Argininosuccinate synthase

Argininosuccinate synthase links aspartate and citrulline via the amino group of aspartate (reaction 3, Figure 29-9) and provides the second nitrogen of urea. The reaction requires ATP and involves intermediate formation of citrullyl-AMP. Subsequent displacement of AMP by aspartate then forms citrulline. 

The down-regulated proteins in HCC tissues have been identified. Park et al. identified aldehyde dehydrogenase 2 (25) and ferritin light chain (32). Kim et al. identified HSP 27, cathepsin D, and others (26). Lim et al. identified cytochrome B5, liver car-boxyesterase, and others (27). Li et al. identified SOD 1, aldolase B, and others (28). Fujii et al. identified galectin-1 (29). Kim et al. identified argininosuccinate synthase, carbamoyl-phosphate s mthase, and others (31). Table 1 shows the summary of the proteins whose expression was different between HCC cancer tissues and non-cancerous tissues. 

Arginine Proline Proline oxidase Ornithine-glutamate transaminase Ornithine transcarbamoylase Argininosuccinate synthase Argininosuccinate lyase... 

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. 

Formation of urea in hepatocytes. NAGS = N-acetylglutamate synthase CPSI = carbamoylphosphate synthase I OCT = ornithine carbamoyltransferase C-OT = citrulline-omithine translocase AS = argininosuccinate synthase AL = argininosuccinate lyase A = arginase. — -> indicates the absolute requirement of N-acetylglutamate for CPSI activity. 

The condensation of citrulline and aspartate to argininosuccinate is catalyzed by argininosuccinate synthase in the cytosol and occurs in two steps. In the initial step, the ureido group is activated by ATP to form the enzyme-bound intermediate adenylylcitrulline. In the second step, nucleophilic attack of the amino group of aspartate displaces AMP and yields argininosuccinate. The overall reaction is shown below ... 

Step 3. Citrulline is transported into the cytoplasm and now condenses with aspartate to produce argininosuccinate. This reaction, which requires energy released by the hydrolysis of ATP, is catalyzed by the enzyme argininosuccinate synthase. 

In this reaction which is catalysed by argininosuccinate synthase the second amino group is introduced in the form of aspartate which reacts with the citrulline formed in Reaction 2 to give argininosuccinate. The reaction requires ATP, which is broken down into AMP and pyrophosphate which is subsequently hydrolysed to inorganic phosphate so that this reaction too is irreversible. 

In the first reaction of the cycle the carbamoyl group is transferred from carbamoyl phosphate to ornithine to yield citrulline. Neither of these amino acids are known to occur in proteins. The remainder of the cycle reactions are cytoplasmic so citrulline is transported by a specific uniport carrier across the inner mitochondrial membrane. In the cytosol, argininosuccinate is formed by a condensation reaction which produces a covalent linkage between the carbonyl carbon atom of citrulline and the amino group of aspartate. This reaction, catalysed by argininosuccinate synthase, is readily reversible but is driven forward by the irreversible... 

Small amounts of urea are secreted in sweat but most is excreted in the urine following filtration by the kidney glomeruli although significant quantities of urea may be passively reabsorbed with water by the proximal tubules on each occasion. Urea is the major nitrogenous constituent of urine when the diet contains normal quantities of protein. Where the diet contains low levels or lacks protein, the decline in urinary urea concentrations reflects the control exercised on carbamoyl-phosphate synthase levels. Defects in the function of carbamoyl-phosphate synthase, ornithine carbamoyltransferase, argininosuccinate synthase and arginase result in hyperammonaemia (elevated blood ammonia concentrations) with concomitant effects on the brain. 

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