Topic 7.6 Enzymes

Question 1

7.6.1 State that metabolic pathways consist of chains and cycles of ______-catalysed reactions.

Answer 1

Metabolic pathways consist of chains and cycles of enzyme-catalysed reactions.

Question 2

7.6.2 Describe the induced-fit model.

Answer 2

• enzymes undergo significant changes in their conformation when substrates combine with their active sites
• Induced-fit model (hand & glove analogy):
  
  • initially, substrate doesn’t fit perfectly into active sit of enzyme
  • when substrate binds to active site, the shape of the active site is changed, and the substrate fits perfectly - induced fit
  • accounts for ability of some enzymes to bind to several substrates - if shape of active site changes when substrate binds, allows for many different but similar substrates to bind to one enzyme

Question 3

7.6.3 Explain that enzymes lower the activation energy of the chemical reactions that they catalyse.

Answer 3

1. chemical environment provided by active site of enzyme for substrate causes changes within substrate molecule, weakening its bonds
2. substrate changed to a transition state (different from transition state when enzyme isn’t involved)
3. activation energy lowers & substrate is altered by rearrangement of existing atoms
4. transformed substrate (product) is released from active site
5. unchanged enzyme is then free to combine with other substrate molecules

Question 4

7.6.4 Explain the difference between competitive and non-competitive inhibition, with reference to one example of each.

Answer 4

Competitive inhibition

• molecule called competitive inhibitor competes directly for active site of enzyme
• substrate has fewer encounters with active site, thus chemical reaction rate ↓
  
  • competitive inhibitor must have similar structure to substrate to function this way
• e.g. use of sulfanilamide to kill bacteria during an infection
  
  • folic acid is essential as coenzyme to bacteria → produced in bacterial cells by enzyme action of PABA
  • sulfanilamide competes with PABA and blocks enzyme
  • human cells don’t use PABA to produce folic acid ∴ unaffected by drug

Non-competitive Inhibition

• AKA allosteric inhibition
• involves inhibitor that doesn’t compete for enzyme’s active site
  
  • inhibitor binds to allosteric site (different from active site)
• binding at allosteric site causes change in shape of enzymes active site, making it non-functional
• e.g. metallic ions (mercury) binding to sulfur groups of component amino acids of many enzymes → results in protein shape changes which causes inhibition of enzyme

Note: may be reversible or irreversible; also examples of allosteric interaction activating an enzyme rather than inhibiting it.

Question 5

7.6.5 Explain the control of metabolic pathways by end-product inhibition, including the role of allosteric sites.

Answer 5

• End-product inhibition prevents cell from wasting chemical resources & energy by making more of a substance than needed.
• metabolic pathways consist of chains/cycles to produce an end-product; each step catalyzed by a specific enzyme.
• when ↑ concentration of end-product, end-product binds with allosteric site of 1^st enzyme → brings inhibition
  
  • prevents build-up of intermediates in cell
    
    • form of negative feedback
• when ↓ concentration/existing end-product is used up by cell, fewer bindings with allosteric site of 1^st enzyme → reactivates enzyme
• enzyme that is inhibited is called an allosteric enzyme