protein/ enzyme exam questions

Question 1

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

Answer 1

1. Substrate binds to the active site/enzyme
2. Active site changes shape (slightly) so it is complementary to
   substrate
3. Reduces activation energy;

Question 2

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.
Adenine
Adenosine diphosphate
Glucose
Ribose

Answer 2

Adenosine diphosphate;

Question 3

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

Answer 3

1. Inhibitor similar shape to substrate;
2. Fits/binds to active site;
3. Prevents/reduces enzyme-substrate complex forming;

Question 4

When bread becomes stale, the structure of some of the starch is
changed. This is called retrograded starch.
Scientists 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.

Answer 4

1. Inhibitor similar shape to substrate;
2. Fits/binds to active site;
3. Prevents/reduces enzyme-substrate complex forming;

Question 5

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

Answer 5

1. Structure is determined by (relative) position of amino acid/R
   group/interactions;
2. Primary structure is sequence/order 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 contains >1 polypeptide chain

Question 6

Explain how the active site of an enzyme causes a high rate of reaction

Answer 6

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;

Question 7

(a) Describe a biochemical test to confirm the presence of protein in a
solution.

Answer 7

1. Add biuret (reagent);
   Accept sodium hydroxide (solution) and copper sulphate (solution)
2. (Positive result) purple/lilac/violet /mauve;

Question 8

Describe how a non-competitive inhibitor can reduce the rate of an
enzyme-controlled reaction.

Answer 8

1. Attaches to the enzyme at a site other than the active site;
2. Changes tertiary structure (of enzyme);
3. (So active site and substrate) no longer complementary so
   less/no substrate can fit/bind;

Question 9

Describe how a peptide bond is formed between two amino acids to form a
dipeptide

Answer 9

1. Condensation (reaction) / loss of water;
2. Between amine / NH2 and carboxyl / COOH;
   Accept between amino (group) and carboxylic / acid

Question 10

Two proteins have the same number and type of amino acids but different
tertiary structures.
Explain why.

Answer 10

1. Different sequence of amino acids
2. Forms ionic / hydrogen / disulfide bonds in different places;

Question 11

The secondary structure of a polypeptide is produced by bonds between
amino acids.
Describe how.

Answer 11

b) 1. Hydrogen bonds;
2. Between NH (group of one amino acid) and C=O (group);

Question 12

Formation of an enzyme-substrate complex increases the rate of reaction.
Explain how.

Answer 12

1. Reduces activation energy;
2. Without enzyme, very few substrates have sufficient energy for
   reaction;

Question 13

Suggest two variables the biochemist controlled when investigating the
effect of temperature on the rate of breakdown of a protein by the protease.

Answer 13

1. Initial / starting substrate concentration
2. Enzyme concentration
3. pH.