UREA CYCLE

INTRODUCTION

Urea is the major end product in Nitrogen metabolism in humans and mammals.

NH3, the product of oxidative deamination reaction, is toxic in even small amount and must be removed from the body.

Urea cycle a.k.a Ornithine cycle is the conversion reactions of NH3 into urea.

This reaction occur in liver (certain occur in cytosol and mitochondria)

The urea is transported to the kidney where it is excreted.

STEPS IN UREA CYCLE

1.SYNTHESIS OF CARBAMOYL PHOSPHATE :

Formation of carbamoyl phosphate by carbamoyl phoshate 1 (CPS l) is driven by the cleavage of 2 molecules of ATP.

CPS l requires N-acetylglutamate as a positive allosteric activator.

NH3 and CO2 incorporated into carbamoyl phosphate, NH3 is provided primarily by the oxidative deamination of glutamate by mitochondrial glutamate dehydrogenase.

Nitrogen atom derived from this NH3 becomes one of the nitrogens of urea.

This step consumes 2 ATP, and rate-limiting.

2. FORMATION OF CITRULLINE

The carbamoyl portion of carbamoyl phosphate is transferred to ornithine by Ornithine transcarbamoylase (OTC) as the high-energy phosphate is released as Pi .

The reactions product, citrulline, is transported to the cytosol. –citrulline produced will transported to the cytosol by a cotransporter.

Ornithine is regenerated with each turn of the urea cycle.

3. SYNTHESIS OF ARGININOSUCCCINATE :

Argininosuccinate synthetase combines with citrulline with aspartate to form argininosuccinate.

The α-amino group of aspartate to the second nitrogen that is ultimately incorporated into urea.

The formation of argininosuccinate is driven by the cleavage of ATP to AMP & pyrophosphate (PPi).

This is the third & final molecule of ATP consumed in the formation of urea.

4. CLEAVAGE OF ARGININOSUCCINATE : Argininosuccinate cleaved by

argininosuccinate lyase to yield arginine and fumarate.

The arginine formed by this reaction serves as the immediate precursor of urea.

Fumarate produced in the urea cycle is hydrated to malate, -fumarate that librated here provides a connecting link with TCA cycle and gluconeogenesis.(see later)

5. FORMATION OF UREA :

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

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

Arginase is activated by Co2+ and Mn2+ Arginase is mostly found in the liver, while the

rest of the enzymes of urea cycle are also present in other tissues.

For this reason, Arginine synthesis may occur to varying degrees in many tissues, but only the liver can ultimately produce urea.

OVERALL REACTION IN UREA CYCLE

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. ATP → AMP + PPi = 2 ATP equivalents

Hence 4 ATP are actually consumed. Overall stoichiometry of the urea cycle : Aspartate + NH3 + CO2 + 3ATP + H2O → urea +

fumarate + 2ADP + AMP + 2Pi + PPi

REGULATION OF THE UREA CYCLE

N-Acetylglutamate is an essential activator for carbamoyl phosphate synthetase I—the rate-limiting step in the urea cycle.

N-Acetylglutamate is synthesized from acetyl coenzyme A and glutamate by N-acetylglutamate synthase, in a reaction for which arginine is an activator.

Therefore, the intrahepatic concentration of N-acetylglutamate increases after ingestion of a protein-rich meal, which provides both a substrate (glutamate) and the regulator of N-acetylglutamate synthesis.

This leads to an increased rate of urea synthesis.

SOURCES OF AMMONIA (FREE NH3) Glutamine- The kidneys generate ammonia from glutamine by

the actions of renal glutaminase and glutamate dehydrogenase. Ammonia is also obtained from the hydrolysis of glutamine by intestinal glutaminase either from the blood or from digestion of dietary protein.

Bacterial action in the intestine- Ammonia is formed from urea by the action of bacterial urease in the lumen of the intestine.

Amines- Amines obtained from the diet, and monoamines that serve as hormones or neurotransmitters, give rise to ammonia by the action of amine oxidase.

Purines and pyrimidines- In the catabolism of purines and pyrimidines, amino groups attached to the rings are released as ammonia.

INTERRELATION BETWEEN UREA CYCLE AND TCA CYCLE

Urea cycle is linked with TCA cycle in three different ways :

1.The production of fumarate in urea cycle is the most important integrating point with TCA cycle. Fumarate is converted to malate and then to OAA in TCA cycle. OAA undergoes transamination to produce aspartate which enters urea cycle. Here, aspartate combines with citrulline to produce arginosuccinate. OAA is an important metabolite which can combine with acetyl CoA to form citrate and get finally oxidized. OAA can also serve as a precursor for the synthesis of glucose (gluconeogenesis).

INTERRELATION BETWEEN UREA CYCLE AND TCA CYCLE

- 4 ATP

+10 ATP

CO2

CO2

H2O

INTERRELATION BETWEEN UREA CYCLE AND TCA CYCLE

2.ATP (10) are generated in the TCA cycle while ATP (4) are utilized for urea synthesis.

3.TCA cycle is an important metabolic pathway for the complete oxidation of various metabolites to CO2 and H2O. The CO2 liberated in TCA cycle (in the mitochondria) can be used in urea cycle.