Urea cycle

Urea Cycle


       At the end of this lecture, student will be able to

      Explain urea cycle

      Explain significance of urea cycle

Overview of amino acid catabolism in mammals


       Urea cycle / Krebs-Henseleit cycle

       Urea is the end product of protein metabolism (amino acid metabolism)

       Nitrogen of amino acids is converted to ammonia

       Ammonia is toxic to the body and hence converted to urea and detoxified

       Urea synthesis is a five-step cyclic process, with five distinct enzymes

       First two enzymes are present in mitochondria while the rest are located in cytosol

       Urea is synthesized in liver and transported to kidneys for excretion in urine

       Urea cycle was elucidated by Hans Krebs and Kurt Henseleit (1932), hence it is known as Krebs-Henseleit cycle

       Urea has two amino (-NH2) groups, one derived from NH3 and the other from aspartate & carbon atom is supplied by CO2

       Interrelation between urea cycle and TCA cycle

Step I: Synthesis of carbamoyl phosphate

       Carbamoyl phosphate synthase I (CPS-l) catalyses the condensation of NH3 ions with CO2 to form carbamoyl phosphate

       This step consumes 2 ATP and is irreversible and rate-limiting

       CPS I requires N-acetylglutamate for its activity

       Takes place in mitochondria

       CPS-I is 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 II. Formation of citrulline

       Citrulline is synthesized from carbamoyl phosphate and ornithine by ornithine transcarbamoylase

       Ornithine is regenerated and used in urea cycle

       Therefore, its role is comparable to that of oxaloacetate in citric acid cycle

       Ornithine and citrulline are basic amino acids

       Citrulline produced in this reaction is transported to cytosol by a transporter system

Step III. Synthesis of arginosuccinate

       Arginosuccinate synthase 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 AMP and pyrophosphate (PPi).

       The latter is immediately broken down to inorganic phosphate (Pi)

Step IV. Cleavage of arginosuccinate

       Arginosuccinase cleaves arginosuccinate to give arginine and fumarate

       Arginine is the immediate precursor for urea

       Fumarate liberated here provides a connecting link with TCA cycle, gluconeogenesis etc

Step V. Formation of urea

       Arginase is the fifth and final enzyme that cleaves arginine to yield urea and ornithine

       Ornithine, so regenerated, enters mitochondria for its reuse in the urea cycle

       Liver can ultimately produce urea

Overall reaction and energetics

       The urea cycle is irreversible and consumes 4 ATP

       2 ATP are utilized for the synthesis of carbamoyl phosphate

       1 ATP is converted to AMP and PPi to produce arginosuccinate which equals to 2 ATP

       Hence 4 ATP are actually consumed

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

Regulation of urea cycle

       The first reaction catalysed by CPS-l is rate limiting reaction in urea synthesis

       CPS-I is allosterically activated by N-acetylglutamate and degraded by a hydrolase

       The rate of urea synthesis in liver is correlated with the concentration of N-acetylglutamate 

       High concentrations of arginine increase NAG, increases the level of NAG in liver, leading to enhanced urea synthesis

Disposal of urea

       Urea produced in the liver freely diffuses and is transported in blood to kidneys, and excreted

       A small amount of urea enters the intestine where it is broken down to CO2 and NH3 by the bacterial enzyme urease, this ammonia is either lost in the faeces or absorbed into the blood.

       ln renal failure, the blood urea level is elevated (uremia), resulting in diffusion of more urea into intestine

       Hyperammonemia (increased blood NH3) is commonly seen in patients of kidney failure

       Major changes in diet can increase the concentrations of individual urea cycle enzymes 10-fold to 20-fold

       Starvation, for example, elevates enzyme levels, presumably to cope with the increased production of ammonia that accompanies enhanced protein degradation

Metabolic disorders are associated with each reaction of the urea cycle

       All the disorders invariably lead to hyperammonemia and toxicity

       The clinical symptoms associated with defect in urea cycle enzymes Include vomiting, lethargy, irritability, ataxia and mental retardation

(1) Defects in any of several enzymes of a metabolic pathway enzyme can result in similar clinical signs and symptoms

(2) The identification of intermediates and of ancillary products that accumulate prior to a metabolic block provides insight into the reaction that is impaired

(3) Precise diagnosis requires quantitative assay of the activity of the enzyme thought to be defective

(4) Rational therapy must be based on an understanding of the underlying biochemical reactions in normal and impaired individuals


       Urea cycle is the end product of protein metabolism

       Urea is synthesized in liver and transport to kidney for excretion in urine

       4 ATP are consumed in urea cycle

       Normal blood urea concentration is 10 – 40 mg/dl

       Hyperammonemia is increase in ammonia level in blood

       CPS-l is rate limiting reaction in urea synthesis

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