2. INTRODUCTION
Urea is the major end product in Nitrogen
metabolism in humans and mammals.
NH3, the product of oxidative deamination
reaction, is toxic in even small amount and must
be removed from the body.
Urea cycle a.k.a Ornithine cycle is the
conversion reactions of NH3 into urea.
This reaction occur in liver (certain occur in
cytosol and mitochondria)
The urea is transported to the kidney where it is
excreted.
5. 1.SYNTHESIS OF CARBAMOYL PHOSPHATE :
Formation of carbamoyl phosphate by
carbamoyl phoshate 1 (CPS l) is driven by
the cleavage of 2 molecules of ATP.
CPS l requires N-acetylglutamate as a positive
allosteric activator.
NH3 and CO2 incorporated into carbamoyl
phosphate, NH3 is provided primarily by the
oxidative deamination of glutamate by
mitochondrial glutamate dehydrogenase.
Nitrogen atom derived from this NH3 becomes
one of the nitrogens of urea.
This step consumes 2 ATP, and rate-limiting.
6.
7. 2. 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 reactions product, citrulline, is transported
to the cytosol. –citrulline produced will transported to the
cytosol by a cotransporter.
Ornithine is regenerated with each turn of the
urea cycle.
8.
9. 3. SYNTHESIS OF ARGININOSUCCCINATE :
Argininosuccinate synthetase combines with
citrulline with aspartate to form
argininosuccinate.
The α-amino group of aspartate to the second
nitrogen that is ultimately incorporated into urea.
The formation of argininosuccinate is driven by
the cleavage of ATP to AMP & pyrophosphate
(PPi).
This is the third & final molecule of ATP
consumed in the formation of urea.
10.
11. 4. CLEAVAGE OF ARGININOSUCCINATE :
Argininosuccinate 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, -fumarate that librated here
provides a connecting link with TCA cycle and
gluconeogenesis.(see later)
12.
13. 5. FORMATION OF UREA :
Arginase is the final enzyme that cleaves arginine
to yield urea and ornithine.
Ornithine, so regenerated, enters mitochondria for
its reuse in the urea cycle.
Arginase is activated by Co2+ and Mn2+
Arginase is mostly found in the liver, while the rest
of the enzymes of urea cycle are also present in
other tissues.
For this reason, Arginine synthesis may occur to
varying degrees in many tissues, but only the liver
can ultimately produce urea.
14.
15. OVERALL REACTION IN UREA CYCLE
The urea cycle is irreversible and consumes 4
ATP.
2 ATP are utilized for the synthesis of carbamoyl
phosphate.
1 ATP is converted to AMP and PPi to produce
arginosuccinate which equals to 2 ATP.
ATP → AMP + PPi = 2 ATP equivalents
Hence 4 ATP are actually consumed.
Overall stoichiometry of the urea cycle :
Aspartate + NH3 + CO2 + 3ATP + H2O → urea + fumarate
+ 2ADP + AMP + 2Pi + PPi
16. REGULATION OF THE UREA CYCLE
N-Acetylglutamate is an essential activator for
carbamoyl phosphate synthetase I—the rate-limiting
step in the urea cycle.
N-Acetylglutamate is synthesized from acetyl
coenzyme A and glutamate by N-acetylglutamate
synthase, in a reaction for which arginine is an
activator.
Therefore, the intrahepatic concentration of N-
acetylglutamate increases after ingestion of a protein-
rich meal, which provides both a substrate
(glutamate) and the regulator of N-acetylglutamate
synthesis.
This leads to an increased rate of urea synthesis.
17. SOURCES OF AMMONIA (FREE NH3)
Glutamine- The kidneys generate ammonia from glutamine
by the actions of renal glutaminase and glutamate
dehydrogenase. Ammonia is also obtained from the hydrolysis of
glutamine by intestinal glutaminase either from the blood or from
digestion of dietary protein.
Bacterial action in the intestine- Ammonia is formed
from urea by the action of bacterial urease in the lumen of the
intestine.
Amines- Amines obtained from the diet, and monoamines that
serve as hormones or neurotransmitters, give rise to ammonia by
the action of amine oxidase.
Purines and pyrimidines- In the catabolism of purines
and pyrimidines, amino groups attached to the rings are released
as ammonia.
18.
19. INTERRELATION BETWEEN UREA CYCLE AND TCA CYCLE
Urea cycle is linked with TCA cycle in three different
ways :
1.The production of fumarate in urea cycle is the most
important integrating point with TCA cycle. Fumarate is
converted to malate and then to OAA in TCA cycle. OAA
undergoes transamination to produce aspartate which
enters urea cycle. Here, aspartate combines with citrulline to
produce arginosuccinate. OAA is an important metabolite
which can combine with acetyl CoA to form citrate and get
finally oxidized. OAA can also serve as a precursor for the
synthesis of glucose (gluconeogenesis).
21. INTERRELATION BETWEEN UREA CYCLE AND TCA CYCLE
2.ATP (10) are generated in the TCA cycle
while ATP (4) are utilized for urea synthesis.
3.TCA cycle is an important metabolic pathway
for the complete oxidation of various
metabolites to CO2 and H2O. The CO2
liberated in TCA cycle (in the mitochondria)
can be used in urea cycle.
