Introduction to Urea Cycle:
The urea cycle is a biochemical pathway that converts toxic ammonia into a non-toxic, water-soluble compound known as urea. Since even small amounts of ammonia are highly toxic to the central nervous system, the liver plays a vital role in rapidly removing ammonia from the blood and converting it to urea. The urea is then transported to the kidneys for excretion in urine.
The urea cycle was first described by Krebs and Henseleit and is also known as the Krebs-Henseleit Cycle. It occurs primarily in the liver and involves both mitochondrial and cytosolic enzymes. The process requires energy and occurs through five key reactions.
- The first two reactions occur in the mitochondria.
- The remaining three reactions occur in the cytosol.
Reactions of the Urea Cycle:
The conversion of ammonia to urea takes place in five steps:
- Formation of Carbamoyl Phosphate
- Formation of Citrulline
- Formation of Argininosuccinate
- Formation of Arginine and Fumarate
- Formation of Urea and Ornithine
In this process, one molecule of ammonia and one molecule of aspartate contribute nitrogen atoms to form urea, while bicarbonate (HCO₃⁻) provides the carbon atom.
Step 1: Formation of Carbamoyl Phosphate
This reaction occurs in the mitochondria and is catalyzed by the enzyme carbamoyl phosphate synthetase I (CPS-I). It involves the condensation of ammonia with bicarbonate using 2 ATP molecules. The enzyme requires N-acetylglutamate as an essential allosteric activator and Mg²⁺ as a cofactor.
The reaction produces carbamoyl phosphate, a high-energy compound that drives subsequent reactions in the urea cycle forward.
Step 2: Formation of Citrulline
The enzyme ornithine transcarbamoylase catalyzes the combination of carbamoyl phosphate and ornithine to form citrulline. This step also occurs in the mitochondria. Citrulline is then transported into the cytosol via a specific transporter across the inner mitochondrial membrane.
Step 3: Formation of Argininosuccinate
In the cytosol, argininosuccinate synthetase catalyzes the condensation of citrulline and aspartate (which donates the second nitrogen atom) to form argininosuccinate. This reaction consumes two high-energy phosphate bonds (from ATP → AMP + PPi). Hydrolysis of PPi to 2 Pi by pyrophosphatase makes the reaction irreversible.
Step 4: Formation of Arginine and Fumarate
The enzyme argininosuccinase (argininosuccinate lyase) cleaves argininosuccinate into arginine and fumarate. Fumarate is an intermediate that can re-enter metabolic pathways such as the TCA cycle or be converted back into aspartate for continued operation of the urea cycle.
Step 5: Formation of Urea and Ornithine
In the final step, the enzyme arginase catalyzes the hydrolysis of arginine to produce urea and regenerate ornithine. Ornithine re-enters the mitochondria to participate in another round of the urea cycle.
Energy Requirement
The urea cycle is an energy-dependent process that requires four high-energy phosphate bonds per molecule of urea synthesized (three ATP molecules are hydrolyzed, with one yielding AMP, equivalent to two high-energy bonds).
Fate of Urea
Urea serves no metabolic function and is the main nitrogenous waste product of protein catabolism in humans. It is released into the bloodstream and excreted by the kidneys through urine.
- About 10–25 g of urea is excreted daily, accounting for 80–90% of total urinary nitrogen.
- Normal blood urea level: 16–36 mg/100 ml.
Fate of Fumarate
Fumarate produced in the urea cycle is recycled back to aspartate through several steps:
- Fumarate → Malate (via fumarase)
- Malate → Pyruvate (via malic enzyme)
- Pyruvate → Oxaloacetate (via pyruvate carboxylase or malate dehydrogenase)
- Oxaloacetate → Aspartate (via transamination with glutamate)
This recycling helps provide one of the nitrogen atoms for urea synthesis.
Regulation of Urea Cycle
The key regulatory enzyme in the urea cycle is carbamoyl phosphate synthetase I (CPS-I), which catalyzes the committed step of urea synthesis. Its activity is controlled allosterically by N-acetylglutamate.
Conditions that increase protein breakdown (e.g., high-protein diet or starvation) elevate N-acetylglutamate synthesis, thus increasing urea production.
Metabolic Disorders of the Urea Cycle
Inherited enzyme deficiencies in the urea cycle lead to impaired conversion of ammonia to urea, resulting in hyperammonemia — toxic accumulation of ammonia in blood. The incidence of urea cycle disorders is approximately 1 in 2500 births. Most are fatal if untreated.
Clinical symptoms: vomiting, irritability, lethargy, seizures, mental retardation, coma, and in severe cases, early death.
Common Disorders Include:
- Hyperammonemia Type I: Caused by deficiency of carbamoyl phosphate synthetase I. The main symptom is severe mental retardation due to ammonia toxicity.
- Hyperammonemia Type II: The most common urea cycle disorder caused by deficiency of ornithine transcarbamoylase. Leads to accumulation of carbamoyl phosphate, which is diverted to pyrimidine synthesis, causing orotic aciduria. Glutamate levels also rise.
- Citrullinemia: Results from absence of argininosuccinate synthetase. Citrulline accumulates in blood and is excreted in urine.
- Argininosuccinic Aciduria: Due to deficiency of argininosuccinase, leading to accumulation of argininosuccinate in blood and urine.
- Hyperargininemia: Caused by deficiency of arginase. Arginine accumulates in blood and urine, though some urea may still be produced by kidney arginase.
- N-Acetylglutamate Synthetase Deficiency: A rare condition due to absence of N-acetylglutamate synthetase. Leads to hyperammonemia and aminoaciduria. Treatment with carbamoyl glutamate (an analog of N-acetylglutamate) can relieve symptoms.
Treatment of Urea Cycle Disorders
Management aims to reduce ammonia levels and prevent its toxic effects on the brain.
- Ammonia removal: Peritoneal dialysis can be used to clear excess ammonia from the bloodstream.
- Dietary control: Restrict protein intake to minimize nitrogen load.
- Medication therapy: Administration of benzoic acid and phenylacetate helps remove nitrogen by forming excretable compounds.
- Supplement therapy: In some cases, supplementation with arginine or citrulline helps bypass enzyme blocks.
Detailed Notes:
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