5.7.6 The Krebs Cycle

Question 1

What is the krebs cycle?

Answer 1

The Krebs cycle (sometimes called the citric acid cycle) consists of a series of enzyme-controlled reactions

The Krebs cycle takes place in the matrix of the mitochondria

2 carbon (2C) Acetyl CoA enters the circular pathway from the link reaction in glucose metabolism
Acetyl CoA formed from fatty acids (after the breakdown of lipids) and amino acids enters directly into the Krebs Cycle from other metabolic pathways

4 carbon (4C) oxaloacetate accepts the 2C acetyl fragment from acetyl CoA to form the 6 carbon (6C) citrate
Coenzyme A is released in this reaction

Citrate is then converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions

Question 2

Describe the regeneration of Oxaloacetate.

Answer 2

Oxaloacetate is regenerated in the Krebs cycle through a series of redox reactions

Decarboxylation of citrate
Releasing 2 CO2 as waste gas

Oxidation (dehydrogenation) of citrate
Releasing H atoms that reduce coenzymes NAD and FAD
8H + 3NAD + FAD → 3NADH + 3H+ + FADH2

Substrate-level phosphorylation
A phosphate is transferred from one of the intermediates to ADP, forming 1 ATP