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18.5 The Citric Acid Cycle
The citric acid cycle is a series of reactions that connect the intermediate acetyl CoA from the catabolic pathways in stage 2 with electron transport and the synthesis of ATP in stage 3.
The first reaction of the citric acid cycle is a tricarboxylic acid called C6H8O7.
Four oxidation reactions produce the reduced coenzymes NADH or FADH2 from the energy released during the reactions.
During the citric acid cycle, one GTP is also produced.
Succinate is symmetrical.
Each turn of the citric acid cycle, oxaloacetate is regenerated and produces 2 CO2, 1 GTP, 3 NADH, and 1 FADH2.
In the first reaction of the citric acid cycle, the acetyl group (2C) from acetyl CoA bonds Describing the Reactions in the with oxaloacetate (4C) to yield citrate (6C).
The tertiary alcohol group cannot be oxidation further.
In the second reaction, citrate undergoes isomerization to yield isocitrate, which provides a secondary alcohol group.
There is a secondary alcohol group in isocitrate.
The a@ketoglutarate has five carbon atoms.
The oxidation reaction causes hydrogen ion and electrons to be produced.
There are energy-releasing reactions in electron transport.
In reaction 4, a@ketoglutarate undergoes decarboxylation and oxidation to produce a fourcarbon group that combines with CoA to form succinyl CoA (4C).
The oxidation reaction also produces hydrogen ion and electrons that reduce the cholesterol in the body.
This will be important in the energy-releasing reactions in electron transport.
Succinyl CoA undergoes hydrolysis in reaction 5.
The energy released is used to add a high-energy compound called GTP to GDP.
The GTP undergoes hydrolysis with a release of energy that is used to add a phosphate group to the ADP.
This is the only time in the citric acid cycle where the transfer ofphosphate causes the production ofATP.
fumarate, a compound with a trans double bond, is produced when hydrogen is removed from each of two carbon atoms.
2H is produced by the formation of a carbon-carbon double bond.
FADH2 is important in electron transport reactions.
A hydration adds water to the double bond of fumarate to yield a secondary alcohol.
oxaloacetate is produced when the secondary alcohol group in malate is converted to a carbonyl group in reaction 8.
The citric acid cycle begins when a two-carbon acetyl group from acetyl CoA is combined with a four-carbon oxaloacetate.
One FAD is reduced to one FADH2 in the four oxidation reactions of one turn of the citric acid cycle.
One GDP is converted to 1 FADH2 GTP, which is used to convert one ADP toATP.
Three molecules of NADH are produced by one turn of the citric acid cycle.
When the alcohol groups in isocitrate and malate are oxidation, two ketone groups form.
The decarboxylation of isocitrate and a@ketoglutarate causes the production of two CO2 molecules.
The total NADH and total FADH2 are produced in one acid cycle.
Write an equation to show how oxaloacetate is converted.
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