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The Krebs Cycle
By Stephanie Worrall
The Krebs Cycle
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Discovered by biochemist Sir Hans Krebs
In the year 1937
Is responsible for the breakdown of pyruvate
Acts as a central metabolic pathway in the
breakdown of foods
• Alternative names; The citric acid cycle and
Tricarboxylic acid cycle
• Occurs in the mitochondrial matrix
Definition of the Krebs cycle
A series of enzyme catalysed reactions that
oxidise the acetyl group of acetyl CoA to 2
molecules of Carbon dioxide. It also produces 1
molecule of ATP by substrate level
phosphorylation, and reduces 3 molecules of
NAD and one molecule of FAD.
1. Cycle begins with the 2C acetyl group (acetate)
attached to Coenzyme A, which combines with
Oxaloacetate (4C compound)
2. Citrate (6C compound) is produced from the
combining of oxaloacetate and acetate
3. Citrate is decarboxylated and dehydrogenated
to form a 5 carbon compound. NAD is reduced
by the pair of hydrogen atoms
4. The 5 carbon compound is then
decarboxylated and dehydrogenated to form a
4 carbon compound and a molecule of
reduced NAD
5. 4 carbon compound is changed into another 4
carbon compound
6. ADP is phosphorylated to produce a molecule
of ATP
7. The second 4 carbon compound is changed into
another 4 carbon compound
8. Coenzyme FAD is reduced – due to accepting a
pair of hydrogen atoms that have been removed
9. The resulting 4-carbon compound (from 7.) is
dehydrogenated, which regenerated
oxaloacetate
10.A molecule of NAD is reduced
Substrate- level phosphorylation
The formation of ATP from ADP and Pᵢ
• Citrate (6C) in converted back to oxaloacetate,
involving 2 types of reactions:
• Decarboxylation Reactions – Carbon atoms are removed from the
Krebs cycle intermediate compounds forming CO2 (Catalysed by
decarboxylase enzymes)
• Dehydrogenation Reactions – Intermediate compounds are oxidised
by the removal of hydrogen atoms ,hydrogen atoms are taken up by
hydrogen acceptor molecules e.g. NAD and another molecule FAD.
(catalysed by dehydrogenase enzymes)
• Reduced NAD and Reduced FAD are oxidised in the electron transport
chain: yielding energy to generate ATP from ADP
• One molecule of ATP is directly synthesised in the Krebs Cycle
• The Krebs cycle has to turn twice for each glucose oxidised ( as 2 molecules
of pyruvate are produced from each glucose )
Summary
• Acetate(2C) Combines with Oxaloacetate(4C) to
form Citrate (6C)
• Citrate (6C) undergoes a series of reactions,
eventually losing 2 Carbon atoms, regenerating
the 4 carbon compound (oxaloacetate)
• The 2 carbon atoms are lost as CO2
• The 6C compound (citrate) is oxidised by the
removal of H atoms
• The H atoms pass to hydrogen acceptor
molecules, 3 molecules of reduced NAD and One
of FAD are formed
Products
6 Reduced NAD
2 Reduced FAD
4 CO2
2 ATP