The Urea Cycle
April 11, 2003
Bryant Miles
I. Ammonia Toxicity
Every amino acid contains at least one amino group.  Therefore every amino acid degradation pathway has a key step where the amino group is removed.  In short amino acid catabolism generates ammonia. Brain tissue is very sensitive to ammonia. Ammonia intoxication produces a comatose state.    Cells get rid of excess ammonia by the reductive amination of α−ketoglutarate to form glutamate by glutamate dehydrogenase and the conversion of glutamate into glutamine by glutamine synthetase.  Both of these enzymes are found in high concentrations in the brain.  A high concentration of ammonia shifts the equilibrium of these two reactions towards glutamine.  Glutamine synthetase use ATP to activate glutamate for amination to form glutamine.  The result is high concentrations of ammonia deplete cells of ATP.  ATP is essential for brain function.
α-Ketoglutarate + NH4+ + NAD(P)H
Glutamate + NAD(P)+
Glutamate + NH4+ +ATP                          Glutamine + ADP + Pi
Glutamate is a neurotransmitter.  Glutamate is also the precursor for γ-aminobutyrate (GABA) which is another important neurotransmitter.  High concentrations of ammonia deplete the concentration of glutamate which produces a similar decrease of GABA which impairs brain function.
Mechanism of Glutamine Synthetase
Most amino acids are catabolized in the liver.  Some of the ammonia produced in amino acid catabolism in the liver is used to synthesize nitrogenous biomolecules such as nucleotides.  The excess ammonia is converted into urea for excretion.
Only the liver and kidneys can convert ammonia into urea.  Other
tissues, such as muscle tissue, catabolize proteins for energy.  The
ammonia produced is quite toxic to a variety of tissues.  Excess
ammonia must be converted into a nontoxic compound that can
function as an amino group carrier in the blood to transport ammonia
from the tissues to the liver where the ammonia will be converted into
urea.  There are two primary carriers of amino group groups in the
blood, glutamine and alanine.
Muscle tissues catabolize amino acids for a source of energy. In the
muscle tissue, excess ammonia is transferred to pyruvate to form
alanine by alanine aminotransferase. Alanine is released into the blood
stream.  The liver absorbs the alanine, deaminates it to reform pyruvate.
Other tissues such as brain tissue take excess ammonia, react it with
glutamate to form glutamine by the enzymatic action of glutamine
synthetase.  Glutamine is then released into the blood stream and
absorbed by the liver or the kidneys.  In the liver and kidneys there is a
glutaminase enzyme that hydrolyzes the glutamine into glutamate.
Under normal conditions the kidneys extract little glutamine out of the
blood stream.  But in metabolic acidosis the kidney increases glutamine
absorption and hydrolysis.  The ammonia produced in the kidneys under
these conditions is not converted into urea but excreted as ammonium
salts.
II. The Urea Cycle
Excess amino groups are channeled into a single excretory end product.
Bony fish excrete ammonia and are thus called ammonoteilic animals. The liver of these bony fish releases ammonia into the blood stream where it is carried to the gills where it is rapidly cleared.
Birds and reptiles excrete uric acid and thus are called uricotelic animals.  Excretion of urea requires lots of water.  If bird secreted urea, they would be weighed down by the large volume of water required and would be unable to fly.  Instead they secrete uric acid in their feces.
Terrestrial animals such as ourselves excrete urea and are called ureotilic animals.
In ureotilic organisms, excess ammonia is channeled to the mitochondria of liver cells and converted into carbamoyl phosphate.  Carbamoyl phosphate enters the nitrogen into the urea cycle.  The urea cycle was discovered by nonother then Hans Krebs.  What a biochemical pioneer he was.  The urea produced in the liver is released into the blood stream.  The kidneys filter out the urea.  The urea is ultimately excreted in the urine.
The urea cycle consists of 4 reactions. The first reaction occurs in the matrix of the mitochondria.  The
subsequent reactions occur in the cytosol.  This is a pathway that spans two cellular compartments.  The first reaction is catalyzed by ornithine transcarbamoylase which transfers a carbamoyl group from carbamoyl phosphate to ornithine to form citrulline.
The second reaction is catalyzed by argininosuccinate synthetase.  This enzyme uses ATP to activate citrulline by forming a citrullyl-AMP intermediate.  This intermediate is attacked by the amino group of an aspartate residue to form argininosuccinate.
The third step is catalyzed by argininosuccinate lyase which cleaves argininosuccinate into fumarate and arginine.
