1.4.2 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 of reaction

Question 3

Explain the specificity of enzymes

Answer 3

• specific tertiary structure determines shape of active site
  
  • dependant 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 4

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

Answer 4

• 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 5

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

Answer 5

• as substrate concentration increases, rate of reaction increases
  
  • substrate concentration = limiting factor (too few substrate molecules to occupy all active sites)
  • more enzymes so- substrate complexes form
• at a certain point, rate of reaction stops increasing / levels off
  
  • enzyme concentration = limiting factor
  • as all active sites saturated / occupied

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 e-s 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 inhibitors increase, 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

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 inhibitors increase, 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 had no effect on rate of reaction as change to active site is permanent