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Urea is the end product of nitrogen metabolism


  • The continuous degradation and synthesis of cellular proteins occur in all forms of life.
  • Each day, humans turn over 1–2% of their total body protein, principally muscle protein.
  • Of the liberated amino acids, approximately 75% are reutilized.
  • Since excess amino acids are not stored, those not immediately incorporated into new protein are rapidly degraded to amphibolic intermediates.
  • The excess nitrogen forms urea (figure-1).

 Fate of amino acids

Figure-1- Fate of Amino acids, the ammonia released from the amino group of the amino acids is detoxified through urea formation and the carbon skeleton is used either for the synthesis of glucose, ketone bodies or is completely oxidized in TCA cycle to provide energy.

Urea formation (Urea cycle)

Characteristics of urea cycle

  • Urea is the major disposal form of amino groups
  • It accounts for 90% of the nitrogen containing components of urine
  • The urea cycle is the sole source of endogenous production of arginine
  • Urea formation takes place in liver,
  • Urea excretion occurs through kidney

Substances required for urea formation

1) Amino acids- 6 amino acids participate in urea formation, which are-

  • Ornithine
  • Citrulline
  • Aspartic acid
  • Argininosuccinic acid
  • Arginine and
  • N-Acetyl Glutamate

Of the six participating amino acids, N-acetyl glutamate functions solely as an enzyme activator. The others serve as carriers of the atoms that ultimately become urea.

2) Energy

  • Synthesis of 1 mol of urea requires 3 mol of ATP

3) Amino group

  • 1 mol each of ammonium ion and of the α-amino nitrogen of aspartate.

4) Enzymes

  • Five enzymes catalyze the reactions of urea cycle

5) Carbon dioxide

  • CO2 and ammonia both are waste products and are eliminated as urea from the body
  • CO2 is added in the form of bicarbonate ion

 Site of urea formation

  • Urea synthesis is a cyclic process.
  • The first two reactions of urea synthesis occur in the matrix of the mitochondrion, the remaining reactions occur in the cytosol

Steps of urea formation

Step-1- Formation of Carbamoyl-Phosphate (figure-2)

  • Condensation of CO2, ammonia, and ATP to form Carbamoyl phosphate is catalyzed by mitochondrial Carbamoyl phosphate synthase I (CPS-1)
  • Formation of Carbamoyl phosphate requires 2 mol of ATP, one of which serves as a phosphoryl donor.
  • Carbamoyl phosphate synthase I, the rate-limiting enzyme of the urea cycle, is active only in the presence of its allosteric activator N-acetyl glutamate, which enhances the affinity of the synthase for ATP.

Step-2- Formation of Citrulline (figure-2)

  • The Carbamoyl group of Carbamoyl phosphate  is transferred to ornithine, forming Citrulline and Ortho Phosphate
  • The reaction is catalyzed by Ornithine trans Carbamoylase
  • Subsequent metabolism of Citrulline take place in the cytosol.
  • Entry of ornithine into mitochondria and exit of citrulline from mitochondria involves mitochondrial inner membrane transport systems

Clinical Significance

  • Ornithine Transcarbamoylase deficiency causes enhanced excretion of Uracil.
  • Excessive excretion of Uracil or its precursor Orotic acid, results from an accumulation of Carbamoyl phosphate in the mitochondria.
  • In the absence of Ornithine Transcarbamoylase, Carbamoyl phosphate accumulates and leaks in to the cytoplasm, where it can be used to make Carbamoyl Aspartate, the first intermediate in the pathway of pyrimidine nucleotide biosynthesis.

Step-3- Formation of Argininosuccinate (figure-2)

  • Argininosuccinate synthase (ASS) links L- Aspartate and Citrulline via the amino group of aspartate and provides the second nitrogen of urea.
  • The reaction requires ATP, production of argino-succinate is an energetically expensive process, since the ATP is split to AMP and pyrophosphate.
  • The pyrophosphate is then cleaved to inorganic phosphate using pyrophosphatase, so the overall reaction costs two equivalents of high energy phosphate per mole.

Step-4- Cleavage of Argininosuccinate (figure-2)

  • Cleavage of argininosuccinate catalyzed by argininosuccinate lyase (ASL), proceeds with retention of nitrogen in arginine and release of the aspartate skeleton as fumarate.
  • Addition of water to fumarate forms L-malate, and subsequent NAD+-dependent oxidation of malate forms oxaloacetate (figure-3).
  • Transamination of oxaloacetate by glutamate aminotransferase then re-forms aspartate. carbon skeleton of aspartate-fumarate thus acts as a carrier of the nitrogen of glutamate into a precursor of urea


 Urea cycle

Figure-2-Steps of urea formation, the first two reactions are mitochondrial and the remaining reactions take place in the cytoplasm.

Relationship of Urea cycle with TCA cycle

 Fate of fumarate

Figure-3- Urea cycle and TCA cycle are linked together through fumarate.

Step-5- Cleavage of Arginine(figure-2)

  • Hydrolytic cleavage of the guanidino group of arginine, catalyzed by liver Arginase (ARG1) releases urea, the other product, Ornithine, reenters liver mitochondria for additional rounds of urea synthesis.
  • Arginine also serves as the precursor of the potent muscle relaxant nitric oxide (NO) in a Ca2+-dependent reaction catalyzed by NO synthase

Regulation of Urea formation

  • The activity of Carbamoyl phosphate synthase I is determined by N-acetyl glutamate, whose steady-state level is dictated by its rate of synthesis from acetyl-CoA and glutamate and its rate of hydrolysis to acetate and glutamate.
  • These reactions are catalyzed by N-acetyl glutamate synthase and N-acetyl glutamate hydrolase, respectively (figure-4).

Formation and degradation of N-Acetyl glutamate 

Figure-4-Formation and hydrolysis of N-Acetyl glutamate

  • Major changes in diet can increase the concentrations of individual urea cycle enzymes 10- to 20-fold.
  • Starvation, for example, elevates enzyme levels, presumably to cope with the increased production of ammonia that accompanies enhanced protein degradation
  • Regulation is also achieved by linkage of mitochondrial glutamate dehydrogenase with CPS-1

Fate of Urea (figure-5)

  • Urea formed in the liver is transported through circulation to kidneys for excretion through urine.
  • It is also transported to intestine where it is decomposed by Urease produced by microbial action.
  • Ammonia liberated by this activity is transported by portal circulation to liver where it is detoxified back to urea.
  • A fraction of ammonia goes to systemic circulation.

 Fate of urea

Figure-5- Urea formed in the liver is mainly excreted through kidney in urine. A fraction of urea is transported to intestine where it is acted upon by bacterial urease. The ammonia thus released either goes through portal circulation to liver for reconversion to urea or enters systemic circulation.


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