Biological Molecules - Enzymes

Question 1

What is an enzyme and how does it work?

Answer 1

• Biological catalysts
• Lower activation energy so speed up the rate of a reaction
• Globular proteins
• (3D) Tertiary structure
• Have a specific active site shape that is complementary to the substrate
• Forms an enzyme-substrate complex

Question 2

How do enzymes lower the activation energy of a reaction?

Answer 2

• By forming new bonds and breaking bonds which requires energy

Question 3

How are enzymes specific?

Answer 3

• They have a specific tertiary structure
• So they have an active site shape that is complementary to the shape of the substrate
• The substrate binds to the active site to produce an enzyme-substrate complex

Question 4

Describe and explain the lock & key model of enzyme action

Answer 4

• Enzyme (lock) has an active site shape that is already complementary to the substrate (key)
• Substrate binding to the active site leads to the formation of the enzyme-substrate complex

Question 5

What are the limitations of the lock & key model?

Answer 5

• States the enzyme is rigid (like a lock) but enzyme structure is flexible not rigid
• Doesn’t explain how other molecules (e.g. activators or inhibitors) can bind to enzyme at sites other than active site & change its activity

Question 6

Describe and explain the induced fit model of enzyme action

Answer 6

• The substrate binds to the active site (which is not completely complementary)
• The active site changes shape to become complementary to the substrate (wraps around the substrate) and puts a strain on the bonds in the substrate
• This allows an enzyme-substrate to form

Question 7

Why is the induced fit model a better explanation?

Answer 7

• It explains how other molecules affect enzyme activity (enzyme is flexible), e.g. activators & inhibitors
• It explains how the activation energy is lowered by putting a strain on the bond

Question 8

Describe & explain the effect of substrate concentration on an enzyme controlled reaction

Answer 8

• As substrate concentration increases, the rate of reaction increases proportionally
• This is because the substrate concentration is limiting. As substrate concentration increases, there are more enzyme substrate collisions so more enzyme-substrate complexes
• As substrate concentration increases further, the rate of reaction plateaus
• This is because all enzyme active sites are filled all of the time. Meaning substrate concentration is no longer limiting, instead the enzyme concentration is

Question 9

Explain in detail, why an increase in temperature causes an increase in the rate of a reaction

Answer 9

• As temperature increases, the rate of reaction also increases
• This is because there is more kinetic energy so more collisions occur, enabling more enzyme-substrate complexes to form
• Until the rate peaks at the optimum temperature ____
• As temperature increases further, the rate of reaction decreases
• This is because hydrogen & ionic bonds are broken so the tertiary structure changes. The active site shape changes so its no longer complementary to the substrate
• The enzyme is denatured so no enzyme-substrate complexes are formed

Question 10

Describe and explain, how an increase or decrease in pH affects the rate of an enzyme-controlled reaction

Answer 10

• As pH decreases (below ___) the rate of reaction also decreases
• This is because there are more H ions so hydrogen & ionic bonds are broken, causing the tertiary structure to change. The active site changes shape so its no longer complementary to the substrate
• The enzyme is denatured so no enzyme-substrate complexes are formed
• The peak pH is ___ because it is the optimum
• As pH increases (above ___) the rate of reaction decreases
• This is because there are more OH ions so hydrogen & ionic bonds break, causing the tertiary structure to change. The active site changes shape so its no longer complementary to the substrate
• The enzyme is denatured so no enzyme-substrate complexes are formed

Question 11

What are the features of competitive inhibitors?

Answer 11

• They have a similar shape to the substrate (not the same shape)
• Binds to active site
• Blocks the substrate from binding with active site
• Less enzyme-substrate complexes
• Increasing substrate concentration increases the rate of reaction

Question 12

What are the features of non-competitive inhibitors?

Answer 12

• Binds to enzyme away from the active site (at an allosteric site)
• Changes the active site shape
• So it is no longer complementary to substrate, so substrate can’t bind
• No enzyme-substrate complexes formed
• Increasing substrate concentration will have no effect on the rate of reaction