The last step is catalyzed by arginase which cleaves arginine to produce urea and ornithine completing the cycle.
The first step of the urea cycle begins in the mitochondrial matrix.  Ammonia is produced in the mitochondrial matrix by the enzymatic activities of glutaminase and glutamate dehydrogenase.
Carbamoyl phosphate synthetase I is the enzyme that takes the ammonia, bicarbonate and 2 molecules of ATP to produce carbamoyl phosphate.  This enzyme activates bicarbonate by the same method used by biotin containing enzymes.  In fact carbamoyl phosphate synthetase is homologous to the biotin family of enzymes.  Carbamoyl phosphate synthetase I activates bicarbonate by phosphorylation with ATP to form carboxyphosphate.  Ammonia then reacts with carboxyphosphate to from a carbamate intermediate.  A second molecule of ATP is used to phosphorylate the carbamate intermediate to form carbamoyl
phosphate.  Carbamoyl phosphate synthetase I is the first committed step of the urea cycle.  As one would expect this enzyme is allosterically regulated.  This enzyme is allosterically activated by N-acetyl
glutamate.  N-acetylglutamate is a precursor for ornithine biosynthesis and is an essential activator for the urea cycle.  N-acetyl glutamate also serves to coordinate related pathways.
Bicarbonate
O
reaction between pvp and amino2N
O
P O
O
-
O -
transcarbamoylase which catalyzes the following reaction.
There is a citrulline transporter that transports citrulline from the matrix of the mitochondria to the cytosol.  There is also an ornithine trasporter that transport ornithine from the cytosol into the matrix of the mitochondria.  The rest of the reactions of the urea cycle occur in the cytosol.
The next step of the urea cycle is catalyzed by argininosuccinate synthetase.  This enzyme uses ATP to
activate citrulline by forming a citrully-AMP intermediate.
-
O 2NH CH C
CH 2O -
O
C O -O
C NH 2NH CH 2CH 2CH 2C
NH 3+H
CO 2-
Argininosuccinate
In the second half of the argininosuccinate synthetase reaction, the α-amino group of aspartate attacks the imino carbon releasing AMP and producing argininosuccinate.
The third step of the urea cycle is catalyzed by argininosuccinate lyase which catalyzes the reaction shown on the left:  The products of this reaction are arginine and fumarate.  The fumarate product is an important link between the urea cycle and the citric acid cycle.
The last reaction of the urea cycle is catalyzed by arginase which catalyzes the hydrolysis of guanidino group to produce urea and ornithine.  Ornthine is then transported back in the matrix of the mitochondria and the cycle is complete.
-
O 2
-
Argininosuccinate
C C C -
O
O
H C
O O -
Arginine
There are two nitrogens in urea.  One came from the ammonia produced
in the matrix in the mitochondria captured in the form of carbamoyl
phosphate.  The second nitrogen was the α−amino group of the aspartate
substrate in the argininosuccinate synthetase reaction.  The sole carbon of urea was originally the carbon of the bicarbonate.
The fumarate formed in the arginosuccinate lyase reaction is an intermediate of the citric acid cycle.  There are a variety of ways this fumarate produced in the cytosol can be transported across the inner mitochondrial membrane into the matrix.  One is the direct
transportation of fumarate across the membrane.  There is a cytostolic fumarase enzyme which converts fumarate into malate.  Malate can be transported across the inner mitochondrial membrane. 
There is also a cytostolic malate dehydrogenase which will convert malate into oxaloacetate.  Oxaloacetate can also be transported into the matrix of the mitochondria.  Oxaloacetate produced this way in the cytosol is a
substrate for PEP carboxylase and proceed on through the gluconeogenic
pathway.    In the mitochondria oxaloacetate is tranaminated by glutamate by
aspartate amino transferase to produce the aspartate which is then
transported into the cytosol where it is required for the argininosuccinate synthetase reaction.  These reactions together are called the aspartate-argininosuccinate shunt.
The summary of the links between the urea cycle and the citric acid cycle is shown below.
The overall reaction of the citric acid cycle is:
2NH 4+ + HCO 3- + 3ATP 4-  Urea + 2 ADP 3- + 4Pi + AMP 2- + 5H +
III. Inherited Deficiencies in the Urea Cycle .
The synthesis of urea in the liver in the primary route of removing excess ammonia.  A deficiency in any
of the enzymes of the urea cycle has devastating consequences.  Deficiencies in the activities of the
23+
2Arginine
2

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