1.2.1 Many proteins are enzymes

Question 1

How do enzymes act as biological catalysts ?

Answer 1

• each enzyme lowers activation energy of reaction it catalyses
• to speed up rate of reaction

Question 2

Describe the induced-fit model of enzyme action

Answer 2

1. Substrate binds to (not completely complementary) active site of enzyme
2. Causing active site to change shape (slightly) so it is complementary to its substrate
3. So enzyme-substrate complex forms
4. Causing bonds in substrate to bend / distort, lowering activation energy

Question 3

Describe how models of enzyme action have changed over time

Answer 3

● Initially lock and key model (now outdated)
○ Active site a fixed shape, complementary to one substrate
● Now induced-fit model

Question 4

Explain the specificity of enzymes

Answer 4

Specific tertiary structure determines shape of active site
○ Dependent on sequence of amino acids
(primary structure)
● Active site is complementary to a specific substrate
● Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex

Question 5

Describe and explain the effect of enzyme concentration on
the rate of enzyme-controlled reactions

Answer 5

● As enzyme concentration increases, rate of reaction increases
○ Enzyme concentration = limiting factor
(excess substrate)
○ More enzymes so more available active
sites
○ So more enzyme-substrate complexes form
● At a certain point, rate of reaction stops increasing / levels off
○ Substrate concentration = limiting factor
(all substrates in use)

Question 6

Describe and explain the effect of temperature on
the rate of enzyme-controlled reactions

Answer 6

● As temperature increases to optimum, rate of reaction increases
○ More kinetic energy
○ So more enzyme-substrate complexes
form
● As temperature exceeds optimum, rate of reaction decreases
○ Enzymes denature - tertiary structure and active site change
shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer enzyme-substrate complexes
form

Question 7

Describe and explain the effect of pH on
the rate of enzyme-controlled reactions

Answer 7

● As pH increases / decreases above / below an optimum, rate of
reaction decreases
○ Enzymes denature - tertiary structure and
active site
change shape
○ As hydrogen / ionic bonds break
○ So active site no longer complementary
○ So fewer enzyme-substrate complexes
form

Question 8

Describe and explain the effect of concentration of competitive inhibitors on
the rate of enzyme-controlled reactions

Answer 8

● As concentration of competitive inhibitor increases, rate of
reaction decreases
○ Similar shape to substrate
○ Competes for / binds to / blocks active
site
○ So substrates can’t bind
○ So fewer enzyme-substrate complexes form
● Increasing substrate concentration reduces effect of inhibitors
(dependent on relative concentrations of substrate and inhibitor)

Question 9

Describe and explain the effect of concentration of
non-competitive inhibitors on the rate of enzyme-controlled reactions

Answer 9

● As concentration of non-competitive inhibitor increases, rate of
reaction decreases
○ Binds to site other than the active site
(allosteric site)
○ Changes enzyme tertiary structure / active
site shape
○ So active site no longer complementary to
substrate
○ So substrates can’t bind
○ So fewer enzyme-substrate complexes
form
● Increasing substrate concentration has no
effect on rate of
reaction as change to active site is
permanent