Urea Cycle: Reactions

Urea   is the major disposal form of amino groups derived from amino acids and accounts for ~90% of the nitrogen-containing components of urine. One nitrogen of the urea molecule is supplied by free ammonia and the other nitrogen by aspartate. [Note: Glutamate is the immediate precursor of both ammonia (through oxidative deamination by GDH) and aspartate nitrogen (through transamination of oxaloacetate by AST).] The carbon and oxygen of urea are derived from CO₂ (as HCO₃⁻). Urea is produced by the liver and then is transported in the blood to the kidneys for excretion in the urine.

Reactions:

The first two reactions leading to the synthesis of urea occur in the mitochondrial matrix, whereas the remaining cycle enzymes are located in the cytosol (Fig. 1). [Note: Gluconeogenesis and heme synthesis also involve both the mitochondrial matrix and the cytosol.]

Figure 1: Reactions of the urea cycle. [Note: An antiporter transports citrulline and ornithine across the inner mitochondrial membrane.] ADP = adenosine diphosphate; AMP = adenosine monophosphate; PPi = pyrophosphate;  Pi = inorganic phosphate; NAD(H) = nicotinamide adenine dinucleotide; MD = malate dehydrogenase.

1. Carbamoyl phosphate formation: Formation of carbamoyl phosphate by carbamoyl phosphate synthetase I (CPS I) is driven by cleavage of two molecules of ATP. Ammonia incorporated into carbamoyl phosphate is provided primarily by the oxidative deamination of glutamate by mitochondrial GDH. Ultimately, the nitrogen atom derived from this ammonia becomes one of the nitrogens of urea. CPS I requires N-acetylglutamate (NAG) as a positive allosteric activator (see Fig. 1). [Note: Carbamoyl phosphate synthetase II participates in the biosynthesis of pyrimidines. It does not require NAG,  uses glutamine as the nitrogen source, and occurs in the cytosol.]

2. Citrulline formation: The carbamoyl portion of carbamoyl phosphate is transferred to ornithine by ornithine transcarbamylase (OTC) as the phosphate is released as inorganic phosphate. The reaction product,  citrulline, is transported to the cytosol. [Note: Ornithine and citrulline move across the inner mitochondrial membrane via an antiporter. These basic amino acids are not incorporated into cellular proteins because there are no codons for them.] Ornithine is regenerated with each turn of the urea cycle, much in the same way that oxaloacetate is regenerated by the reactions of the tricarboxylic acid (TCA) cycle.

3. Argininosuccinate formation: Argininosuccinate synthetase combines citrulline with aspartate to form argininosuccinate. The α-amino group of aspartate provides the second nitrogen that is ultimately incorporated into urea. The formation of argininosuccinate is driven by the cleavage of ATP to adenosine monophosphate and pyrophosphate. This is the third and final molecule of ATP consumed in the formation of urea.

4. Argininosuccinate cleavage: Argininosuccinate is cleaved by argininosuccinate lyase to yield arginine and fumarate. The arginine serves as the immediate precursor of urea. The fumarate is hydrated to malate, providing a link with several metabolic pathways. Malate can be oxidized by malate dehydrogenase to oxaloacetate, which can be transaminated to aspartate  and enter the urea cycle (see Fig. 1). Alternatively, malate can be transported into mitochondria via the malate–aspartate shuttle, reenter the TCA cycle, and get oxidized to oxaloacetate, which can be used for gluconeogenesis. [Note: Malate oxidation generates NADH for oxidative phosphorylation, thereby reducing the energy cost of the urea cycle.]

5. Arginine cleavage to ornithine and urea: Arginase-I hydrolyzes arginine to ornithine and urea and is virtually exclusive to the liver. Therefore,  only the liver can cleave arginine, thereby synthesizing urea, whereas other tissues, such as the kidney, can synthesize arginine from citrulline.  [Note: Arginase-II in kidneys controls arginine availability for nitric oxide synthesis.]

6. Fate of urea: Urea diffuses from the liver and is transported in the blood to the kidneys, where it is filtered and excreted in the urine. A portion of the urea diffuses from the blood into the intestine and is cleaved to CO2 and ammonia by bacterial urease. The ammonia is partly lost in the feces and is partly reabsorbed into the blood. In patients with kidney failure, plasma urea levels are elevated, promoting a greater transfer of urea from blood into the gut. The intestinal action of urease on this urea becomes a clinically important source of ammonia,  contributing to the hyperammonemia often seen in these patients. Oral administration of antibiotics reduces the number of intestinal bacteria responsible for this ammonia production.