Carbamoyl-phosphate synthase

Carbamoyl Phosphate Synthase I Initiates Urea Biosynthesis... 

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... 

Carbamoyl Phosphate Synthase I Is the Pacemaker Enzyme of the Urea Cycle... 

The activity of carbamoyl phosphate synthase I is determined by A -acetylglutamate, whose steady-state level is dictated by its rate of synthesis from acetyl-CoA and glutamate and its rate of hydrolysis to acetate and glutamate. These reactions are catalyzed by A -acetylglu-tamate synthase and A -acetylglutamate hydrolase, respectively. Major changes in diet can increase the concentrations of individual urea cycle enzymes 10-fold to 20-fold. Starvation, for example, elevates enzyme levels, presumably to cope with the increased production... 

Hyperammonemia Type 1. A consequence of carbamoyl phosphate synthase I deficiency (reaction 1, Figure 29-9), this relatively infrequent condition (estimated frequency 1 62,000) probably is familial. 

Changes in enzyme levels and allosteric regulation of carbamoyl phosphate synthase by A -acetylglutamate regulate urea biosynthesis. 

Figure 34-7 summarizes the roles of the intermediates and enzymes of pyrimidine nucleotide biosynthesis. The catalyst for the initial reaction is cytosolic carbamoyl phosphate synthase II, a different enzyme from the mitochondrial carbamoyl phosphate synthase I of urea synthesis (Figure 29-9). Compartmentation thus provides two independent pools of carbamoyl phosphate. PRPP, an early participant in purine nucleotide synthesis (Figure 34-2), is a much later participant in pyrimidine biosynthesis.

Some of the results obtained by differential centrifugation showed enzyme distribution between different cell fractions which were difficult to interpret. Enzymes like carbamoyl phosphate synthase or isocitrate dehydrogenase were found both in mitochondria and in the soluble fraction of the cell. This led to detailed kinetic studies with purified enzymes which indicated there might be populations of enzymes with slightly different properties (isozymes) catalyzing similar reactions in different compartments or in different cell types. Variations in kinetic behavior appeared to tailor the enzyme appropriately to the particular compartment or cell where the reaction took place. 

The enzyme carbamoyl phosphate synthase (CPS) is a control point in the process. Stage 3. The urea cycle (Figure 6.7)... 

Carbonic anhydrase (CA, also called carbonate dehydratase) is an enzyme found in most human tissues. As well as its renal role in regulating pH homeostasis (described below) CA is required in other tissues to generate bicarbonate needed as a co-substrate for carboxylase enzymes, for example pyruvate carboxylase and acetyl-CoA carboxylase, and some synthase enzymes such as carbamoyl phosphate synthases I and II. At least 12 isoenzymes of CA (CA I—XII) have been identified with molecular masses varying between 29 000 and 58 000 some isoenzymes are found free in the cytosol, others are membrane-bound and two are mitochondrial. 

The rate of urea formation is mainly controlled by reaction [1]. N-acetyl glutamate, as an allosteric effector, activates carbamoyl-phosphate synthase. In turn, the concentration of acetyl glutamate depends on arginine and ATP levels, as well as other factors. 

The regulation of bacterial aspartate carbamoyltransferase by ATP and CTP has been particularly well studied, and is discussed on p. 116. In animals, in contrast to prokaryotes, it is not ACTase but carbamoyl-phosphate synthase that is the key enzyme in pyrimidine synthesis. It is activated by ATP and PRPP and inhibited by UTP. 

ATP synthase beta chain Carbamoyl-phosphate synthase... 

Table 2.1.9 Changes of blood amino acids in various primary inherited defects and as a result of secondary changes. ASA Argininosuccinic acid, CPS carbamoyl phosphate synthase, LPI Lysinuric protein intolerance, MAD multiple acyl-CoA dehydrogenation, MSUD maple syrup urine disease, NAGS N-acetylglutamate synthase, NKH nonketotic hyperglycinemia, NTBC 2-(2-nitro-4-3 trifluoro-methylbenzoyl)-1,3-cyclohexanedione, OCT Ornithine carbamoyltransferase,...

Rubio, V. Cervera, J. The carbamoyl-phosphate synthase family and carbamate kinase structure-function studies. Biochem. Soc. Trans., 23, 879-883 (1995)... 

The mechanism of formation of carbamoyl phosphate. The reaction involves three steps, all of which take place on the same enzyme, carbamoyl phosphate synthase. 

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. 

The pathway for UMP synthesis is shown in figure 23.13. It starts with the synthesis of carbamoyl phosphate, catalyzed by carbamoyl phosphate synthase. This enzyme is present in microorganisms and in the cytosol of all eukaryotic cells... 

Low activities of orotidine phosphate decarboxylase and (usually) orotate phosphoribosyltransferase are associated with a genetic disease in children that is characterized by abnormal growth, megaloblastic anemia, and the excretion of large amounts of orotate. When affected children are fed a pyrimidine nucleoside, usually uridine, the anemia decreases and the excretion of orotate diminishes. A likely explanation for the improvement is that the ingested uridine is phosphorylated to UMP, which is then converted to other pyrimidine nucleotides so that nucleic acid and protein synthesis can resume. In addition, the increased intracellular concentrations of pyrimidine nucleotides inhibit carbamoyl phosphate synthase, the first enzyme in the. naibwav of aro-tate synthesis. 

In eukaryotes, carbamoyl phosphate synthase is inhibited by pyrimidine nucleotides and stimulated by purine nucleotides it appears to be the most important site of feedback inhibition of pyrimidine nucleotide biosynthesis in mammalian tissues. It has been suggested that under some conditions, orotate phosphoribosyltransferase may be a regulatory site as well. 

How do you explain the observation that pyrimidine biosynthesis in bacteria is regulated at the level of aspartate carbamoyltransferase, whereas most of the regulation in humans is at the level of carbamoyl phosphate synthase ... 

Carbamoyl phosphate synthase contributes to two processes (a) the initial enzyme in the biosynthesis of pyrimidines and (b) a component in the synthesis of arginine biosynthesis or the urea cycle. In bacteria both of these processes occur within the same compartment. In human beings the carbamoyl phosphate synthase involved in the urea cycle is contained in... 

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