The Krebs cycle

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Learning objective
describe the Krebs cycle

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Success criteria
1.Knows the Krebs cycle
2.Describes the Krebs cycle
3.Correctly identifies incoming

and outgoing products of the Krebs cycle
4.Explains the role of the Krebs cycle in energy metabolism

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Terminology
Acetyl-CoA, Citric acid cycle, Citrate – 6C, Isocitrat – 6C,

Alfa – Ketoglutarat 5C, Succinyl – CoA – 4C, Succinat – 4C, Fumarate – 4C, Malat – 4C, Oxalacetat - 4C, ATP, NADH, FADH, CO2, Alfa – Ketoclutarat synthase, Fumarate reductase,

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The Krebs cycle
The Krebs cycle (also known as the citric acid

cycle or tricarboxylic acid cycle) was discovered in 1937 by Hans Krebs.
The Krebs cycle is a closed pathway of enzyme controlled reactions.
■Acetyl coenzyme A combines with a four-carbon compound (oxaloacetate) to form a six-carbon compound (citrate).
■The citrate is decarboxylated and dehydrogenated in a series of steps, to yield carbon dioxide, which is given off as a waste gas, and hydrogens which are accepted by the carriers NAD and FAD.
■Oxaloacetate is regenerated to combine with another acetyl coenzyme A.

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the Krebs cycle
• Two molecules of carbon dioxide are given off

in separate decarboxylation reactions.
• A molecule of ATP is formed as part 1 of the reactions of the cycle - as with glycolysis, this ATP synthesis is 'at substrate level' too.
• Three molecules of reduced NAD are formed.
• One molecule of another hydrogen accepter - FAD (flavin adenine dinucleotide) is reduced. (NAD is the chief hydrogen- carrying coenzyme of respiration but FAD is another coenzyme with this role in the Krebs cycle).

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