1. The urea cycle converts toxic ammonia produced during protein catabolism into urea in the liver. 2. The cycle involves five enzymatic steps, with the first two occurring in the mitochondria and the rest in the cytosol. 3. Key reactions include the conversion of ammonia and bicarbonate to carbamoyl phosphate, then the addition of carbamoyl phosphate to ornithine to form citrulline. 4. Citrulline then reacts with other compounds via several enzymes to ultimately form urea and ornithine, with ornithine recycled into the mitochondria to repeat the cycle.

1. UREA FORMATION
300 Level
BY:
MRS A. O. FADIPE
BSc, AIMLS, NIST, PGDE, MSc.

2. UREA FORMATION

3. INTRODUCTION
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Almost 80% of the nitrogen excreted is in
the form of urea which is produced
through a series of reactions in occuring
in the cytosol and mitochondrial matrix of
the liver cells.
These reactions are collectively called the
UREA CYCLE or KREBS –HENSELEIT
CYCLE.

4. CONTD
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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.

5. UREA FORMATION
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The overall reaction for urea formation
from ammonia is as follows:
2 ammonia + CO2 + 3 ATP → urea + water
+3 ADP

6. STEPS IN THE UREA CYCLE
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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.

7. CONTD
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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

8. CONTD
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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.

9. IN THE MITOCHONDRIAL
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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

10. IN THE CYTOPLASM
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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

11. CONTD
• 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.

12. PATHOLOGY OF U/C
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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

13. UREA CYCLE DEFECTS
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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.

14. SUMMARY OF STEPS INVOLVED
IN UREA FORMATION
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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 ) .

15. 2ND STEP
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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.

16. 3RD STEP
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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.

17. 4TH STEP
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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

18. 5TH STEP
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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: