3. INTRODUCTION
• Almost 80% of the nitrogen excreted is in the
form of urea which is produced through a
series of reactions in occurring in the cytosol
and mitochondrial matrix of the liver cells.
• These reactions are collectively called the
UREA CYCLE or KREBS –HENSELEIT
CYCLE.
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4. • Ammonia is a toxic product of nitrogen
metabolism which should be removed from
the body.
• The urea cycle or ornithine cycle converts
excess ammonia into urea in the
mitochondrial of liver cells.
• The urea formed then enters the blood
stream, is been filtered by the kidneys and is
ultimately excreted in the urine.
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5. UREA FORMATION
• The overall reaction for urea formation from
ammonia is as follows:
• 2 ammonia + CO2 + 3 ATP → urea + water +3
ADP
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6. STEPS IN THE UREA CYCLE
• The urea cycle is a series of five reactions
catalysed by several key enzymes .
• The first two steps in the cycle take place in
the mitochondrial matrix while the rest of the
steps take place in the cytosol.
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7. • Ammonia is formed by the breakdown of amino
acids/gut bacteria.
• The mitochondrial stage:
• Carbamoyl phosphate is formed from ammonia
and bicarbonate, by carbamoyl phosphate
synthetase (CPS).
• Ornithine transcarbamoylase (OTC) condenses
carbamoyl phosphate and ornithine to
form Citrulline.
• Citrulline is then transported to the cytosol by
SLC25A15
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8. The cytosolic stage:
• Argininosuccinate synthetase (AS) condenses citrulline
and aspartate to form Argininosuccinate.
• Argininosuccinate is broken down into arginine and
fumarate by Argininosuccinatelyase (AL).
• Arginine is broken down into urea and ornithine by
arginase.
• Ornithine translocase transports ornithine into the
mitochondria.
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9. IN THE MITOCHONDRIAL
The Mitochondrial stage:
• The first two steps of the urea cycle occur in the
mitochondria of the cell. First, the enzyme CPS takes
ammonia and bicarbonate and forms carbamoyl phosphate
with the use of adenosine triphosphate (ATP). This is the
step in the cycle that determines how fast the cycle
progresses. N-acetylglucosamine is also required for CPS
to function, and functions as a regulator for the formation
of urea.
• OTC then condenses carbamoyl phosphate and ornithine,
which forms citrulline. This citrulline is then moved out of
the mitochondria into the cytosol of the cell by the
transporter SLC25A15
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10. IN THE CYTOPLASM
The cytosolic stage:
• AS takes the citrulline formed in the mitochondrial stage and
condenses it with aspartate to form Argininosuccinate. This
occurs by the formation of an intermediate, Citrulline-AMP.
• Argininosuccinate is then broken into arginine and fumarate by
AL. Fumarate is then incorporated into another metabolic
cycle, the tricarboxylic acid (TCA) cycle. The TCA cycle,
which is also known as the citric acid cycle or the Krebs cycle,
can then reform aspartate, which is used by AS.
• Arginine is then further broken down into urea and ornithine
by arginase. Arginine can also be acquired from the diet, and
this can also be taken in by the liver cells and broken down
into urea and ornithine by arginase
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11. • The ornithine is then transported into the
mitochondria by ornithine translocase. There,
it is used by OTC again to form citrulline. The
citrulline is then processed to form urea and
ornithine again, and the cycle continues.
During the cycle, urea is the only new product
which is formed, while all other molecules
used in the cycle are recycled.
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12. PATHOLOGY OF U/C
What happens when the urea cycle goes wrong?
• If there is a problem with the urea cycle, then the level
of ammonia in the blood will rise, causing
hyperammonemia. Ammonia is able to cross the
barrier between the bloodstream and the brain. Once it
enters the brain, it can stop the TCA cycle by depleting
one of the metabolites, α-ketoglutarate. This means that
these brain cells cannot make energy, ultimately leading
to their death. This eventually will lead to neurological
problems, which can be as severe as irreversible brain
damage
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13. UREA CYCLE DEFECTS
• Problems can arise as a result of damage to the
liver (cirrhosis) or by an inherited defect in one of
the above enzymes. In both cases, the level of
ammonia rises with the potential consequences
previously detailed.
• Inherited urea cycle defects are part of “inborn
errors in metabolism”, and are known as “urea
cycle disorders.” Symptoms usually present in
newborns around 24-48 hours after birth. The
most severe symptoms are seen in newborns with
a defect in CPS.
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14. SUMMARY OF STEPS INVOLVED
IN UREA FORMATION
• In the first step ,ammonia produced in the
mitochondrial is converted to Carbomyl
phosphate by an enzyme called Carbomyl
synthetase I
• NH3 + Co2 + 2 ATP → CarbomylPhosphate +
2ADP +Pi
• The 2nd step is the transfer of a carbomyl group
from carbomylphosphate to ornithine to form
citrulline.
• This step is catalysed by the enzyme ornithine
transcarbomylase ( OTC ) .
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15. 2ND STEP
• The reaction is given as follows:
• Carbomylphosphate + ornithine → citrulline +
pi.
• The citrulline thus formed is released into the
cytosol for use in the rest of the steps of the
cycle.
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16. 3RD STEP
• The 3rd steps is catalysed by an enzyme called
arginosuccinate synthetase which uses
citrulline and ATP to form a citrully AMP
intermediate which reacts with an amino group
from aspartate to produce arginosuccinate.
• Citrulline + ATP + Aspartate →
Arginosuccinate + AMP Ppi.
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17. 4TH STEP
• The 4th step is the cleavage of Arginosuccinate
to form fumarate and arginine.
Arginosuccinatelyase is the enzyme catalysing
the reaction. It is represented thus:
• ARGINOSUCCINATE → ARGININE +
FUMARATE
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18. 5TH STEP
• Arginine → urea + ornithine
• The overall reaction is given as :
• 2NH3 + Co2 + 3 ATP → Urea +2 ADP+ AMP
+ PPi + 2Pi
• The 5th and the last step of urea cycle, arginine
is hydrolysed to form urea and ornithine. This
is catalysed by arginase and is given as
follows:
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