Urea Cycle

Overview of Nitrogen Metabolism

• The catabolism of amino acids (AAs) involves transamination and deamination reactions, which result in the release of ammonia (NH₃). 
• The regulation of nitrogen levels within the body is crucial due to the toxicity of ammonia. 
• Excess nitrogen in the form of ammonium is then bonded to AAs via transamination reactions and transported to the liver and kidneys. 
  • The most important AAs in nitrogen transport are glutamate and alanine.
• Within the kidneys, ammonium ions are released through glutamine deamination and glutamine and directly excreted into the urine.  
• Within the liver, the amino groups from alanine and glutamate are transferred via aminotransferases, resulting in ammonia and aspartate, which are shunted into the urea cycle:
  • Involves 1 feeder reaction (incorporation of ammonium and CO₂) and 4 cycle reactions (creating 1 molecule of urea)
  • Overall reaction equation:
    NH₃ + CO₂ + aspartate + 3 ATP + 2 H₂O → urea + fumarate + 2 ADP + 2 P_i + AMP + PP_i
  • Defects in any of the cycle’s catalyzing enzymes result in hyperammonemia.

Transport of Nitrogen in the Blood

Nitrogen atoms are produced in skeletal musculature via the catabolism of proteins. They are bound to AAs in the form of amino groups → bloodstream → liver.

• Nitrogen can be synthesized in the liver from intermediate byproducts of the citric acid cycle.
• Nitrogen can also be produced in the intestines by bacteria or via the breakdown of dietary protein, then shunted directly to the liver via the venous portal system.

Glutamate (or glutamic acid) is essential for nitrogen transport to the kidneys and liver.

• Forms in peripheral cells by transamination to an α-ketoglutarate 
• Can receive another amino group via glutamine synthetase → glutamine
  • In the kidneys, glutaminase deaminates glutamine, releasing ammonium ions into the urine.
  • In the liver, glutamine releases ammonium ions via glutamate dehydrogenase, which are shunted into the urea cycle.
  • Can also be converted into alanine via transamination

Alanine is essential for nitrogen transport from the muscles to the liver.

• Forms when glutamate transfers its amino group to pyruvate → alanine
• This reaction reverts within the liver:
  • Resulting pyruvate is used in gluconeogenesis → glucose, which is released into the bloodstream and used again in skeletal muscle (glucose-alanine or Cahill cycle).
  • Resulting glutamate can be used to release ammonia via deamination OR convert oxaloacetate into aspartate via transamination; both products are shunted into the urea cycle.

Steps of the Urea Cycle

• Urea cycle takes place exclusively in the liver, within the mitochondria and cytosol of hepatocytes. 
• Nitrogen atoms reach the liver in the form of amino groups bound to AAs (alanine or glutamate). 
• Involves 1 feeder reaction and 4 cycle reactions, and requires 3 ATP

Reaction steps of the urea cycle

• Step 1 or feeder reaction: ammonia (NH₃) + CO₂ → carbamoyl phosphate 
  • In the mitochondria, catalyzed by carbamoyl phosphate synthetase, requires N-acetylglutamate as an activator and 2 ATP
  • Rate-limiting reaction
• Step 2: carbamoyl phosphate + ornithine → citrulline
  • In the mitochondria, catalyzed by ornithine carbamoyltransferase
• Step 3: citrulline + aspartate → argininosuccinate 
  • In the cytosol, catalyzed by argininosuccinate synthetase, requires 1 ATP
• Step 4: argininosuccinate → arginine + fumarate
  • In the cytosol, catalyzed by argininosuccinate lyase
  • Fumarate either enters the CAC or transforms into oxaloacetate, which can be turned into aspartate and re-enter the urea cycle (aspartate cycle). 
• Step 5: arginine + H₂O → urea + ornithine
  • In cytosol, catalyzed by arginase
  • Urea enters the bloodstream.
  • Ornithine is transported back into the mitochondrial matrix → step 2.
• Net reaction per cycle: 2 NH₃ + CO₂ + 3 ATP + H₂O → urea + 2 ADP + 4 P_i + AMP

Elimination of Urea

• Approximately 30 g of urea are produced → bloodstream every day 
  • Varies greatly according to diet
• Urea is soluble in water → the kidneys → excreted in the urine
  • Biggest portion of the nitrogen-containing compounds is in the urine
• Urea is filtered via the glomerular capillaries and partially reabsorbed (can be measured in the laboratory).

Clinical Relevance

Congenital deficiencies of the urea cycle

• Ornithine transcarbamylase (OTC) deficiency: X-linked recessive condition considered to be the most common type of urea cycle disorder in humans. Patients suffering from this congenital disorder lose the ability to eliminate ammonia due to a lack of OTC. The high levels of ammonia caused by this condition produce a coma in newborns within 72 hours of birth.

Acquired conditions that affect the urea cycle

• Acute and chronic liver diseases: caused by hepatic parenchymal necrosis leading to hepatic encephalopathy, attributed to hyperammonemia because of the liver’s inability to metabolize ammonia by the urea cycle. The liver’s synthetic functions are defective, so there is often a concomitant coagulopathy. Jaundice occurs because of the impaired conjugation of bilirubin and excretion of the conjugated product.
  • Acute liver failure: usually caused by hepatotoxins or viral infections. In the USA and UK, acetaminophen overdose is the most common cause, while viral hepatitis predominates in Asia.
  • Advanced liver failure in cirrhosis: a condition caused by chronic damage and fibrosis, with liver insufficiency evident in the advanced stages. Common causes of cirrhosis include chronic viral hepatitis (hepatitis B, C), alcoholic liver disease, hemochromatosis, and nonalcoholic fatty liver disease.