Urea cycle carbamoyl phosphate formation

Carbamoyl phosphate synthetases. The first of the individual steps in the urea cycle is the formation of carbamoyl phosphate.163 Carbon dioxide and ammonia equilibrate spontaneously with carbamic acid ... 

As we see, the energy for the synthesis of one molecule of carbamoyl phosphate is derived from two molecules of ATP. Figure 14.9 summarizes the reactions of the urea cycle. The net reaction for carbamoyl phosphate formation and the urea cycle is... 

Condensation of CO2, ammonia, and ATP to form carbamoyl phosphate is catalyzed by mitochondrial carbamoyl phosphate synthase I (reaction 1, Figure 29-9). A cytosolic form of this enzyme, carbamoyl phosphate synthase II, uses glutamine rather than ammonia as the nitrogen donor and functions in pyrimidine biosynthesis (see Chapter 34). Carbamoyl phosphate synthase I, the rate-hmiting enzyme of the urea cycle, is active only in the presence of its allosteric activator JV-acetylglutamate, which enhances the affinity of the synthase for ATP. Formation of carbamoyl phosphate requires 2 mol of ATP, one of which serves as a phosphate donor. Conversion of the second ATP to AMP and pyrophosphate, coupled to the hydrolysis of pyrophosphate to orthophosphate, provides the driving... 

The activity of the urea cycle is regulated at the level of enzyme synthesis and by allosteric regulation of the enzyme that catalyzes the formation of carbamoyl phosphate. 

Formation of citrulline Ornithine and citrulline are basic amino acids that participate in the urea cycle. [Note They are not incorporated into cellular proteins, because there are no codons for these amino acids (see p. 429).] Ornithine is regenerated with each turn of the urea cycle, much in the same way that oxaloacetate is regenerated by the reactions of the citric acid cycle (see p 109). The release of the high-energy phosphateof carbamoyl phosphate as inorganic phosphate drives the reaction in the forward direction. The reaction product, citrulline, is trans ported to the cytosol. 

CPSI catalyzes the formation of carbamoyl phosphate from bicarbonate, ammonium, and two adenosine triphosphate molecules (Fig. 18-1).This first step of the urea cycle occurs in the mitochondrial matrix and assimilates the first of the two nitrogen atoms that will eventually be found in urea. While two ATP molecules are hydrolyzed, there is formation of a lower energy bond in carbamoyl phosphate. CPSI is a homodimer that accounts for 15-30% of the total protein mass in liver mitochondria. jV-Acetylglutamate (NAG) is an essential allosteric activator of CPSI activity, and magnesium ions are also required for its activity. 

Urea is synthesized in the liver by a series of reactions known as the urea cycle (Fig. 15-13). One nitrogen is derived from ammonium, the second from aspartate and the carbon is derived from C02. The synthesis of urea requires the formation of carbamoyl phosphate and the four enzymatic reactions of the urea cycle. Some of the reactions take place in the mitochondria and some in the cytoplasm. The enzymes involved in the synthesis of urea are discussed below. 

Ornithine and citrulline are amino acids, but they are not used as building blocks of proteins. The formation of NH4 + by glutamate dehydrogenase, its incorporation into carbamoyl phosphate, and the subsequent synthesis of citrulline take place in the mitochondrial matrix. In contrast, the next three reactions of the urea cycle, which lead to the formation of urea, take place in the cytosol. 

In eukaryotic cells, two separate pools of carbamoyl phosphate are synthesized by different enzymes located at different sites. Carbamoyl phosphate synthetase I (CPS I) is located in the inner membrane of mitochondria in the liver and, to lesser extent, in the kidneys and small intestine. It supplies carbamoyl phosphate for the urea cycle. CPS 1 is specific for ammonia as nitrogen donor and requires N-acetylglutamate as activator. Carbamoyl phosphate synthetase II (CPS II) is present in the cytosol. It supplies carbamoyl phosphate for pyrimidine nucleotide biosynthesis and uses the amido group of glutamine as nitrogen donor. The presence of physically separated CPSs in eukaryotes probably reflects the need for independent regulation of pyrimidine biosynthesis and urea formation, despite the fact that both pathways require carbamoyl phosphate. In prokaryotes, one CPS serves both pathways. 

The Urea Cycle Begins with the Formation of Carbamoyl Phosphate... 

The answer is c. (Murray, pp 307—346. Scriver, pp 1909-1964. Sack, pp 121-138. Wilson, pp 287-317.) The steps of the urea cycle are divided between the mitochondrial matrix and cytosol of liver cells in mammals. The formation of ammonia, its reaction with carbon dioxide to produce carbamoyl phosphate, and the conversion to citrulline occur in the matrix of mitochondria. Citrulline diffuses out of the mitochondria, and the next three steps of the cycle, which result in the formation of urea, all take place in the cytosol. Peroxisomes have single membranes, in contrast to the double membranes of mitochondria. They house catalase and enzymes for medium- to long-chain fatty acid oxidation. 

Urea is synthesized in the liver in the urea cycle. The first step is formation of carbamoyl phosphate from ammonia, C02, and ATP. This is followed by a number of other steps, including formation of citrulline, argininosuccinate, and arginine, which is split to urea plus ornithine. The second nitrogen of urea is donated by aspartate in the formation of argininosuccinate. 

The urea cycle converts ammonium ions into urea, which is less toxic. The sources of the atoms are shown in color and the intracellular locations of the reactions are indicated. Citrulline, formed in the reaction between ornithine and carbamoyl phosphate, is transported out of the mitochondrion and into the cytoplasm. Ornithine, a substrate for the formation of citrulline, is transported from the cytoplasm into the mitochondrion. 

Figure 16.S illustrates the reactions and the com-partmentalization of the enzymes of the urea cycle. The first reaction in urea biosynthesis is the mitochondrial formation of carbamoyl phosphate, the substrate of the urea cycle. The reaction utilizes an ammonium (NH4 ) ion, delivered into the mitochondrion as glutamate by the action of both the glutamate-aspartate (Section 11.3) and the glutamate-hydroxyl ion antiport carriers. Oxidative deamination of glutamate by glutamate dehydrogenase releases an NH4 ion.

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