1.4.2 Enzymes

Question 1

How do enzymes help reactions to proceed quickly at body temperatures?

Answer 1

Lower activation energy (needed to start a reaction).

Question 2

Describe the induced-fit model of enzyme action.

Answer 2

Before the reaction, the active site is not fully complementary to the substrate, but the shape of active site changes as the substrate binds. The enzyme-substrate complex stresses / distorts / bends the bonds in the substrate, lowering the activation energy required for the reaction (so that is can proceed).

Question 3

What factors affect the rate of enzyme-controlled reactions?

Answer 3

enzyme concentration
substrate concentration
concentration of competitive and of non-competitive inhibitors
pH
temperature

Question 4

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

Answer 4

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, because the enzyme-substrate complex places stress on bonds in the substrate.

Question 5

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

Answer 5

1.  Competitive inhibitor (or maybe just a part of the competitive inhibitor) has a similar shape to the substrate, so is complementary to the active site;

2.  Competitive inhibitor competes with substrate for active site;

3.   Less substrate attaches to the active site, so fewer enzyme-substrate complexes form

Question 6

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

Answer 6

1.   Attaches to the enzyme at a site other than the active site (allosteric site).

2.   Changes tertiary structure of enzyme resulting in a change in shape of the active site.

3.   Active site and substrate no longer complementary, so fewer enzyme-substrate complexes form.

4.   Cannot be overcome by adding more substrate.

Question 7

Two different enzymes can act on the same molecule. Suggest how.

Answer 7

EITHER
Active sites are different shapes;
So complementary to DIFFERENT PARTS of the substrate, forming different enzyme-substrate complexes;

OR
Two different enzymes have active sites with similar tertiary structures;
So both form enzyme-substrate complexes with the same substrate (in the SAME part of the substrate);

Question 8

Some enzymes only become able to bind to their substrates when phosphorylated. Use your knowledge of enzyme structure to suggest why.

Answer 8

Phosphorylation changes the tertiary structure of the enzyme, so the active site becomes complementary to the substrate.

Question 9

Describe and explain the effect of increasing substrate concentration on the rate of an enzyme controlled reaction.

Answer 9

At low concentrations, increasing the concentration increases the rate of reaction (shown by the gradient of a graph) because more enzyme-substrate complexes are formed. Substrate concentration is the limiting factor.

At high concentrations, increasing the concentration has no effect on the rate of reaction (shown by the plateau of a graph) because all enzymes are involved in enzyme-substrate complexes at any one time, so there are no free active sites. Enzyme concentration is the limiting factor.