Purine glutamine

Antimetabolites may be further classified as inhibitors of pyrimidine, purine, or glutamine metaboHsm. The compounds are cell cycle dependent. 

One example of a naturally occurring diazirine, duazomycin A (137 Scheme 11.20), has been reported, isolated in 1985 from a Streptomyces species during a screen for herbicidal compounds [196], It was fotind to inhibit de novo starch synthesis and it was suggested that this is due to direct inhibition of protein synthesis. Duazomycin A is structurally related to 6-diazo-5-oxo-L-norleucine (138), also reported as a natural product from Streptomyces [197], which acts as a glutamine antagonist and inhibits purine biosynthesis [198],... 

Glutamine as an important source for the synthesis of purines, pyrimidines and amino sugars is essential for most cell lines, too. High concentrations of glutamine may also effect cell growth indirectly as the major end-products are lactate and ammonia [15]. Both known to be toxic metabohtes. 

Antifolate Drugs or Glutamine Analogs Block Purine Nucleotide Biosynthesis... 

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

Several reactions of IMP biosynthesis require folate derivatives and glutamine. Consequently, antifolate drugs and glutamine analogs inhibit purine biosynthesis. 

Such enzymes catalyse the condensation of specific compounds, accompanied by the breakdown of a pyrophosphate bond in adenosine triphosphate (10.64). Adenosine is the condensation product of a pentose (D-ribofuranose) and a purine (adenine). Scheme 10.15 shows the action of glutamine synthetase on a mixture of L-glutamic acid (10.65) and... 

The amino acids glycine, aspartate, and glutamine are used in purine synthesis. 

Glutamine Purine and pyrimidine nucleotides, amino sugars... 

Tumour cells also require glutamine as a fuel for energy generation and as a precursor for the synthesis of purine and pyrimidine nucleotides for DNA and RNA synthesis. The roles and importance of glutamine in tumour cells and possible competition between the cells for glutamine are discussed in Chapter 21. The pathway for the metabolism of glutamine is similar to that in the immune cells. 

Figure 17.38 A simple diagram illustrating the two roles of glutamine (i) generation of ATP, via glutaminolysis, (ii) formation of purine and pyrimidine nucleotides, for the synthesis of nucleic acids in proliferating cells (Chapter 20). ( represents the carbon atoms of glutamine, one of which is released as CO2 and the others are converted to aspartate, via part of the Krebs cycle (Chapter 9). (N) represents the amide nitrogen of glutamine.

Figure 20.10 The positions in the pathway for de novo purine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. The pathway begins with glucose which provides ribose 5-phosphate, via the pentose phosphate pathway (Chapter 6). Glutamine provides its amide nitrogen in two reactions formation of 5-phosphoribosylamine and formation of guanosine monophosphate (GMP) from xantho-sine 5-phosphate (XMP).

Orotic acid in the diet (usually at a concentration of 1 per cent) can induce a deficiency of adenine and pyridine nucleotides in rat liver (but not in mouse or chick liver). The consequence is to inhibit secretion of lipoprotein into the blood, followed by the depression of plasma lipids, then in the accumulation of triglycerides and cholesterol in the liver (fatty liver) [141 — 161], This effect is not prevented by folic acid, vitamin B12, choline, methionine or inositol [141, 144], but can be prevented or rapidly reversed by the addition of a small amount of adenine to the diets [146, 147, 149, 152, 162]. The action of orotic acid can also be inhibited by calcium lactate in combination with lactose [163]. It was originally believed that the adenine deficiency produced by orotic acid was caused by an inhibition of the reaction of PRPP with glutamine in the de novo purine synthesis, since large amounts of PRPP are utilized for the conversion of orotic acid to uridine-5 -phosphate. However, incorporation studies of glycine-1- C in livers of orotic acid-fed rats revealed that the inhibition is caused rather by a depletion of the PRPP available for reaction with glutamine than by an effect on the condensation itself [160]. 

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. 

The synthesis of the purine ring is more complex. The only major component is glycine, which donates C-4 and C-5, as well as N-7. All of the other atoms in the ring are incorporated individually. C-6 comes from HCOa . Amide groups from glutamine provide the atoms N-3 and N-9. The amino group donor for the inclusion of N-1 is aspartate, which is converted into fumarate in the process, in the same way as in the urea cycle (see p. 182). Finally, the carbon atoms C-2 and C-8 are derived from formyl groups in N °-formyl-tetrahydrofolate (see p. 108). 

Figure 10-1. Overview of purine synthesis. Details of the first two reactions and sources of the atoms of the purine ring in inosine 5 -monophosphate (IMP) are shown. PRPP, 5 -phosphoribosyl-1-pyrophosphate Gin, glutamine Gly, glycine Asp, aspartate THF, tetrahydrofolate.

FIGURE 22-20 Biosynthesis of histidine in bacteria and plants. Atoms derived from PRPP and ATP are shaded red and blue, respectively. Two of the histidine nitrogens are derived from glutamine and glutamate (green). Note that the derivative of ATP remaining after step (AICAR) is an intermediate in purine biosynthesis (see Fig. 22-33, step ), so ATP is rapidly regenerated. 

Phosphoribosyl pyrophosphate (PRPP) is important in both, and in these pathways the structure of ribose is retained in the product nucleotide, in contrast to its fate in the tryptophan and histidine biosynthetic pathways discussed earlier. An amino acid is an important precursor in each type of pathway glycine for purines and aspartate for pyrimidines. Glutamine again is the most important source of amino groups—in five different steps in the de novo pathways. Aspartate is also used as the source of an amino group in the purine pathways, in two steps. 

In the first committed step of the pathway an amino group donated by glutamine is attached at C-l of PRPP (Fig. 22-33). The resulting 5-phosphoribosylamine is highly unstable, with a half-life of 30 seconds at pH 7.5. The purine ring is subsequently built up on this structure. The pathway described here is identical in all organisms, with the exception of one step that differs in higher eukaryotes as noted below. 

Three major feedback mechanisms cooperate in regulating the overall rate of de novo purine nucleotide synthesis and the relative rates of formation of the two end products, adenylate and guanylate (Fig. 22-35). The first mechanism is exerted on the first reaction that is unique to purine synthesis—transfer of an amino group to PRPP to form 5-phosphoribosylamine. This reaction is catalyzed by the allosteric enzyme glutamine-PRPP amidotransferase, which is inhibited by the end products IMP, AMP, and GMP. AMP and GMP act synergisti-cally in this concerted inhibition. Thus, whenever either AMP or GMP accumulates to excess, the first step in its biosynthesis from PRPP is partially inhibited. 

The purine ring system is built up step-by-step beginning with 5-phosphoribosylamine. The amino acids glutamine, glycine, and aspartate... 

The atoms of the purine ring are contributed by a number of compounds, including amino acids (aspartic acid, glycine, and glutamine), CO2, and N10-formyltetrahydrofolate (Figure 22.5). The purine ring is constructed by a series of reactions that add the donated carbons and nitrogens to a preformed ribose 5-phosphate. (See p. 145 for a discussion of ribose 5-phosphate synthesis by the HMP pathway.)... 

The atoms of a purine are contributed by amino acids (aspartic acid, glutamine, and glycine), CO2, and N10-formyl tetrahydrofolic acid. 

Synthesis of 5 phosphoribosylamine from PRPP and glutamine is catalized by glutamine phosphoribosyl pyrophosphate amidotransferase. This enzyme is inhibited by the purine 5 -nucleotides, AMP, GMP, and IMP—the end-products of the pathway. This is the committed step in purine nucleotide biosynthesis. 

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