Asparagine synthetase reaction

Figure 28-4. The asparagine synthetase reaction. Note similarities to and differences from the glutamine synthetase reaction (Figure 28-2).

In this case, one can easily measure the PPj-ATP exchange if one varies the concentration of Mg ATP at different fixed levels of acid, and with PP( held constant, there are four patterns that can be observed (Table 6). Thus by measuring the exchange of product P with its corresponding substrate, one can teU whether the pattern is random or ordered, and, if ordered, whether P picks up label from substrate A or B. This method was applied initially in asparagine synthetase reaction (Cedar Schwartz, 1969) and the corresponding equations were written in detail (Santi et al, 1974). 

A three-substrate, four-product enzyme reaction scheme in which the first two substrates bind in an ordered fashion followed by the release of the first product. The third substrate then binds and the remaining three products are released in order (thus, E + A EA is followed by EA + B (EAB FP) F -I- P and then F + C (EC EQRS) ERS + Q followed by ERS ES + R and ES E + S). There are indications that asparagine synthetase from certain sources may have this reaction scheme. 

Fig. 1.3 Reactions showing synthesis of glutamate in brain. Aspartate aminotransferase (1) glu-taminase (2) glutamate dehydrogenase (3) GABA aminotransferase (4) alanine aminotransferase (5) ornithine aminotransferase (6) Al-pyrroline 5-carboxylic acid dehydrogenase (7) and asparagine synthetase (8)...

Earlier preparations of L-asparagina.se contained endotoxins from E. coU. but the.se are absent in the purer new preparations. Clearance of the en /.yme from plasma is due to an immunological reaction in which it combines with protein. Ms reaction tnay lead to. sensiti /.iition in some patients. Patients who cannot tolerate L-aspuragina.se from E. coli fliiglit be treated by the preparation from Envinia caruto-inm. " Tumor resistance is based on the development of asparagine. synthetase by the tumor cells. ... 

Asparagine Synthetase A Similar Amidation Reaction (Diagram)... 

L-[amide-Asparagine has been prepared in radiochemical yields of 10-20%, in a reaction catalyzed by asparagine synthetase purified from extracts of rat Novikoff hepatoma (29) or from Escherichia coU (30). Neither enzyme preparation is presently available from commercial sources. 

Several aaRS-like proteins are involved in metabobc pathways (1). For example, E. coli asparagine synthase, an aspartyl-tRNA synthetase (AspRS)-like enzyme, catalyzes the synthesis of asparagine from aspartate and ATP. A paralog of LysRS-II, called PoxA/GenX, is important for pyruvate oxidase activity in E. coli and Salmonella typhimurium and for virulence in S. typhimurium. The E. coli biotin synthetase/repressor protein (BirA), which has a domain that resembles structurally the seryl-tRNA synthetase (SerRS) catalytic domain, activates biotin to modify posttranslationaUy various metabolic proteins involved in carboxylation and decarboxylation. BirA can also bind DNA and regulate its own transcription using biotin as a corepressor. A histidyl-tRNA synthetase (HisRS)-hke protein from Lactococcus lactis, HisZ is involved in the allosteric activation of the phosphoribosyl-transferase reaction. 

Glutamine (and probably asparagine) can be destroyed by two independent routes. The enzyme catalysing direct deamidation to glutamate is activated by phosphate, suggesting a mechanism related to that of glutamine synthetase. The second path involves transamination (equation 30) and subsequent deamidation (equation 31) of a-ketoglutaramic acid. The two reactions are catalysed by separate... 

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