questions

• Describe how to test for proteins in a sample.

  Biuret test confirms presence of peptide bond

  1. Add equal volume of sodium hydroxide to sample at room temperature.
  2. Add drops of dilute copper (Il) sulfate solution. Swirl to mix. (steps 1 & 2 make Biuret reagent)
  3. Positive result: colour changes from blue to purpleNegative result: solution remains blue.
• How many amino acids are there and how do they differ from one another?

  • 20
  • differ only by side 'R' group
• Describe the 2 types of secondary protein structure. a-helix:

  • 
    
  • all N-H bonds on same side of protein chain
  • spiral shape
  • H-bonds parallel to helical axis
  • ß-pleated sheet:
  • N-H & C=0 groups alternate from one side to the other
• Define 'tertiary structure' of a protein. Name the bonds present.

  3D structure formed by further folding of polypeptide

  • disulfide bridges
  • ionic bonds
  • hydrogen bonds
• Describe each type of bond in the tertiary structure of proteins.

  • Disulfide bridges: strong covalent S-S bonds between molecules of the amino acid cysteine
  • lonic bonds: relatively strong bonds between chargedR groups (pH changes cause these bonds to break)
  • Hydrogen bonds: numerous & easily broken
• Define 'quaternary structure' of a protein.

  • Functional proteins may consist of more than one polypeptide.
  • Precise 3D structure held together by the same types of bond as tertiary structure.
  • May involve addition of prosthetic groups e.g metal ions or phosphate groups.
• Describe the structure and function of globular proteins.

  • Spherical & compact.
  • Hydrophilic R groups face outwards & hydrophobicR groups face inwards = usually water-soluble.
  • Involved in metabolic processes e.g. enzymes & haemoglobin
• Describe the structure and function of fibrous proteins.

  • Can form long chains or fibres
  • insoluble in water.
  • Useful for structure and support e.g. collagen in skin.
• Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst.

  Answer: 

  1. Substrate binds to the active site/enzyme
  2. Active site changes shape (slightly) so it is complementary to substrate
  3. Reduces activation energy;
• Explain how the active site of an enzyme causes a high rate of reaction.

  1.Lowers activation energy;

  2.Induced fit causes active site to change shape;

  3. enzyme-substrate complex causes bonds to form
• Formation of an enzyme-substrate complex increases the rate of reaction.

  Explain how.

  1. Reduces activation energy;
  2. Due to bending bonds
• Explain how a change in the primary structure of a globular protein may result in a different three-

  dimensional structure. [3]

  Sequence of amino acids changes; tertiary structure changes/

  folds in a different way;

  bonds form in different places (not peptide);
• Describe and explain how an increase in temperature affects the rate of an

  enzyme controlled reaction.

  [5]

  Rate of reaction increases;

  Increasing temperature increases rate of kinetic energy;

  substrate enters active site more often; Up to optimum;

  Rate of reaction decreases;

  High temperatures cause denaturation/ loss of tertiary structure/3D structure;

  By breaking specified ( didulfide ,ionic, hydrogen) bonds (not peptide bond); Active site altered/substrate cannot bind
• An enzyme catalyses only one reaction. Explain why [2]

  (Enzyme has) active site; Only substrate fits (the active site);
• Describe the induced fit model of enzyme action. [2]

  Active site / enzyme not

  complementary;

  Active site changes (shape) / is flexible;

  (Change in enzyme allows) substrate to fit / E-S complex to form;
• Describe one way that the lock and key model is different from the induced fit

  model.

  Active site does not change (shape) / is fixed (shape) / is rigid
• Explain why the rate of

  reaction of an enzyme is low away from its optimum pH.

  [3]

  (change in pH) leads to breaking of bonds holding tertiary structure; substrate will not bind withactive site; fewer/no ES complexes formed;
• A protein molecule contains 150 amino acids. What is the total number of peptide bonds in this molecule? [1]

  149
• Describe how the structure of a protein depends on the amino acids it contains.

  1. Structure is determined by position of amino acid
  2. Primary structure is sequence of amino acids;
  3. Secondary structure formed by hydrogen bonding (between amino acids)
  4. Tertiary structure formed by interactions (between R groups);
  5. Creates active site in enzymes
  6. Quaternary structure formed by bonds between polypeptides;
• Explain how the active site of an enzyme causes a high rate of reaction.

  1. Lowers activation energy;
  2. Induced fit causes active site (of enzyme) to change shape;
  3. (So) enzyme-substrate complex causes bonds to form/break;
• A dipeptide consists of two amino acids joined by a peptide bond. Dipeptides may differ in the type of amino acids they contain. Describe two other ways in which all dipeptides are similar and one way in which they might differ.

  • similarities; Amine(group at end); Carboxyl (group at end);
  • differences; Variable/different R group
• Describe how a peptide bond is formed between two amino acids to form a dipeptide.

  • Condensation (reaction)
  • Between amine and carboxyl
• The secondary structure of a polypeptide is produced by bonds between amino acids. Describe how.

  • Hydrogen bonds
  • Forming β pleated sheets / α helix;
• Two proteins have the same number and type of amino acids but different tertiary structures. Explain why

  • Different primary structure
  • Forms ionic / hydrogen / disulfide bonds in different places
• Formation of an enzyme-substrate complex increases the rate of reaction. Explain how.

  • Reduces activation energy;
  • Without enzyme, very few substrates have sufficient energy for reaction
• Suggest two variables the biochemist controlled when investigating the effect of temperature on the rate of breakdown of a protein by the protease.

  • Enzyme concentration
  • pH
• Link the primary structure of a protein to its precise 3D shape (3)

  • Primary structure = unique sequence of amino acids
  • Each amino acid has an R group in a specific place and the ionic, hydrogen and disulfide bonds form between them
  • This caused the polypeptide to fold into a specific tertiary structure