Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:6.3.5.5 (CPS)
 1,262 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Three conserved histidine residues, His-243, His-781, and His-788, located within the large subunit of carbamoyl phosphate synthetase from Escherichia coli were identified by sequence identity comparisons. These three histidine residues were individually mutated to asparagine residues. The H243N mutant enzyme was found to be critical for carbamoyl phosphate synthesis as the mutant protein was unable to synthesize carbamoyl phosphate at a significant rate (< 1/1500). By analysis of the effects of this mutation on the partial reactions catalyzed by CPS, it was determined that this mutation blocked the formation of the carbamate intermediate from carboxyphosphate and ammonia. The H781N mutant enzyme had an order of magnitude reduction for both the rate of carbamoyl phosphate formation and ATP synthesis which is consistent with the proposal that the carboxyl-terminal half of the large subunit is primarily involved in the phosphorylation of the putative carbamate intermediate. This mutation also reduced the effects of the allosteric activator ornithine on the Km parameters for ATP in the overall biosynthetic reaction and ADP in the ATP synthesis reaction. The H788N mutant enzyme is a functional protein which maintains the ability to synthesize carbamoyl phosphate at a rate comparable to that of the wild-type enzyme. The effects of this mutation are 10-fold reductions of the ATP synthetase and the bicarbonate-dependent ATPase activities with substantial increases in the Km values for ATP in the full biosynthetic reaction and for ADP in the ATP synthesis reaction.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Differential roles for three conserved histidine residues within the large subunit of carbamoyl phosphate synthetase. 841 43

Carbamoyl phosphate synthetase from Escherichia coli catalyzes the production of carbamoyl phosphate from two molecules of Mg2+ATP, one molecule of bicarbonate, and one molecule of glutamine. The enzyme consists of two polypeptide chains referred to as the large and small subunits. While the large subunit provides the active sites responsible for the binding of nucleotides and other effector ligands, the small subunit contains those amino acid residues that catalyze the hydrolysis of glutamine to glutamate and ammonia. From both amino acid sequence analyses and structural studies it is now known that the small subunit belongs to the class I amidotransferase family of enzymes. Numerous biochemical studies have suggested that the reaction mechanism of the small subunit proceeds through the formation of the glutamyl thioester intermediate and that both Cys 269 and His 353 are critical for catalysis. Here we describe the X-ray crystallographic structure of carbamoyl phosphate synthetase from E. coli in which His 353 has been replaced with an asparagine residue. Crystals employed in the investigation were grown in the presence of glutamine, and the model has been refined to a crystallographic R-factor of 19.1% for all measured X-ray data from 30 to 1.8 A resolution. The active site of the small subunit clearly contains a covalently bound thioester intermediate at Cys 269, and indeed, this investigation provides the first direct structural observation of an enzyme intermediate in the amidotransferase family.
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PMID:Carbamoyl phosphate synthetase: caught in the act of glutamine hydrolysis. 963 22

Human carbamoyl-phosphate synthetase (hCPS) has evolved three features that allow it to remove excess, potentially neurotoxic ammonia via the urea cycle: inability to use glutamine as an alternative nitrogen donor; a K(m) for ammonia 100-fold lower than for CPSs that also use glutamine; and required allosteric activation by N-acetylglutamate (AGA), a sensor of excess amino acids. To determine the structural features of hCPS that allow its unique functioning, we have developed the first recombinant expression system for hCPS, utilizing Schizosaccharomyces pombe. Of several common single-nucleotide polymorphisms identified in the gene encoding hCPS, only the one resulting in substitution of threonine at position 1406 with asparagine has been linked to phenotypic effects. We have expressed and characterized both variants of hCPS. The asparagine polymorph, hCPS_N, consistently displayed inferior catalytic properties, but the K(m) and k(cat) values for overall and partial reactions varied only by a factor of 1.7 or less. We have designed and characterized an hCPS construction from which the N-terminal domain A is deleted. hCPS_DeltaA was competent to bind AGA, demonstrating that domain A does not contain the AGA binding site. Thus, the site at the C/D boundary previously identified by AGA analogue labelling appears to be the functionally significant initial binding site for AGA. However, hCPS_DeltaA was not able to fully assume the catalytically competent conformation, with specific activity of CP formation decreased 700-fold.
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PMID:Human carbamoyl-phosphate synthetase: insight into N-acetylglutamate interaction and the functional effects of a common single nucleotide polymorphism. 1867 23

This study was aimed at investigating the physiological role of ferredoxin-glutamate synthases (EC 1.4.1.7), NADH-glutamate synthase (EC 1.4.1.14) and carbamoylphosphate synthetase (EC 6.3.5.5) in Arabidopsis. Phenotypic analysis revealed a high level of photorespiratory ammonium, glutamine/glutamate and asparagine/aspartate in the GLU1 mutant lacking the major ferredoxin-glutamate synthase, indicating that excess photorespiratory ammonium was detoxified into amino acids for transport out of the veins. Consistent with these results, promoter analysis and in situ hybridization demonstrated that GLU1 and GLU2 were expressed in the mesophyll and phloem companion cell-sieve element complex. However, these phenotypic changes were not detected in the GLU2 mutant defective in the second ferredoxin-glutamate synthase gene. The impairment in primary ammonium assimilation in the GLT mutant under nonphotorespiratory high-CO(2) conditions underlined the importance of NADH-glutamate synthase for amino acid trafficking, given that this gene only accounted for 3% of total glutamate synthase activity. The excess ammonium from either endogenous photorespiration or the exogenous medium was shifted to arginine. The promoter analysis and slight effects on overall arginine synthesis in the T-DNA insertion mutant in the single carbamoylphosphate synthetase large subunit gene indicated that carbamoylphosphate synthetase located in the chloroplasts was not limiting for ammonium assimilation into arginine. The data provided evidence that ferredoxin-glutamate synthases, NADH-glutamate synthase and carbamoylphosphate synthetase play specific physiological roles in ammonium assimilation in the mesophyll and phloem for the synthesis and transport of glutamine, glutamate, arginine, and derived amino acids.
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PMID:Assimilation of excess ammonium into amino acids and nitrogen translocation in Arabidopsis thaliana--roles of glutamate synthases and carbamoylphosphate synthetase in leaves. 1955 10