- 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 CO2 to 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 Mn2+.
- 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.
NH4+ + CO2 + Aspartate + 3ATP → Urea + Fumarate + 2ADP + 2Pi + AMP +PPi