Argininosuccinate, urea cycle

One step in the urea cycle for ridding the body of ammonia is the conversion of argininosuccinate to the amino acid arginine plus fumarate. Propose a mechanism for the reaction, and show the structure of arginine. 

As we noted in Chapter 16, the enzymes of many metabolic pathways are clustered (p. 605), with the product of one enzyme reaction being channeled directly to the next enzyme in the pathway. In the urea cycle, the mitochondrial and cytosolic enzymes appear to be clustered in this way. The citrulline transported out of the mitochondrion is not diluted into the general pool of metabolites in the cytosol but is passed directly to the active site of argininosuccinate synthetase. This channeling between enzymes continues for argininosuccinate, arginine, and ornithine. Only urea is released into the general cytosolic pool of metabolites. 

The interconnected cycles have been called the "Krebs bicycle." The pathways linking the citric acid and urea cycles are called the aspartate-argininosuccinate shunt these effectively link the fates of the amino groups and the carbon skeletons of amino acids. The interconnections are even more elaborate than the arrows suggest. For... 

If we consider the urea cycle in isolation, we see that the synthesis of one molecule of urea requires four high-energy phosphate groups (Fig. 18-10). Two ATP molecules are required to make carbamoyl phosphate, and one ATP to make argininosuccinate—the latter ATP undergoing a pyrophosphate cleavage to AMP and PPj, which is hydrolyzed to two Pj. The overall equation of the urea cycle is... 

Cleavage of argininosuccinate Argininosuccinate is cleaved 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, providing a link with sev eral metabolic pathways. For example, the malate can be trans ported into the mitochondria via the malate shuttle and reenter... 

The complete urea cycle as it occurs in the mammalian liver requires five enzymes Argininosuccinate synthase, arginase, and argininosuccinate lyase (which function in the cytosol), and ornithine transcarbamoylase, and carbamoyl phosphate synthase (which function in the mitochondria). Additional specific transport proteins are required for the mitochondrial uptake of L-ornithine, NH3, and HC03 and for the release of L-citrulline. 

In the urea cycle ammonia is first combined with C02 to form carbamoyl phosphate. This then combines with ornithine to form citrulline. Citrulline then condenses with aspartate, the source of the second nitrogen atom in urea, to form argininosuccinate. This compound is in turn split to arginine and fumarate, and the arginine then splits to form urea and regenerate ornithine The first two reactions take place in the mitochondria of liver cells, the remaining three in the cytosol. 

Fig. 1. The urea cycle. The enzymes involved in this cycle are (1) carbamoyl phosphate synthetase (2) ornithine transcarbamoylase (3) argininosuccinate synthetase (4) arginosuccinase and (5) arginase.

Argininosuccinate synthetase (the second enzyme of the urea cycle) and the remaining two... 

Figure 6.10 The urea cycle. The enzymes of the urea cycle include, 1 carbamoyl phosphate synthetase-I, 2 ornithine transcarbamoylase, 3 argininosuccinate synthetase, 4 argininosuc-cinase, 5 arginase.

What about the other enzymes in the urea cycle Ornithine transcarbamoylase is homologous to aspartate transcarbamoylase and the structures of their catalytic subunits are quite similar (Figure 23.18). Thus, two consecutive steps in the pyrimidine biosynthetic pathway were adapted for urea synthesis. The next step in the urea cycle is the addition of aspartate to citrulline to form argininosuccinate, and the subsequent step is the removal of fumarate. These two steps together accomplish the net addition of an amino group to citrulline to form arginine. Remarkably, these steps are analogous to two consecutive steps in the purine biosynthetic pathway (Section 25.2 3). 

The enzymes that catalyze these steps are homologous to argininosuccinate synthetase and argininosuccinase, respectively. Thus, four of the five enzymes in the urea cycle were adapted from enzymes taking part in nucleotide biosynthesis. The remaining enzyme, arginase, appears to be an ancient enzyme found in all domains of life. 

Figure 24-2 The urea cycle pathway. CPS I, Carbamyl phosphate synthetase I N-acetyigiutamate as positive allosteric effector OTC, ornithine transcarbamyiase MS, argininosuccinate synthetase Ai, argininosuccinate iyase AR, arginase ADP, adenosine diphosphate, ATf adenosine triphosphate, P, inorganic phosphate.

Fia. 2. Pathways of ammonia uptake, including urea cycle. CP, carbamyl phosphate ASA, argininosuccinate KG, ketoglutarate. 

Fig. 5. Diagrammatio representation of intracellular localization of urea cycle pathway showing diffusion pattern. CPS, carbamyl phosphate synthetase ASL, argininosuccinate lyase ASA, argininosucoinate.

Qualitative or semiquantitative screening by paper chromatography of the urine for amino acids is accomplished by the usual methods. In infants and children, as in adults, arginine and ornithine are normally present in the urine only in very small amounts, 1 mg or less being excreted in 24 hours (R9). Citrulline may be present in similar amounts, but it is usually absent in the urine. The other intermediate of the urea cycle, argininosuccinic acid, is also found in normal urine in amounts of up to 2 mg per day (P2) although it is not normally detectable in blood. 

The specific syndrome arising from a severe deficiency of ornithine transcarbamylase has been termed hyperammonemia (L2) (Fig. 9). Next to argininosuccinic aciduria, this in the most frequently reported of the enzymatic disturbances of the urea cycle eleven proved examples... 

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