Urea Cycle – Steps, Disorders and Significance

The steps of the urea cycle generally consist of the five reactions in the mitochondrial matrix and the cytosol. The urea cycle also known as the ornithine cycle or the Krebs-Henseleit cycle is a series of different biochemical reactions that produce urea from ammonia in the ureotelic organisms.

The whole process takes place in the mitochondria of the liver cells. This cycle is a major metabolic pathway to remove nitrogen from the body with ammonia and carbon dioxide. A urea cycle needs 4 ATP to complete the whole process which is irreversible. In this article, we are going to discuss the different steps of the Urea Cycle in detail.

What is Urea Cycle?

Definition: The urea cycle is a process that helps to remove the harmful gases i.e. ammonia and carbon dioxide from the body. It is a five step biochemical reaction that mainly produces the urea. The whole process take place in the mitochondria of the liver cells.

The urea cycle is dependent on the ATP. It starts in the mitochondria of hepatocytes and ends in the cytoplasm. When the urea cycle does not work properly, the ammonia accumulates in the body and can create so many harmful diseases like Lethargy, Slurred speech, Cerebral edema, and Asterixis. The liver turns ammonia into urea. It goes to the kidneys through the blood and is removed from the body as urine.

Read More: What is Mitochondria? Structure, Diagram, & Functions

What are the 5 Steps of Urea Cycle?

The urea cycle begins in liver cells and finishes in the cytoplasm. The final product is then transported to the kidney for removal from the body as urine. Here are the five steps involved in the urine cycle as mentioned below.

First Step of the Urea Cycle: Carbamoyl Phosphate Synthesis

Mitochondrial carbamoyl phosphate synthetase I (CPS I) combines NH4+ ions with CO2 to create carbamoyl phosphate. It is a crucial step in pyrimidine production. This process is irreversible and uses 2 ATP in total. CPS I relies on N-acetyl glutamate to perform the process. Carbamoyl phosphate synthetase II (CPS-II) is an enzyme that is located in the cytosol. It uses the amino group from glutamine and does not require N-acetyl glutamate (NAG).

Here are the related reactions as mentioned below:

CO+ NH+ 2ATP produces carbamoyl phosphate +2ADP + P

Synthesis of NAG: glutamate + acetyl-CoA NAG

Second Step of the Urea Cycle: Citrulline Formation

In this step, Ornithine transcarbamoylase (OTC) helps make citrulline to from carbamoyl phosphate and ornithine. Ornithine can be reused in the urea cycle. Citrulline moves from the mitochondria of the liver cell to the cytoplasm through ornithine translocase.

Here is the related reaction as mentioned below:

NH4 + CO2 +2ATP → Carbamoyl Phosphate → Citrulline

Third Step of the Urea Cycle: Argininosuccinate Synthesis

In this step, Argininosuccinate synthase combines the citrulline and aspartate and generates argininosuccinate. The process adds the second amino group of urea. This breaks down the ATP into AMP and pyrophosphate (PPi). After this, it quickly turns into the inorganic phosphate (Pi).

Here is the related reaction as mentioned below:

Citrulline + Aspartate + ATP → Argininosuccinate + AMP + PPi

Aspartate forms when oxaloacetate and glutamate reacts together. This combination requires vitamin B6.

Here is the related reaction as mentioned below:

Oxaloacetate + glutamate à aspartate + alpha-ketoglutarate

Fourth Step of the Urea Cycle: Argininosuccinate Cleavage

Argininosuccinate is broken by the arginosuccinase into arginine and fumarate. Urea turns into arginine. After that, Argininosuccinate turns into arginine through argininosuccinate lyase. This reaction also releases fumarate. It helps to generate the NADH in the mitochondria during the TCA cycle. At last it breaks down into tyrosine.

Here is the related reaction as mentioned below:

Argininosuccinate → Arginine + Fumarate

Final Step of the Urea Cycle: Urea Formation

Arginase is the final enzyme. It breaks down arginine to make urea and ornithine. The recycled ornithine goes back into the mitochondria that will reuse in the urea cycle. Arginase activates both CO2 and Mn2+ during this process.

Here is the related reaction as mentioned below:

Arginine + HO urea + ornithine

Read More: ATP Synthesis in Mitochondria

Urea Cycle Diagram

Here is the diagram of the Urea Cycle as mentioned below:

Regulation of Urea Cycle

Regulation of the urea cycle involves with N-acetyl glutamate (NAG) and Substrate Concentrations. Let us discuss.

N-acetyl glutamate (NAG)

• NAG activates CPS I enzyme.
• NAG produced by the acetyl CoA and glutamate by using NAG synthase.
• Arginine and glutamic acid help speed up this process. Glutamic acid plays a double role as both a starting material and a booster for the urea cycle.

Substrate Concentrations

• Most enzymes work depends on how much of their substrates they have. Arginase is excluded from this process.

