Cofactors, Coenzymes and Prosthetic Groups

What is a cofactor?

A non-protein component required for some enzymes to function.

What is a coenzyme?

An organic cofactor that assists enzyme activity.

What is the key difference between cofactors and coenzymes?

Coenzymes are organic molecules; cofactors may be inorganic.

Why do some enzymes require cofactors?

To complete the active site or transfer atoms or groups during reactions.

What type of molecules are inorganic cofactors?

Mineral ions obtained from the diet.

Give examples of inorganic cofactors

Iron, calcium, chloride, zinc ions.

What role does chloride ion play in amylase?

It helps form the correctly shaped active site.

Why is amylase dependent on chloride ions?

Without chloride, the enzyme cannot function effectively.

Where are coenzymes obtained from?

Vitamins in the diet.

What vitamin is used to make NAD?

Vitamin B3.

What is the role of NAD?

Transfers hydrogen atoms during respiration.

What is NADP used for?

Hydrogen transfer in photosynthesis.

Which vitamin is used to make coenzyme A?

Vitamin B5.

What is the role of coenzyme A?

Essential in the breakdown of fatty acids and carbohydrates in respiration.

How do coenzymes assist metabolic pathways?

They transfer atoms or chemical groups between reactions.

What is a prosthetic group?

A non-protein component permanently bound to an enzyme.

How do prosthetic groups differ from cofactors?

They are tightly bound and form part of the enzyme’s structure.

Give an example of a prosthetic group

Iron ion in haemoglobin or zinc ion in carbonic anhydrase.

What enzyme contains zinc as a prosthetic group?

Carbonic anhydrase.

Why is zinc important in carbonic anhydrase?

It forms part of the enzyme’s structure and active site.

Define an apoenzyme

The inactive protein portion of an enzyme without its cofactor.

Define a holoenzyme

The active enzyme formed when the cofactor binds to the apoenzyme.

Why is the apoenzyme inactive?

The active site is incomplete or incorrectly shaped.

What happens when a cofactor binds to an apoenzyme?

The enzyme becomes active.

What is precursor activation?

The activation of enzymes produced in an inactive form.

Why are some enzymes made as inactive precursors?

To prevent damage to cells or tissues.

What is another name for inactive precursor enzymes?

Zymogens or proenzymes.

How can precursor enzymes be activated?

By cofactors, protease action, or changes in pH or temperature.

What happens during protease activation of enzymes?

Specific bonds are cleaved, changing tertiary structure.

Give an example of enzyme activation by pH

Pepsinogen converting to pepsin in the stomach.

Why is pepsin released as pepsinogen?

To prevent digestion of body tissues.

What condition activates pepsinogen?

The acidic pH of the stomach.

AO3: Explain why precursor enzymes are important in digestion

They protect tissues from self-digestion.

What is factor X?

An enzyme involved in blood clotting.

What vitamin is required to activate factor X?

Vitamin K.

What does activated factor X catalyse?

The conversion of prothrombin to thrombin.

How is prothrombin activated?

By cleavage of bonds altering its tertiary structure.

What type of enzyme is thrombin?

A protease.

What reaction does thrombin catalyse?

Conversion of fibrinogen to fibrin.

Why is fibrin important?

It forms fibres that create a blood clot.

What is a cascade system?

A sequence of enzyme activations.

Why is the blood-clotting cascade important?

It allows rapid and amplified response to injury.

AO3: Explain how enzyme activation controls metabolic pathways

Only required enzymes are activated when needed.

AO3: Compare prosthetic groups and coenzymes

Prosthetic groups are permanent; coenzymes are temporary carriers.

AO3: Explain why vitamins are essential in small amounts

They are required to make coenzymes.

AO3: Explain why lack of a cofactor stops enzyme activity

The active site cannot function correctly.