PROTEINS

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PROTEINS

• C, H, O, N, (S)
• Polymers made from chains of amino acids
• 20 amino acids used
• Linked by a peptide bond

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In addition fibrous proteins (collagen) form
structural components in cells and tissues
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Amino Acids

• Central carbon has attached
• Amine group
• Acid group
• Functional group (R) determines nature of amino
  acid
• R groups fall into 4 categories
• Non-polar - only carbons chains or aromatic
  rings (methionine has sulphur)
• Uncharged Polar- carbons with amine groups (NH2)
  or - hydroxyl groups (OH)
• Acidic - carboxylic acid groups (COOH) ionizes to
  negative charge COO-
• Basic - terminal amine groups (not next to CO)
  ionizes to NH3

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Peptide bond

• Amino acids joined by a peptide bond
• Condensation reaction between
• COOH of 1st amino acid and NH2 of 2nd amino acid

• Chains are called peptides (short)/ polypeptides
  (longer)
• Peptide bond is rigid
• Bonds either side can rotate
• Introduces flexibility allowing proteins to take
  up variety of shapes.

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Protein Structure

• 4 levels
• Primary
• Secondary
• Tertiary
• Quaternary

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PRIMARY STRUCTURE
PRIMARY STRUCTURE DETERMINES other levels of
STRUCTURE

• Order in which amino acids are linked together
• Written starting at the N (amino) terminus
• e.g.
• Arg-Lys-Phe-Glu-Ser-Gly-
• R K F E S G

N
C
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SECONDARY STRUCTURE

• Two possible shapes in the protein chain each
  stabilised by Hydrogen bonds
• ?-pleated sheet
• ?-helix

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?-pleated sheet

• Peptide chains arranged side by side
• Held together by H-bonds between the two chains
• Parallel (chains running same direction)
• N ? C
• N ? C
• Antiparallel (chains in opposite directions)
• N ? C
• C ? N

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?-pleated sheet

• Silk
• Resistant to stretch (very strong)

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?-helix

• Peptide chain coils into a helix
• Held by H-bond between N-H group and the CO 4
  residue away
• 4 residues per turn

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?-helix

• Hair/Wool (keratin)
• Stretchy (er)
• ?-helices coiled together to form a superhelix
• For horn/hoof more disulphide bridges are present
  (covalent)

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Tertiary Structure

• The overall folded shape of a protein held
  together by (usually) weak forces.
• Hydrogen bonding which doesnt form secondary
  structure
• Hydrophobic interactions
• Place non-polar amino acids inside protein
• Polar amino acids on surface
• Van der Waals forces
• Ionic interactions (strong)
• Disulphide bridges (strong)
• Covalent bond between cysteine residues

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• With reference to bonding, explain why enzyme
  activity decreases as you increase the
  temperature above the optimum, and as you move pH
  away from the optimum.

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Proteins fold to take up their shape
Shape is determined by primary structure order
of hydrophobic/ hydrophilic amino acids
relative positions of polar/charged amino
acids. Loss of tertiary structure is called
denaturation.
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Proteins are 3D
Lysozyme
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Primary structure determines tertiary structure
Mutation acid (polar) for non polar changes
folding pattern
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Prosthetic groups

• Some proteins have permanently bound non protein
  groups, called prosthetic groups
• e.g. myoglobin haemoglobin bind to a porphyrin
  (haem) chelating an Iron atom
• e.g. Chlorophyll has a similar prosthetic group
  chelating Mg
• The protein without its prosthetic group is
  called an apoprotein, with its group it is called
  a holoprotein

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Co-factors/ Co-enzymes

• Other proteins have inorganic ions temporarily
  bound to them
• E.g. copper/ zinc on enzymes
• Others have carbon containing molecules
  temporarily attached
• e.g. Coenzyme A, NAD, FAD

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Quaternary Structure

• Only present if protein has more than one
  polypeptide chain
• Describes the shape adopted by the interacting
  polypeptide chains

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Nucleic Acids

• DNA
• Deoxyribonucleic Acid
• RNA
• Ribonucleic Acid
• Video

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Nucleic Acids

• Summary Knowledge
• DNA deoxyribose sugar,
• RNA ribose sugar
• DNA double stranded (antiparallel)
• RNA single stranded
• DNA thymine,
• RNA uracil
• A double (hydrogen) bonds to T (A 2 T)
• G triple (hydrogen) bonds to C (G 3 C)
• G A purines (small word, big molecule A Giant)
• C,T U pyrimidines (big word, small molecule)

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