Urea cycle

March 15, 2019

Urea is considered as the end product of protein metabolism (amino acid metabolism).
The nitrogen of amino acids gets converted to ammonia which is toxic to the body.
It is then converted to urea and detoxified.
Such urea accounts for 80-90% of the nitrogen containing substances which is excreted in the urine.
It is synthesized in the liver and transported to the kidneys for excretion through urine.
The urea cycle is elucidated by Hans Krebs and Kurt Henseleit in 1932 which was the first metabolic cycle.
Due to this reason, this cycle is also called Krebs-Henseleit cycle.
However, the individual reactions were described later on by Ratner and Cohen.
Urea has two amino groups (-NH₂), one derived from NH₃ and the other from aspartate where carbon atom is supplied by CO₂.
Urea synthesis is a five step cyclic process which is guided by five distinct enzymes.
The first two enzymes are present in mitochondria and the rest enzymes are localized in cytosol.
The various steps of urea cycle are given below.

A) Synthesis of carbamoyl phosphate

Carbamoyl phosphate synthetase I (CPS I) of mitochondria catalyzes the condensation of NH₄⁺ ions with CO₂to form carbamoyl phosphate.
This step consumes two ATP and is irreversible and rate limiting.
CPS I requires N-acetylglutamate for its activity.
There is another enzyme CPS II, involved in pyrimidine synthesis (present in cytosol) which accepts amino group from glutamine and does not require N-acetylglutamate for its activity.

B) Formation of citrulline

Citrulline is synthesized from carbamoyl phosphate and ornithine by ornithine transcarbamoylase.
Ornithine is again generated and used in urea cycle.
Therefore, its role can be compared to the role of oxaloacetate in citric acid cycle.
Ornithine and citrulline are basic amino acids which are never found in protein structure due to lack of codons.
A transporter system transports citrulline to the cytosol which is formed in this reaction.

C) Synthesis of argininosuccinate

Argininosuccinate synthetase condenses citrulline with aspartate to produce arginosuccinate.
The second amino group of urea is incorporated in this reaction.
This step requires ATP which is cleaved to form AMP and pyrophosphate (PPi).
The latter is broken down immediately to inorganic phosphate (Pi).

D) Cleavage of argininosuccinate

Argininosuccinase cleaves argininosuccinate to give free arginine and fumarate.
Arginine is the immediate precursor for urea where fumarate liberated here provides a connecting link with TCA cycle, gluconeogenesis, etc.
This is the only reversible reaction of the urea cycle.

E) Formation of urea

Arginase is the fifth and final enzyme that helps in cleaving arginine to yield urea and ornithine.
Ornithine produced again enters mitochondria for its reuse, in the urea cycle.
Arginase is activated by Co²⁺ and Mn^2+.
Ornithine and lysine compete with arginine which is competitive inhibition.
Arginase is mostly found in the liver, while the rest of the enzymes are present in other tissues.
For this reason, arginine synthesis may occur to varying degrees in many tissues.
But it is the liver that can ultimately produce urea.

Overall reaction and energetics

The urea cycle is irreversible and consumes 4 ATP.
Two ATPs are utilized for the synthesis of carbamoyl phosphate.
One ATP is converted to AMP and PPi to produce arginino-succinate which equals to 2 ATP.
Hence 4 ATP are actually consumed.
However, the urea cycle also causes a net conversion of oxaloacetate to fumarate via aspartate.
The regeneration of oxaloacetate produces NADH in the malate dehydrogenase reaction.
Each NADH molecule can generate up-to 5ATP during mitochondrial respiration greatly reducing the overall cost of urea synthesis.