Krebs Cycle
Introduction of Krebs Cycle :
The Krebs cycle, also known as the citric acid cycle or the
tricarboxylic acid cycle, is a series of biochemical reactions that occur in
the mitochondria of eukaryotic cells and in the cytoplasm of prokaryotic cells.
The cycle plays a crucial role in cellular respiration, the process by which
cells convert nutrients into energy in the form of ATP (adenosine
triphosphate).
The Krebs cycle is named after Sir Hans Adolf Krebs, a German-born British biochemist who first described the cycle in 1937. Krebs was awarded the Nobel Prize in Physiology or Medicine in 1953 for his work on the cycle.
The purpose of the Krebs cycle is to generate energy in the form of ATP from the breakdown of carbohydrates, fats, and proteins. The cycle starts with the reaction between acetyl-CoA and oxaloacetate, which produces citrate. This sets off a series of reactions that involve the oxidation, decarboxylation, and rearrangement of carbon compounds, ultimately leading to the regeneration of oxaloacetate and the production of ATP, NADH, and FADH2.
The ATP generated during the Krebs cycle is used to power cellular processes, while the NADH and FADH2 molecules are used in the electron transport chain to produce even more ATP. The Krebs cycle is essential for the proper functioning of aerobic organisms, as it is the primary pathway for the production of ATP in the presence of oxygen.
Krebs Cycle Steps :
The Krebs cycle is a complex series of biochemical
reactions, involving a total of eight steps that occur in the mitochondria of
eukaryotic cells and in the cytoplasm of prokaryotic cells. Here are the steps
of the Krebs cycle in more detail:
Step 1: Formation of Citrate:
The first step of the Krebs cycle involves the combination
of acetyl-CoA, which is produced by the breakdown of glucose or fatty acids,
with oxaloacetate, a four-carbon molecule, to form citrate. This reaction is
catalyzed by the enzyme citrate synthase. Citrate is a six-carbon molecule that
is also known as citric acid.
Step 2: Isomerization of Citrate:
In the second step, the enzyme aconitase isomerizes citrate
into isocitrate. Isocitrate is still a six-carbon molecule, but it has a
different arrangement of atoms.
Step 3: Oxidation of Isocitrate:
The third step involves the oxidation of isocitrate to
alpha-ketoglutarate, a five-carbon molecule. This reaction is catalyzed by the
enzyme isocitrate dehydrogenase, and it produces NADH and releases a molecule
of carbon dioxide (CO2).
Step 4: Oxidation of Alpha-Ketoglutarate:
In the fourth step, alpha-ketoglutarate is oxidized to
produce succinyl-CoA, a four-carbon molecule. This reaction is also catalyzed
by an enzyme called alpha-ketoglutarate dehydrogenase, and it produces NADH and
releases a molecule of CO2.
Step 5: Substrate-level Phosphorylation:
In the fifth step, succinyl-CoA is converted into succinate,
a four-carbon molecule. This reaction is catalyzed by the enzyme succinyl-CoA
synthetase and also produces ATP by substrate-level phosphorylation.
Step 6: Oxidation of Succinate:
The sixth step involves the oxidation of succinate to
fumarate, a four-carbon molecule. This reaction is catalyzed by the enzyme
succinate dehydrogenase, which is also a component of the electron transport
chain. This reaction also produces FADH2.
Step 7: Hydration of Fumarate:
In the seventh step, fumarate is hydrated to form malate, a
four-carbon molecule. This reaction is catalyzed by the enzyme fumarase.
Step 8: Oxidation of Malate:
In the final step of the Krebs cycle, malate is oxidized to
regenerate oxaloacetate, which can then combine with another acetyl-CoA
molecule to start the cycle again. This reaction is catalyzed by the enzyme
malate dehydrogenase and produces NADH.
In summary, the Krebs cycle is a series of biochemical
reactions that starts with the combination of acetyl-CoA and oxaloacetate to
form citrate, and ends with the regeneration of oxaloacetate. The cycle
produces ATP, NADH, and FADH2, which are important sources of energy for the
cell.
Krebs Cycle Diagram:
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