Krebs cycle
The Krebs cycle, also known as the citric acid cycle or the
tricarboxylic acid cycle, is a fundamental metabolic pathway in all aerobic
organisms that generates energy by oxidizing acetyl-CoA to carbon dioxide. It
occurs within the mitochondria of eukaryotic cells and in the cytosol of
prokaryotic cells.
The Krebs cycle is composed of eight sequential enzymatic
reactions that work together to convert acetyl-CoA, which is derived from the
breakdown of carbohydrates, fats, and proteins, into carbon dioxide and
energy-rich molecules such as ATP, NADH, and FADH2.
The first step in the Krebs cycle is the condensation of
acetyl-CoA with oxaloacetate, which is catalyzed by the enzyme citrate synthase
to form citrate. Citrate is then converted into isocitrate by the enzyme
aconitase, which involves the removal of a water molecule and the addition of a
hydroxyl group.
Isocitrate is then oxidized to alpha-ketoglutarate by the
enzyme isocitrate dehydrogenase, producing one molecule of carbon dioxide and
two molecules of NADH. Alpha-ketoglutarate is then oxidized to succinyl-CoA by
the enzyme alpha-ketoglutarate dehydrogenase, which produces one molecule of
carbon dioxide and one molecule of NADH.
Succinyl-CoA is then converted into succinate by the enzyme
succinyl-CoA synthetase, which generates one molecule of GTP or ATP. Succinate
is then oxidized to fumarate by the enzyme succinate dehydrogenase, which
produces one molecule of FADH2.
Fumarate is also
doused to form malate by the enzyme
fumarase . Malate is then oxidized to oxaloacetate by the enzyme malate
dehydrogenase, producing one molecule of NADH.
The oxaloacetate generated at the end of the Krebs cycle can
then combine with another acetyl-CoA molecule to initiate another round of the
cycle.
The energy-rich molecules generated during the Krebs cycle,
such as NADH and FADH2, are then used in the electron transport chain to
generate ATP, the primary energy currency of the cell.
In summary, the Krebs cycle is a complex series of reactions
that generates energy by oxidizing acetyl-CoA to carbon dioxide and energy-rich
molecules such as ATP, NADH, and FADH2. It is a vital metabolic pathway that
plays a central role in cellular respiration and energy production in all
aerobic organisms.
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