Protein and Enzymes

• Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst.

  1. Substrate binds to the active site of enzyme and enzyme-substrate complex forms;

  2. Active site changes shape slightly so it is complementary to substrate and distorts bonds in the substrate;

  3. Reduces activation energy;
• Scientists investigated the action of the enzyme ATP synthase.

  They made reaction mixtures each containing:

  • ATP synthase

  • buffer (to control pH)

  • substrates.

  One of the substrates required in these reaction mixtures is inorganic phosphate (Pi).

  Which other substrate the scientists must add to the reaction mixtures to produce ATP?

  Adenosine diphosphate
• A competitive inhibitor decreases the rate of an enzyme-controlled reaction. Explain how.

  1. Inhibitor similar shape to substrate;

  2. Binds to active site;

  3. Prevents enzyme-substrate complex forming;
• When bread becomes stale, the structure of some of the starch is changed. This changed starch is called retrograded starch. Scientists have suggested retrograded starch is a competitive inhibitor of amylase in the small intestine. Assuming the scientists are correct, suggest how eating stale bread could help to reduce weight gain.

  1. Less hydrolysis of starch;

  2. To maltose;

  3. So less absorption of glucose
• Describe how the structure of a protein depends on the amino acids it contains.

  1. Structure is determined by relative position of amino acid interactions;

  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
• Describe how amino acids join to form a polypeptide so there is always NH2 at one end and COOH at the other end.

  1. One amine group joins to a carboxyl group to form a peptide bond;

  2. So in chain there is a free amine group at one end and a free carboxyl group at the other
• 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;
• Describe a biochemical test to confirm the presence of protein in a solution.

  1. Add biuret reagent;

  2. Positive result purple
• A dipeptide consists of two amino acids joined by a peptide bond. Dipeptides may differ in the type of amino acids they contain.

  Describe other ways in which all dipeptides are similar and one way in which they might differ.

  Similarities

  1. Amine group at end;

  2. Carboxyl group at end;

  3. Two R groups;

  4. All contain C and H and N and O;

  Difference

  5. R groups;
• Describe how a non-competitive inhibitor can reduce the rate of an enzyme-controlled reaction.

  1. Attaches to the enzyme at a site other than the active site;

  2. Changes shape of the active site

  3. So active site and substrate no longer complementary so no substrate can bind;
• Describe how a peptide bond is formed between two amino acids to form a dipeptide.

  1. Condensation reaction resulting in loss of water;

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

  1. Hydrogen bonds;

  Between amine group of one amino acid and carboxyl group and forming beta pleated sheets or alpha helix;
• Two proteins have the same number and type of amino acids but different tertiary structures. Explain why.

  1. Different sequence of amino acids which results in different primary structure;

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

  1. Reduces activation energy;

  2. Due to weakening bonds
• The genetic code is described as degenerate. What is meant by this?

  More than one codon codes for a single amino acid;