Glutamine amide transfer reactions

Table 5-III lists the glutamine amide transfer reactions of purine and pyrimidine biosynthesis de novo, and of purine and pyrimidine ribonucleotide interconversion, and several more that occur in other areas of metabolism. All have features in common 1, 3).

The possible function of this enzyme in the regulation of the pathway is discussed below. It is listed with glutamine amide transfer reactions in Table 5-III and with other phosphoiibo ltransferases in Table 5-IV. 

As would be expected for a glutamine amide transfer reaction (Chapter 5), the animal enzyme prefers glutamine as amino group donor, but can use ammonia to a lesser extent. 

The results of these [ N]N2 labeling experiments thus show that isolated heterocysts are capable of reducing N2 to and that the NH4 was assimilated into glutamine. That is, activity of glutamine synthetase is coupled to nitrogenase in these cells. It is possible that a faction of the [ N] glutamine was catabolized in heterocysts, but due to the relatively low amount of total assimilated we were unable to detect the products. We have consistently observed in cyanobacteria that approximately 10 -fold hi er levels of incorporation of can be attained with NU4 than with [ N]N2 as substrate. Therefore, to study metabolism of NH4 and glutamine further, we incubated isolated heterocysts with NH4 and other substrates of amidation and amide transfer reactions. 

Glutamine also supplies an amino function to start off purine nucleotide biosynthesis. This complex little reaction is again an Sn2 reaction on PRPP, but only an amino group from the amide of glutamine is transferred. The product of the enzymic reaction is thus 5-phosphoribosylamine. 

Asparagine, the amide of aspartate, is not formed directly from aspartate and NHj. Instead, the amido group of glutamine is transferred by amido group transfer during an ATP-requiring reaction catalyzed by asparagine synthase ... 

In addition to the transphosphorylation reactions discussed in Chapter 4, there are several general types of carbon and nitrogen transfer reactions which also occur in purine and pyrimidine nucleotide biosynthesis and interconversion. Among these are one-carbon and phosphoribosyl transfer reactions, amino group transfer from glutamine and aspartate, and amide syntheses. In most of these processes carbon-nitrogen bonds... 

Guanine formation occurs following oxidation in position 2, to form xanthylic acid." This oxidation has been demonstrated with a DPN-requiring enzyme from bone marrow. Subsequent animation occurs through different mechanisms in animal and bacterial systems. In bone marrow extracts the amide of glutamine is transferred, while in bacterial extracts ammonia is utilized more readily than glutamine. The utilization of ammonia by the bacterial system is accompanied by the hydrolysis of ATP to AMP and PP. In addition to these reactions, it is known that adenine and guanine can be interconverted, but the enzymatic mechanisms are not known. 

Ammonia and a number of (but not all) amino acids in high concentrations are inhibitory. The transfer of ammonia was proved by using N Hs in the medium and finding the labeled nitrogen, after incubation with the enzyme, in the amide radical.The enzyme extract used effected the transfer between asparagine or glutamine and ammonia or hydroxamic acid, but with no other amides. The reaction was not inhibited by cyanide, fluoride, or iodoacetate. Acetone powders prepared from extracts of mammalian tissues also were effective they required activation by Mn++, and their activity was enhanced further by phosphate they had no glutaminase activity. Addition of ATP or ADP was without effect but this was probably because the enzyme system was not sufficiently purified to show this dependence (see below). 

The first reaction in purine biosynthesis is the transfer of the amide from glutamine to PRPP with release of pyrophosphate. The product is phosphoribosylamine (PRA). 

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