The Role of Coenzymes

Define Coenzyme.

• A small, organic, non-protein molecule that binds to an enzyme and is required for its activity (often by carrying chemical groups between reactions).

Which stages of respiration involve NAD?

• Glycolysis

• Link Reaction

• Krebs Cycle

• Oxidative Phosphorylation (as the donor)

Which stages of respiration involve FAD?

• Only the Krebs Cycle (and Oxidative Phosphorylation).

What is the specific function of Coenzyme A?

• o carry acetyl groups (2C) from the Link Reaction into the Krebs Cycle.

How many hydrogen atoms does NAD accept to become Reduced NAD?

• Two hydrogen atoms (releasing one proton into solution and keeping the other + 2 electrons).

Why do coenzymes need to be re-oxidised at the Electron Transport Chain?

• To unload their Hydrogen atoms so they can return to the earlier stages (Glycolysis/Krebs) and pick up more hydrogen. If they aren't re-oxidised, respiration stops.

Which coenzyme is derived from Vitamin B3 (niacin)?

• NAD.

Difference in ATP yield between Reduced NAD and Reduced FAD?

• Reduced NAD: Results in approx. 2.5 - 3 ATP.

• Reduced FAD: Results in approx. 1.5 - 2 ATP (enters ETC later).

Explain the role of coenzymes in the Krebs cycle. (3 marks)

• NAD acts as a hydrogen carrier / accepts hydrogen during dehydrogenation (1).

• FAD acts as a hydrogen carrier / accepts hydrogen during dehydrogenation (1).

• Coenzyme A carries the acetyl group to combine with oxaloacetate (to enter the cycle) (1).

Describe the differences between the coenzymes NAD and FAD. (3 marks)

• Stages: NAD is used in Glycolysis, Link, and Krebs; FAD is only used in Krebs (1).

• ATP Yield: Reduced NAD results in more ATP production (3 ATP) than Reduced FAD (2 ATP) (1).

• Location in ETC: Reduced NAD donates electrons to Complex I (start of chain); Reduced FAD donates to Complex II (further down) (1).

Explain why the Link Reaction cannot proceed if the Electron Transport Chain stops working (e.g., due to lack of oxygen). (3 marks)

• The Link Reaction requires oxidised NAD to accept hydrogen (1).

• NAD is re-oxidised (regenerated) at the Electron Transport Chain (1).

• If the ETC stops, there is no oxidised NAD available, so the enzymes in the Link Reaction cannot function (1).

Suggest why Coenzyme A is essential for the oxidation of fatty acids. (2 marks)

• Fatty acids are broken down into 2-carbon acetyl groups (beta-oxidation) (1).

• Coenzyme A is required to bind to these acetyl groups (forming Acetyl CoA) to transport them into the Krebs Cycle for oxidation (1).