  together by sulphur bridges.

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PROTEIN DENATURATION

• Globular proteins are held in their 3D form by a
  variety of bonds(hydrogen bonds, ionic bonds,
  covalent bonds) between R-groups
• When these bonds are disrupted, the shape of the
  protein changesit falls apart
• This usually means that is cannot accomplish its
  function

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• A number of agents can denature proteins
• Changes in pH
• changes in salt concentration
• changes in temperature (higher temperatures
  reduce the strength of hydrogen bonds)
• presence of reducing agents
• None of these agents breaks peptide bonds, so the
  primary structure of a protein remains intact
  when it is denatured.
• When a protein is denatured, it loses its
  function.