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The Citric Acid Cycle. Ashadi Sasongko. Essential facts of the citric acid cycle. • The citric acid cycle is an eight-step reaction. • It requires 8 enzymes. • The final product is oxaloacetate . • Three NADH molecules are produced. • One GTP molecule is produced.

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The citric acid cycle

The Citric Acid Cycle

AshadiSasongko


Essential facts of the citric acid cycle
Essential facts of the citric acid cycle

• The citric acid cycle is an eight-step reaction.

• It requires 8 enzymes.

• The final product is oxaloacetate.

• Three NADH molecules are produced.

• One GTP molecule is produced.

• One FADH2 molecule is produced.

• The cycle is accompanied by the liberation of 2 CO2 molecules.


E ight enzymes utilized in the citric acid cycle
Eight enzymes utilized in the citric acid cycle

1. Citrate synthase

2. Aconitase

3. Isocitratedehydrogenase

4. a-Ketoglutaratedehydrogenase

5. Succinyl-CoAsynthetase

6. Succinatedehydrogenase

7. Fumarase

8. Malatedehydrogenase


Step 1 acetyl coa citrate
Step 1: Acetyl-CoACitrate

  • In the first step of the cycle, the enzyme citrate synthase catalyzes the condensation of acetyl-CoAwith oxaloacetate.


Step 2 citrate isocitrate
Step 2: Citrate Isocitrate

  • In step 2 of the cycle, there are two substeps used to generate isocitrate, which is easier to oxidize.


Step 3 isocitrate ketoglutarate
Step 3: Isocitrate α-Ketoglutarate

  • The next step in the citric acid cycle also has two substeps or phases. First, isocitrate is oxidized by the enzyme isocitratedehydrogenase producing oxalosuccinate. Like cis-aconitatein step 2, this molecule is an intermediate that never dissociates from the enzyme.


Step 4 ketoglutarate succinyl coa
Step 4: α-KetoglutarateSuccinyl-CoA

  • The next step in the reaction, which is catalyzed by a-ketoglutaratedehydrogenase, decarboxylatesa-ketoglutarateproducing succinyl-CoA.


Step 5 succinyl coa succinate
Step 5: Succinyl-CoASuccinate

  • In this step, a high energy GTP molecule is produced. This is substrate level phosphorylation. The reaction is catalyzed by the succinyl-CoAsynthetase, and the coenzyme-A molecule consumed in the production of succinyl-CoAis released.


Step 6 succinate fumarate
Step 6: SuccinateFumarate

  • In this step, succcinatedehydrogenase oxidizes the succinate molecule producing fumarate.


Step 7 fumarate l malate
Step 7: FumarateL-Malate

  • In this step, fumaratehydratase or fumarase catalyzes a reversible reaction in which fumarateis transformed into malate.


Step 8 malate oxaloacetate
Step 8: MalateOxaloacetate

  • In this last step, the molecule oxaloacetateis regenerated from malate. The enzyme which catalyzes this step is malatedehydrogenase, and 1 NADH molecule is produced


What is metabolomics what is metabonomics
What is Metabolomics?What is Metabonomics?

  • The ensemble of metabolites in an organism is known as its metabolome.

  • In 1999 the term metabonomics was devised to describe “the multiparametric, quantitative study of dynamic metabolome responses in living systems to physiological and pathophysiological stimulation or genetic modification”.

  • However, a competing term appeared in 2001: metabolomics, that was generally defined as: the “comprehensive and quantitative analysis of all metabolites…”


The urine samples obtained from heat stressed rats contained altered concentration of citrate (2.66), succinate ( 2.42), 2-oxoglutrate ( 2.98), phenylalanine ( 3.26, 4.06, 7.30, 7.34), creatinine ( 3.02, 4.06), hippurate (7.54, 7.82) and formate ( 8.46) in comparison with control rats


The urine samples obtained from heat stressed rats contained altered concentration of citrate (2.66), succinate ( 2.42), 2-oxoglutrate ( 2.98), phenylalanine ( 3.26, 4.06, 7.30, 7.34), creatinine ( 3.02, 4.06), hippurate (7.54, 7.82) and formate ( 8.46) in comparison with control rats


The results of the present study showed significant effects on the metabolites involved in several pathways such as tricarboxylic acid (TCA) cycle (citrate, 2-oxoglutrate, succinate)


  • The urinary excretion levels of citrate, 2-oxoglutrate, and on the metabolites involved in several pathways such as succinate were decreased. As reported earlier, irrespective of any kind of physiological stress, there is increased energy consumption and protection against internal and external stress is provided by allostatis. During heat exposure, increased energy consumption is expected. However, in our studies the decrease in TCA cycle metabolites can be explained by two stages.

  • Initially, during heat exposure, TCA cycle is accelerated due to enhanced adrenergic nerve activity. As soon as the rats are returned back to metabolic cages, they appeared less active indicating initiation of recovery process and thus slower energy consumption period. The short-term exposure to acute heat stress followed by long room temperature recovery process leads to overall lower level of TCA cycle metabolites in twelve hours urine sample. Hence, alteration of the TCA cycle is an important part of metabolic regulatory and compensatory mechanism in response to heat stress exposure.


conclusion on the metabolites involved in several pathways such as

  • NMR based metabonomic studies in conjugation with statistical analysis permits non-invasive and simultaneous monitoring of various metabolic pathways revealing a subtle interplay of functional metabolites and pathways leading to an understanding of the systemic response to external stimuli such as heat stress.

  • The results provides a new insight into the changes induced by heat stress at metabolic level showing a decrease in metabolites involved in TCA cycle (succinate, 2-oxoglutrate and citrate) and catecholamine metabolic pathway (phenylalanine).


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