Download The Citric

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.
• One FADH2 molecule is produced.
• The cycle is accompanied by the liberation of 2
CO2 molecules.
Eight enzymes utilized in the citric acid cycle
1. Citrate synthase
2. Aconitase
3. Isocitrate dehydrogenase
4. a-Ketoglutarate dehydrogenase
5. Succinyl-CoA synthetase
6. Succinate dehydrogenase
7. Fumarase
8. Malate dehydrogenase
Step 1: Acetyl-CoA  Citrate

In the first step of the cycle, the enzyme citrate
synthase catalyzes the condensation of acetylCoA with oxaloacetate.
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  α-Ketoglutarate

The next step in the citric acid cycle also has two substeps
or phases. First, isocitrate is oxidized by the enzyme
isocitrate dehydrogenase producing oxalosuccinate. Like
cis-aconitate in step 2, this molecule is an intermediate that
never dissociates from the enzyme.
Step 4: α-Ketoglutarate  Succinyl-CoA

The next step in the reaction, which is catalyzed
by a-ketoglutarate dehydrogenase,
decarboxylates a-ketoglutarate producing
succinyl-CoA.
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-CoA synthetase, and the
coenzyme-A molecule consumed in the production of
succinyl-CoA is released.
Step 6: Succinate  Fumarate

In this step, succcinate dehydrogenase oxidizes the
succinate molecule producing fumarate.
Step 7: Fumarate  L-Malate

In this step, fumarate
hydratase or fumarase
catalyzes a reversible
reaction in which
fumarate is
transformed into
malate.
Step 8: Malate  Oxaloacetate

In this last step, the molecule oxaloacetate is
regenerated from malate. The enzyme which
catalyzes this step is malate dehydrogenase, and 1
NADH molecule is produced
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 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

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).