1. The urea cycle is a series of enzymatic reactions that occurs primarily in the liver to convert toxic ammonia produced from amino acid catabolism into urea for excretion. 2. The cycle involves five principal reactions: carbamoyl phosphate synthesis, citrulline synthesis, argininosuccinate synthesis, argininosuccinate cleavage, and arginine cleavage into ornithine and urea. 3. The urea cycle serves two major biological roles - detoxification of ammonia into urea and biosynthesis of the amino acid arginine from ornithine in tissues like liver, kidney, and intestine.

1. Mahpara Gondal
Pharm-D
MS-Pharmaceutical Chemistry
Rashid Latif College of Pharmacy

2.  In mammalian tissues, α-NH2 group of amino
acids, derived either from the diet or
breakdown of tissue proteins, ultimately is
converted first to NH3 and then to urea and is
excreted in the urine.
 The formation of urea involves the action of
several enzymes

3.  Urea is the characteristic end-product of amino
acid N-catabolism in human beings and
ureotelic organisms.
 Urea synthesis is replaced,
• By uric acid formation in uricotelic organism,
e.g.
Reptiles and birds,
• By NH3 in ammonotelic organism, e.g. bony
fish.

4.  Transamination is a reversible reaction in which
α-NH2 group of one amino acid is transferred
to a α-ketoacid resulting in formation of a new
amino acid and a new ketoacid.
 The general process of transamination may be
represented as further:

5. Donor amino acid (I) thus becomes a new ketoacid (I)
after losing the α-NH2 group, and the recipient ketoacid
(II) becomes a new amino acid (II) after receiving the NH2
group.

6.  The process represents only an
intermolecular transfer of NH2 group without
the splitting out of NH3. Ammonia formation
does not take place by transamination
reaction.
 Deamination is the process by which N– of
amino acid is removed as NH3.
Types: It can be of 2 types:
 A. Oxidative deamination
 B. Non-oxidative deamination.

7.  Liver promptly removes the NH3 from the
portal blood, so that blood leaving the liver is
virtually NH3-free.
 This is essential since even small quantities of
NH3 are toxic to CNS.
 Normal Blood Ammonia Level
 In man, normal blood level of NH3 varies from 40
to 70 μg/100 ml. Free NH+ 4 (ammonium ion)
concentration of fresh plasma is less than 20 μg
per 100 ml. Such low concentrations suggest that
the mechanism for removal for this highly toxic
substance is extremely efficient.

8.  Features of NH3 intoxication: The symptoms
of NH3 intoxication include:
• Slurring of speech
• Blurring of vision
• In severe cases follows to coma and death.
 These resemble those of syndrome of hepatic
coma, where blood and brain NH3 levels are
elevated.

9.  Metabolic fate of NH3 in the body: Three
main fates:
 1. Mainly NH3 is converted to urea (urea cycle)
 2. Formation of Glutamine
 3. Amination of α-ketoacid to form α-amino acid

10.  The removal of excess of NH3 derived from
amino acid catabolism in the tissues or from
bacterial action in the gut is accomplished by
the production of urea which is excreted in the
urine.
 Characteristic Features
• It is a cyclic process, five reactions which
involves ornithine, citrulline, arginine and
aspartic acid.
• Site of synthesis: urea formation takes place in
liver in mammals and all of the enzymes involved

11.  Stages
The reactions of urea cycle can be studied in five
sequential enzymatic reactions.
 • Reaction 1: Synthesis of carbamoyl-
phosphate
 • Reaction 2: Synthesis of citrulline
 • Reaction 3: Synthesis of argininosuccinate
 • Reaction 4: Cleavage of argininosuccinate
 • Reaction 5: Cleavage of arginine to for
ornithine and urea

12.  Reaction 1: Synthesis of Carbamoyl-P
(Mitochondrial)
 In this reaction, HCO– 3, NH+ 4 and phosphate
derived from ATP reacts to form carbamoyl-P
(also called Carbamyl-P).
 The reaction is catalysed by the mitochondrial-
enzyme Carbamoyl phosphate synthetase 1.

13. .

14.  Reaction 2: Synthesis of Citrulline:
(Mitochondrial)

15.  Ornithine transcarbamoylase enzyme, also
called as ornithine carbamoyl transferase is
found associated with carbamoylphosphate
synthetase I in the mitochondrial matrix.
 It catalyses the nucleophilic addition of
ornithine to the carbonyl group of carbamoyl-P
to produce Citrulline.
 During this reaction, the δ-NH2 group of
ornithine attaches to the carbonyl group of
carbamoyl-P and the phosphate group (Pi) is
released.

16.  Reaction 3: Synthesis of Argininosuccinate:
(cytosolic):

17.  After citrulline has been transported to the
cystosol, it condenses with Aspartate to form
argininosuccinate in an ATP-dependant
reaction catalysed by argininosuccinate
synthetase.

18.  Reaction 4: Cleavage of Argininosuccinate:
(Cytosolic)

19.  In this reaction of urea cycle, the enzyme
argininosuccinase also known as
Argininosuccinate Lyase catalyses
conversion of Argininosuccinate to arginine and
fumarate.
 The urea cycle is linked to the TCA cycle
through the production of fumarate. Amino
acid catabolism, is therefore directly
coupled to energy production

20.  Reaction 5: Cleavage of Arginine to
Ornithine and Urea

21.  The last reaction of the urea cycle completes
the cycle.
 It is catalysed by the enzyme arginase, which
is found only in the liver cells. Arginase
catalyses hydrolysis of the guanidine group of
arginine, releasing urea and regenerating
ornithine. Ornithine now enters mitochondrion
through inner mitochondrial membrane by a
specific transport protein.

23.  1. Detoxification of NH3: Major biological role of
this pathway is the detoxication of NH3. Toxic
ammonia is converted into a nontoxic substance
urea and excreted in urine.
 2. Biosynthesis of arginine: The urea cycle also
serves for the biosynthesis of arginine from
ornithine in liver, kidney and intestinal mucosa.
Kidney and intestinal mucosa probably contribute
most of the body arginine because they possess
all the urea cycle enzymes except arginase.
Hence they can form upto arginine and cannot
form urea.
 The arginine is used for protein synthesis.

24. .