Who do we thank for this great discovery ? No other than the man himself Sir Hans Krebs in 1937. This brought major advances in biochemistry , not only does the cycle breakdown pyruvate but it also acts a central pathway for which other food materials pass in order for them to be broken down.
The krebs cycle has many different names including tricarboxylic acid (TCA) cycle and also citric acid cycle.
All the reactions along with the enzymes that catalyse them occur in the matrix of the mitochondria. In the beginning a 2C acetyl group which is attached to acetyl CoA combines with a 4C compound oxaloacetate. The CoA is then released again to get more acetyl groups for the reaction. When you combine a 2C group with a 4C group you get a 6C compound called citrate.
This citrate will be converted back into oxaloacetate, after under going a series of steps, these include two reactions.
Decarboxylation – this is catalysed by decarboxylase enzymes and involves carbon atoms being remove from the Krebs cycle intermediate compounds, forming carbon dioxide.
Dehyrogenation – this is catalysed by dehydrogenase enzymes and involves the intermediates being oxidised by taking up hydrogen atoms. They are taken up by hydrogen acceptor molecules such as NAD, along with FAD (flavin adenine dinucleotide)
These two compounds NAD and FAD will be oxidised in the electron transport chain to yield energy that would synthesis ATP from ADP. One molecule of the ATP is also directly synthesised in the cycle.
Recap of the events that occur in the krebs cycle :
1. 2C acetyl group joins with a 4C compound and produces 6C citrate
2. After a series of reactions the 6C will lose 2 carbons and regenerate the 4C compound.
3. The 2C that were lost , are lost as CO2 a waste product of respiration
4. 6C compound also being oxidised by removal of H atoms
5. The H atoms pass to hydrogen acceptor molecules: 3 molecules of reduced NAD and 1 of reduced FAD are formed.
6. A molecule of ATP is synthesised directly, by substrate level phosphorylation.