2.1.4 Enzymes – Coenzymes and Cofactors

Question 1

What are cofactors?

Answer 1

Cofactors are non-protein molecules that some enzymes can only work in the presence of.

Question 2

What is a prosthetic group?

Answer 2

Prosthetic groups are cofactors that permanently bind to the enzyme forming part of the enzyme.
e.g. carbonic anhydrase requires a cofactor in the form of a zinc ion (Zn2+). Without the zinc, it cannot catalyse carbon dioxide and water into carbonic acid.

Question 3

Give examples of cofactors that are not prosthetic groups.

Answer 3

Some enzymes require cofactors that do not permanently bind to the enzyme.
e.g. amylase can only digest starch into maltose if chloride ions (Cl-) are present.
e.g. calcium ions (Ca2+) needed to catalyse the reaction of fibrinogen into fibrin by the enzyme thrombin during blood clotting.

Question 4

How do cofactors work?

Answer 4

• They can act as co-substrates: they bind to the substrate making it the correct shape to fit into the active site of the enzyme.
• The can alter the charge: they alter the charge distribution on the surface of the substrate or active site, making temporary bonds that make forming the enzyme-substrate complex easier.

Question 5

What is a coenzyme?

Answer 5

These are a sub-type of cofactor and are small non-protein organic molecules. They bind temporarily to the active site of the enzyme just before or at the same time as the substrate. They are chemically changed in the reaction and need to be recycled.

Question 6

Give two examples of coenzymes

Answer 6

• Coenzyme A - needed in many metabolic pathways, including for aerobic respiration.
• Vitamin B3 (nicotinamide e.g. NAD) - needed for the enzyme pyruvate dehydrogenase to work during respiration.