Energetics and Overall Reaction of Urea Cycle

The urea cycle is a one-directional process that need 4 ATP to complete the whole process. Formation of the carbamoyl phosphate uses two ATPs. One ATP converts into AMP and PPi to create argininosuccinate. It is equal to two ATPs. So, in total, the whole process takes 4 ATP to perform. Here is the whole reaction of the urea cycle as mentioned below.

NH4+ + CO2 + Aspartate + 3ATP → Urea+ Fumarate + 2ADP + 2 Pi + AMP + PPi

What is the Significance of Urea Cycle?

Here are the major significance of the urea cycle as mentioned below.

• The urea cycle is also known as the “Ammonia Detox Cycle”. It removes the harmful ammonia and CO2 from the body.
• The cycle turns the harmful ammonia into urea.
• This cycle also creates arginine. It is an amino acid that is needed for our body.
• Arginine helps balance blood pH, which depends on the levels of dissolved CO2 and HCO3.
• Ornithine is a part of this cycle. It is a starting point for making polyamines like spermidine and spermine.
• Ammonia comes from digesting protein and it is harmful to our body parts, especially for the brain.

Urea Cycle Disorders

Urea cycle disorders (UCDs) are genetic conditions. It can cause ammonia to build up in the blood. These disorders are caused by inherited deficiencies in one of the six enzymes or two transporters of the urea cycle pathway. In simple words, these disorders affect the functions of proteins and enzymes that removes ammonia from our blood. Here are some common Urea Cycle Disorders as mentioned below.

Ornithine Transcarbamylase (OTC) Deficiency

• Ornithine transcarbamylase deficiency is the only X-linked enzyme deficiency. It is recessive in nature.
• It is the most common enzyme deficiency in the urea cycle.
• This deficiency leads to increased levels of carbamoyl phosphate.
• High level of carbamoyl phosphate is redirected into the pyrimidine synthesis pathway.
• Carbamoyl phosphate is converted into orotic acid that is a precursor of all pyrimidines.
• Increased levels of orotic acid in the blood and urine result from this deficiency.
• Orotic aciduria can present as orange crystals.

Argininosuccinate Synthetase Deficiency

• This disorder is an another common enzyme deficiency in the urea cycle.
• It is inherited in a process of autosomal recessive manner.
• Patients with this deficiency will have high levels of citrulline.

Carbamoyl Phosphate Synthetase I (CPS I) Deficiency 

• CPS I deficiency is a fatal in infancy.
• It does not cause increased orotic acid levels.
• OTC deficiency and CPS I deficiency both lower urea and raise ammonia levels.
• OTC deficiency leads to high orotic acid levels.
• OTC deficiency and hereditary orotic aciduria both have high orotic acid levels.
• OTC deficiency also raises ammonia levels.

Conclusion – Urea Cycle

The urea cycle is a set of metabolic events in the liver that convert ammonia to urea. The urea cycle consists of six enzymes that eliminate nitrogen generated during amino acid metabolism. They convert it into urea, which dissolves in the urine. This article explains the importance and regulation of the urea cycle. There are several problems associated with defective urea cycle enzyme action.

FAQs on Urea Cycle

What are the Steps Involved in Urea Cycle?

Carbamoyl phosphate is formed from ammonia and bicarbonate, by carbamoyl phosphate synthetase (CPS). Ornithine transcarbamoylase (OTC) condenses carbamoyl phosphate and ornithine to form citrulline.

What is Step One of Urea Cycle?

The first step, which is also rate-limiting, involves the conversion of CO and ammonia into carbamoyl phosphate via the enzyme carbamoyl phosphate synthetase I (CPS I). Ammonia is the source of the first amine group of urea.

What is the Second Step of Urea Cycle?

Arginine undergoes hydrolysis to yield urea and ornithine in the presence of arginase. Ornithine is transported back to the mitochondria, which is used up in the second step of the cycle to form citrulline by combining with carbamoyl phosphate.

Is the First Step of the Urea Cycle is the Reversible Reaction?

The only reversible reaction of the urea cycle is catalyzed by carbamoyl phosphate synthetase I. The reversible reaction is the one where argininosuccinase cleaves argininosuccinate into arginine and fumarate.

Why is Urea Cycle called Bicycle?

In the urea cycle 2 ATPs are used in the first reaction. Another ATP is converted to AMP and PPi,which is equivalent to 2 ATPs. The urea cycle consumes 4 high energy phosphate bonds. The urea cycle and TCA cycle are interlinked, and so, it is called as “urea bicycle”.

How many ATP is used in Urea Cycle?

One urea cycle required three ATP molecules to convert the toxic ammonia to molecular urea. It also requires one molecule of carbon-monoxide and one molecule of aspartic acid.

Who Invented Urea Cycle?

In 1932, Hans Krebs and his assistant Kurt Henseleit discovered the steps of urea synthesis in mammals.

Which of the Following Steps are ATP Dependent in Urea Cycle?

ATP is required for the conversion of carbamoyl phosphate to citrulline and for the conversion of argininosuccinate to fumarate and arginine in the urea cycle.