Page 353 - Biology_F5

Biology for Advanced Level Secondary Schools

(a) Formation of carbamoyl phosphate This reaction incorporates the second
Before the cycle, ammonia (NH ) from nitrogen from aspartate.
3
metabolism of nitrogen containing
compounds combine with carbon dioxide (d) Cleavage of argininosuccinate
(CO ) gas from respiration in a solution The cleavage of argininosuccinate
2
form (ammonium ions and bicarbonate involves the enzyme argininosuccinase, an
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ions respectively) resulting into the intermediate enzyme in the urea synthesis
formation of carbamoyl phosphate, by the pathway whose function is imperative
help of the enzyme carbamoyl phosphate to the continuation of the cycle. It acts
synthetase-I. The reaction occurs in the reversibly to cleave argininosuccinate into
mitochondria of the liver cells, and it a free arginine and fumarate. The arginine
requires 2 ATP molecules. continues with the cycle in the other stage,
whereas fumarate enters the Tricarboxylic
(b) Synthesis of citrulline Acid (TCA) cycle, which is also known
The carbamoyl phosphate formed in the as Kreb’s cycle. The linkage between
first step enters the ornithine cycle and TCA cycle and urea cycle is known as the
combines with ornithine resulting in Kreb’s bi-cycle.
the synthesis of citrulline, aided by an
enzyme citrulline synthase or ornithine (e) Cleavage of arginine
transcarbamoylase. In the reaction the Arginine is hydrolysed into ornithine and
phosphate group is released. Citrulline urea under the influence of the enzyme
can easily pass through the mitochondrial arginase; hence, arginine is known as a
membrane, thus it diffuses from the semi-essential amino acid. Though it is
mitochondrion into cytosol (cytoplasm) synthesised in the body, it is not available
of liver cells. for protein synthesis. Ornithine is

regenerated in this step and the urea cycle
(c) Synthesis of argininosuccinate completes by the formation of urea. The
In the cytosol, citrulline combines with
the amino group of aspartate under ornithine produced is transported back to
condensation reaction catalysed by the mitochondria to start the cycle again,
enzyme argininosuccinate synthetase while urea is transported to the kidney
to form argininosuccinate. It requires through blood vessels to be excreted.
ATP which is hydrolysed to adenosine Thus, the urea cycle brings two amino
monophosphate (AMP) resulting in the groups (NH ) and hydrogen carbonate ions
2
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utilisation of two high energy bonds. (HCO ) together to form urea.
3
Magnesium ions (Mg ) act as cofactors.
2+

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