Download 2. Lect. Urea cycle

UREA CYCLE
.
UREA CYCLE
The urea cycle is the major mechanism for utilization
and excretion of ammonia (excretion of ammonia in
form of urea in the kidneys).
In urea cycle: urea will be produced. Since humans
cannot metabolize Urea, it is transported to the
kidneys for filtration and excretion.
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MECHANISM OF AMMONIA TOXICITY
Ammonia is a very toxic substance, and usually excreted in
the form of urea.
Metabolic disorders that arise from abnormal function of
enzymes Of urea synthesis (urea cycle) are fatal and cause
coma “due to ATP depletion”, specially when ammonia
concentration is high.
High concentration of ammonia sequesters α-ketoglutarate
in form of Glutamate, leading to:
- depletion of TCA cycle intermediates
- and reducing ATP production
.
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Two mechanisms are available in
humans for the transport of
ammonia from the peripheral
tissues to the liver for its
ultimate conversion to urea.
THE FIRST, found in most tissues,
uses glutamine synthetase to
combine ammonia (NH3) with
glutamate to form glutamine—a
nontoxic transport form of
ammonia . The glutamine is
transported in the blood to the
liver where it is cleaved by
glutaminase to produce
glutamate and free ammonia
Ammonia transporter (glutamine)
50% of circulating amino acid molecules are
glutamine “the ammonia Transporter”.
The glutamine is a nitrogen donor for several classes
of Molecules including: purine bases, and the amino
group of cystine.
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THE SECOND transport
mechanism, used primarily by
muscle, involves
transamination of pyruvate (the
end product of aerobic
glycolysis) to form alanine.
Alanine is transported by the
blood to the liver, where it is
converted to pyruvate, again by
transamination. In the liver, the
pathway of gluconeogenesis
can use the pyruvate to
synthesize glucose, which can
enter the blood and be used by
muscle—a pathway called the
glucose-alanine cycle.
AMMONIA IS RELEASED IN LIVER
The main destination of glutamine and alanine in the
blood is the liver, where ammonia is released by alanine
aminotransferase, glutaminase, and glutamate
dehydrogenase.
Glutamate dehydrogenase not only releases ammonia
but also produces NADH and α-keto-glutarate as source
of energy.
REACTIONS
OF THE UREA
CYCLE
• .
Reactions of the cycle
• The first two reactions leading to the synthesis
of urea occur in the mitochondria, whereas the
remaining cycle enzymes are located in the
cytosol.
• [Note: Gluconeogenesis and heme synthesis also
involve both the mitochondrial matrix and the cytosol
The precursors of urea cycle: ammonium ion and
bicarbonate, Both will form carbamoyl phosphate.
Carbamoyl phosphate and citrulline, the first two
intermediates of urea cycle are synthesized in
mitochondria, while the rest of the reactions in the
cytosol.
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1+2+3 Formation of carbamoyl phosphate:
• Formation of carbamoyl phosphate by carbamoyl
phosphate synthetase I is driven by cleavage of
two molecules of ATP. Ammonia incorporated into
carbamoyl phosphate is provided primarily by the
oxidative deamination of glutamate by
mitochondrial
glutamate
dehydrogenase.
Ultimately, the nitrogen atom derived from this
ammonia becomes one of the nitrogens of urea.
Carbamoyl phosphate synthetase I requires Nacetylglutamate as a positive allosteric activator.
4.Formation of citrulline:
• The carbamoyl portion of carbamoyl phosphate is
transferred to ornithine by ornithine transcarbamoylase (OTC) as the high-energy phosphate is
released as Pi. The reaction product, citrulline, is
transported to the cytosol. Ornithine is
regenerated with each turn of the urea cycle(6)
much in the same way that oxaloacetate is
regenerated by the reactions of the citric acid
cycle .
