4.4 - Cofactors, Coenzymes, Prosthetic Groups

Question 1

Define cofactor and coenzyme

Answer 1

​Cofactor: Inorganicsubstaces that fit to the active site of an enzyme toactivate the enzyme

Coenzyme:Large organic substances that take part in an enzyme controlled reaction

• Usually transfer reactants between enzymes
  
  • E.g. coenzyme A - used in aerobic respiration
  • E.g. NAD - transports hydrogen atoms to the inner mitochondrial membrane

Question 2

How are coenzymes obtained?

Answer 2

Derived from vitamins (class of organic molecules)

Vit B3 synthesises NAD - coenzyme responsible for transfer of H atoms between molecules in respiration

Vit B5 - makes coenzyme A to break down fatty acids and carbs in respiration.

Question 3

What are prosthetic groups?

Answer 3

Cofactors - Required by some enzymes to carry out catalytic function

• Zn²⁺ a prosthetic group for carbonic anhydrase - enzyme used in CO₂ metabolism
• Cl^- a prosthetic group for amylase - enzyme used in carbohyrate digestion

Question 4

What are inactive precursors?

Answer 4

• Enzymes in the inactive state
• This is because some enzymes are harmful, they cause damage within cells or tissues
• Or when an enzyme’s actions need to be controlled and only activated under certain conditions.
• Activated by the addition of a cofactor

Question 5

Define

Apoenzyme

Holoenzyme

Zymogen/Proenzyme

Answer 5

Apoenzyme - name for precursor enzyme before addition of cofactor

Holoenzyme - name for precursor enzyme after addition of a cofactor

Zymogen/Proenzyme

• Some changes in the tertiary structure are brought about by another enzyme cleaving (e.g. splitting) bonds.
• Known as zymogens

Question 6

How else can precursor proteins be activated?

Answer 6

• For example, change in pH or temp.
• Pepsinogen released into stomach, acidic pH changes it to pepsin enzyme.