CITRIC ACID CYCLE / KREB CYCLE:
Regarding the reaction of living body, which provides
energy for acetic acid or acetyl equivalent ozone-based phosphate bonds (such
as ATP) for storage – it is also called the citric acid cycle, tricarboxylic
The citric acid cycle also known as the tricarboxylic
acid cycle (TCA cycle), the Krebs cycle, or it is a series of enzyme catalyzed
chemical reactions, which has central importance in all living cells that use
oxygen. In eukaryotic cells, matrix of the mitochondria contain the citric acid
cycle present in then. In aerobic organisms,a metabolic pathway involved the
conversion of carbohydrates, fats and proteins into carbon dioxide and water so
that it generate a form of usable energy.
The citric acid cycle is also called cellular
respiration, the process where your body gets energy from the food you eat. The
citric acid cycle has the same citric acid that is found in oranges and other citrus
where the simple sugar such as glucose is broken down, also present in the
mitochondria the pyruvate that is formed as a product is transformed into
acetyl CoA .
CoA is modified in the mitochondria to generate energy precursors in
preparation for the next step.
In the process of
oxidative phosphorylation, the electron are transformed to citric acid cycle
and phosphorylation of ADP, and ATP is formed. This process is also occur the
How does it happen?
The carbon made atom and molecules are enter and
circulate through the citric acid cycle For understanding the working of citric
acid cycle first need to determine how the carbon atoms are rearranged through
the cycle. Molecules that are electron shuttles, gain the energy that is
released by stepwise rearrangement and subtraction of carbon in the form of
electron that is accept by molecule are called electron shuttles. Electron
shuttles like NAD+ and FADH small organic molecule, that transport high
energy electrons to the point where they need to be by gaining electrons
through reduction and through oxidation losing electrons. The electrons that
are transported by electron shuttles are later to be used to generate ATP.
NADH: An energy shuttle which is deliver high energy electron to the electron
transport chain and eventually produce the 2-3 ATP as a end product when the energy
shuttle don’t have the high energy electron it is going to oxidized or losing
electron and finally have the positive charge is called NAD+
FADH2: It carries high energy electron to the electron transport chain finally
they have power of production 1-2 ATP. The FAD is the oxidized form of FADH2.
High energy molecules:
ATP: It is main type of energy that is used by the cell
and is generate by phosphorylation of ADP.
GTP: GTP is similar to ATP, which can be easily converted
to ATP in the cell.
Citric acid cycle-REACTIONS:
There are many reactions which are involved in Kreb cycle or Citric acid
In the first reaction of citric acid cycle the
oxaloacetate react with acetyle-CoA to form citric acid.The first reaction is catalyzed by
citrate synthase enzyme. Once the two molecules are joined the water molecule
attack at acetyl group main release the coenzyme A from this complex.
The enzyme citrate synthase is present in nearly all
living cells and act as a pace-making enzyme in the citric acid cycle’s first
step. Citrate synthase is present in eukaryotes but is is made up of nuclear
DNA instead of mitochondrial. It is synthesized from cytoplasmic ribosomes,
then transported into the mitochondrial matrix. Citrate synthase is commonly
present as a quantitative enzyme marker for the presence of intact
mitochondria. .Oxaloacetate is regenerated after the completion of one kreb
REACTION 2: Formation of Isocitrate:
The next reaction of the Kreb cycle is catalysed by acontinase
enzyme. In this reaction overall two H2O molecules are produced one
water molecule is removed and other water molecule is put back into another
location. The overall effect of this reaction is that the shifting of -OH group
from position 3 to the position 4. The yield that get is isocitrate molecule.
(aconitate hydratase) is an enzyme that catalyzes the stereo-specific isomerization
of citrate to isocitrate via cis-aconitate in the tricarboxylic acid cycle, a
of Isocitrate to ?-Ketoglutarate:
Two events occur in citric acid cycle’s third
reaction. In the first reaction the formation of NADH from NAD. The enzyme that
catalyzed this reaction is isocitrate dehydrogenase enzyme.This catalyze the
oxidation of -OH group at 4 position of isocitrate and yield is intermediate
futher in the second event carbon-dioxide is removed and yield is
is an enzyme that have role in the citric acid cycle to catalyzes the third
step of the cycle: the oxidative decarboxylation of isocitrate, producing
alpha-ketoglutarate (a-ketoglutarate) and CO2 while converting NAD+ to NADH.
REACTION 4: Oxidation
of ?-Ketoglutarate to Succinyl-CoA:
In this reaction the CO2 is removed from
the alpha ketoglutarate and another molecule that is coenzyme A is added at
this place. The decarboxylation is happened due to reduction of NAD into NADH. The
enzyme that catalyse this reaction is alpha-ketoglutarate dehydrogenase. The
conversion mechanism is same as in the previous steps pyruvate metabolism. The
resulting molecule that generate is succinyl-CoA.