Citric Acid Cycle
The most significant metabolic route for the
body's energy source is the citric acid cycle, also known as the Krebs cycle or
the tricarboxylic acid—TCA cycle. Approximately 65–70% of ATP is produced
during the Krebs cycle. Acetyl CoA is effectively oxidised to produce CO2 and
H2O as part of the citric acid cycle. Approximately two thirds of the oxygen
the body takes in is used in this cycle. The cycle is referred to as the TCA
cycle because tricarboxylic acids (citrate, cisaconitate, and isocitrate) are
involved in the beginning of the cycle.
TCA cycle—the central metabolic pathway
The last common oxidative pathway for amino
acids, lipids, and carbs is the citric acid cycle. This cycle produces many
of the intermediates needed for the synthesis of amino acids, glucose,
heme, and other compounds in addition to energy. The most significant core pathway
that either directly or indirectly connects practically all of the distinct metabolic pathways is
the Krebs cycle.
Location of TCA cycle
The TCA cycle enzymes are found near the electron transport chain in the mitochondrial matrix. This makes it possible for oxidative phosphorylation to produce ATP without interference.
TCA cycle—an overview
In essence, the Krebs cycle is the reaction of a four carbon oxaloacetate and a
two carbon acetyl CoA to create citrate, a six carbon tricarboxylic acid. The
two carbons are converted to CO2 in the ensuing processes, and oxaloacetate is
recycled and renewed. It is believed that oxaloacetate catalyzes the citric
acid cycle. A diagram of the Krebs cycle is shown.
TCA cycle—an open cycle
Krebs cycle is a cyclic process. It shouldn't
be seen as a closed circle, though, as numerous compounds come into and go out
of it. A busy traffic circle on a major highway with numerous linking lanes is
similar to a TCA cycle. Every pathway's intermediary that links to another is a
road!
To generate energy, the Krebs cycle requires
four B-complex vitamins. Thiamine (as TPP) functions as D-ketoglutarate
dehydrogenase's cofactor. 2. As a cofactor for succinate dehydrogenase,
riboflavin (as FAD). 3. Isocitrate dehydrogenase, D-ketoglutarate
dehydrogenase, and malate dehydrogenase use niacin (as NAD+) as an electron
acceptor. 4. Coenzyme A, or pantothenic acid, is linked to active carboxylic
acid residues, such as acetyl and succinyl coenzymes.
Requirement
of O2 by TCA cycle
Oxygen does not directly participate in the Krebs cycle. But the cycle can only function in an aerobic environment. This is because the ETC can only regenerate NAD+ and FAD (from NADH and FADH2, respectively), which are essential for the cycle's activity, in the presence of O2. Consequently, the citric acid cycle is exclusively aerobic, whereas glycolysis can function in both aerobic and anaerobic environments.
References: ‘Biochemistry’ by Satyanarayana and Chakrapani
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