Synthesis of argininosuccinate:
• Argininosuccinate synthetase combines citrulline
with aspartate to form argininosuccinate. The αamino group of aspartate provides the second
nitrogen that is ultimately incorporated into
urea. The formation of argininosuccinate is
driven by the cleavage of ATP to adenosine
monophosphate (AMP) and pyrophosphate. This
is the third and final molecule of ATP consumed
in the formation of urea.
Cleavage of argininosuccinate:
• Argininosuccinate is cleaved by argininosuccinate
lyase to yield arginine and fumarate. The arginine
formed by this reaction serves as the immediate
precursor of urea. Fumarate produced in the
urea cycle is hydrated to malate, providing a link
with several metabolic pathways.
For example, the malate can be transported into the
mitochondria via the malate shuttle, reenter the
tricarboxylic acid cycle, and get oxidized to oxaloacetate
(OAA), which can be used for gluconeogenesis .
Alternatively, the OAA can be converted to aspartate via
transamination , and can enter the urea cycle.
Cleavage of arginine to ornithine and
urea:
• Arginase cleaves arginine to ornithine and
urea, and occurs almost exclusively in the liver.
Thus, whereas other tissues, such as the
kidney, can synthesize arginine by these
reactions, only the liver can cleave arginine
and, thereby, synthesize urea.
Urea cycle: location and source of atoms
a. Urea synthesis takes
place mostly in the liver.
b. One N atom of urea
comes from Asp (blue).
c. One N atom comes from
NH4+ (green).
d. One C atom comes from
CO2 (red).
e. Ornithine acts as a carrier
of various atoms in the
process of synthesizing
urea.
Fate of urea:
Urea diffuses from the liver, and is transported in the
blood to the kidneys, where it is filtered and excreted in
the urine. A portion of the urea diffuses from the blood
into the intestine, and is cleaved to CO2 and NH3 by
bacterial urease. This ammonia is partly lost in the feces,
and is partly reabsorbed into the blood.
Fate of urea:
• In patients with kidney failure, plasma urea
levels are elevated, promoting a greater
transfer of urea from blood into the gut. The
intestinal action of urease on this urea
becomes a clinically important source of
ammonia,
contributing
to
the
hyperammonemia often seen in these patients.
Oral administration of NEOMYCIN reduces the
number of intestinal bacteria responsible for
this NH3 production.
BENEFIT OF GIVING NEOMYCIN TO HEPATO COMROMISED PATIENT
Oral
administration
of NEOMYCIN
reduces the
number of
intestinal
bacteria
responsible for
this NH3
production.
Regulation of the urea cycle
N-Acetylglutamate is an essential activator for carbamoyl phosphate
synthetase I—the rate-limiting step in the urea cycle. NAcetylglutamate is synthesized from acetyl coenzyme A and
glutamate by N-acetylglutamate synthase , in a reaction for which
arginine is an activator.
The synthesis of N- Acetylglutamate from glutamic acid & acetyl COA
is ↑ by high protein diet & a.as especially arginine → ↑rate of urea
cycle
This leads to an increased rate of urea synthesis.
Acetate
GLUTAMATE
Acetyl CO A
a.as esp arginine
Synthetase ++
N- ACETYL GLUTAMATE
CO A
ALLOSTERICALLY ++
carbamoyl phosphate synthetase 1
Overall stoichiometry of the urea cycle
• Aspartate + NH3 + CO2 + 3 ATP + H2O ~
• urea + fumarate + 2 ADP + AMP + 2 Pi + PPi
• Four high-energy phosphate bonds are consumed in the
synthesis of each molecule of urea; therefore, the
synthesis of urea is irreversible. [Note: The ATP is
replenished by oxidative phosphorylation.]
Word
Enzyme
Can
Carbamoyl Phosphate
Synthetase 1
Our
Aunts
Aim
Accurately
Ornithine
Transcarbamoylase
Argininosuccinate
Synthetase
Argininosuccinate
Lyase
Arginase 1