Purine feedback control

The first step of this sequence, which is not unique to de novo purine nucleotide biosynthesis, is the synthesis of 5-phosphoribosylpyrophosphate (PRPP) from ribose-5-phosphate and adenosine triphosphate. Phosphoribosyl-pyrophosphate synthetase, the enzyme that catalyses this reaction [278], is under feedback control by adenosine triphosphate [279]. Cordycepin interferes with thede novo pathway [229, 280, 281), and cordycepin triphosphate inhibits the synthesis of PRPP in extracts from Ehrlich ascites tumour cells [282]. Formycin [283], probably as the triphosphate, 9-0-D-xylofuranosyladenine [157] triphosphate, and decoyinine (LXXlll) [284-286] (p. 89) also inhibit the synthesis of PRPP in tumour cells, and this is held to be the blockade most important to their cytotoxic action. It has been suggested but not established that tubercidin (triphosphate) may also be an inhibitor of this reaction [193]. 

That the model is potentially realistic was demonstrated by a recent isolation of a cultured cell line that overproduces purines and excretes large amounts of them into the culture medium. The cell was further shown to have an altered PRibPP synthetase that overproduced PRibPP. The mutant enzyme has normal catalytic properties but was not subject to feedback control by AMP, ADP, or ATP (G5). 

Wingaarden, J.B. Ashton, D.H. (1959). The regulation of activity of phosphoribosylpyrophosphate amidotransferase by purine ribonucleotides a potential feedback control of purine biosynthesis. ]. Biol. Chem., 234, 1492-6. 

In mammalian systems, the initial evidence for feedback control was obtained in in vivo systems by showing that purines were effective inhibitors of the accumulation of an early intermediate, formyl-GAR [51-54]. A purified enzyme preparation has been obtained from cell culture of a mouse tumor [55]. It shows a sigmoidal saturation curve with PRPP (n = 1.9). All nucleotides are inhibitory, the most potent being dGDP. The inhibitory patterns were altered by Mg, diphosphate and triphosphate nucleotides (except GTP) becoming less inhibitory with increase in Mg concentration. 

VIII. Feedback Control of Purine and Pyrimidine Biosynthesis. 443... 

The mdstence of a feedback control mechanism for purine biosynthesis was suggested by the observation that unlabeled purines inhibited purine theris de novo from labeled precursors in several systems (SB, 1B4,445-447). Initially, it was postulated that the feedback inhibition by preformed purines may have involved a shuntii of PRPP from the de novo route of purine biosyntheris and utilization of PRPP for nucleotide formation from the purine base and PRPP. PRPP was an obligatory intermediate in the synthesis of purines de novo. Since the nucleotide was still formed albeit by a different route, this would not be considered a feedback mechanism. It is probable, however, that the accumulation of the newly-formed nucleotide from PRPP and base inhibited the de novo route. 

Servomechanisms play important roles in the regulation of several other pathways of mammalian metabolism, including the synthesis of purines (Wyngaarden and Kelley, 1978), pyrimidines (Levine et al., 1974), porphyrins (Meyer and Schmid, 1978), and sialic acids (Komfeld et al., 1964). A detailed discussion of each instance is beyond the scope of this chapter. Some examples of feedback control are cited in Table 1. 

Purine and pyrimidine biosynthesis parallel one another mole for mole, suggesting coordinated control of their biosynthesis. Several sites of cross-regulation characterize purine and pyrimidine nucleotide biosynthesis. The PRPP synthase reaction (reaction 1, Figure 34-2), which forms a precursor essential for both processes, is feedback-inhibited by both purine and pyrimidine nucleotides. 

Synthesis of purines is under complex control.273 Some of the mechanisms found in bacteria are outlined in Fig. 25-16. Both feedback inhibition and activation are involved. Very important is the fact that GTP is needed in the synthesis of ATP, and that ATP is needed for synthesis of GTP. This kind of control ensures that an excess of either nucleotide will not be formed for long. In bacteria all of the final end product nucleotides inhibit the initial reaction of step a in Fig. 25-15. 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

Carbamyl-L-aspartate is the key precursor in the biosynthesis of pyrimidines. The enzyme aspartate transcarbamylase is inhibited by several pyrimidine nucleotides, notably cytidine triphosphate, and is activated by ATP, a purine nucleotide. Thus the enzyme is under feedback regulation, and controls the relative concentration of pyrimidine and purine nucleotides. 

The rates of these two complementary reactions can control the amount of either AMP or GMP present in the cell. Each of these reactions is feedback-inhibited by its nucleotide product. Thus, if more adenosine nucleotides exist than guanosine nucleotides, the synthesis of AMP slows down until the purine nucleotides balance. 

Pyrimidine synthesis is controlled at the first committed step. ATP stimulates the aspartate transcarbamoylase reaction, while CTP inhibits it. CTP is a feedback inhibitor of the pathway, and ATP is a feed-forward activator. This regulation ensures that a balanced supply of purines and pyrimidines exists for RNA and synthesis. 

A serious genetic disorder is associated with the salvage pathways, the Lesch-Nyhan syndrome. It is believed that it is caused by a failure to control the de novo purine biosynthetic pathway. In the Lesch-Nyhan syndrome, the enzyme HGPRTase is severely depressed. Because the de novo pathway is controlled largely via feedback effects of purine nucleotides, the pathway is derepressed and excessive quantities of purine nucleotides and their degradation product, uric acid, are accumulated. This results is neurologic effects, self-mutilation, and mental retardation. 

The synthesis of purine nucleotides is controlled by feedback inhibition at several sites (Figure 25.16). 

Figure 25.16. Control of Purine Biosynthesis. Feedback inhibition controls both the overall rate of purine biosynthesis and the balance between AMP and GMP production.

On the other hand, many of the purine nucleoside analogues act as feedback inhibitors of de novo purine synthesis, but only after conversion to their 5 -phosphates. The reaction under regulatory control is the first committed... 

The Synthesis of Purine Nucleotides Is Controlled by Feedback Inhibition at Several Sites... 

The answer is c. (Ivlurray, pp 375— /O I. Scrivt i, pp 2513—2570. Sack, pp 121—138. Wilson, pp 287—320.1 Several control sites exist in the path of purine synthesis where feedback inhibition occurs, AMP, GMP, or IMP may inhibit the first step of the pathway, which is the synthesis ol 5-phosphoribosyl-l-pyrophosphate (PRPP). PRPP synthetase is specifically inhibited. All three nucleotides can inhibit glutamine PRPP aminotranslerase, which catalyzes the second step of the. pathway. AMP blocks the conversion ol IMP to adenylosuccinate. GMP inhibits the lormation ol xanthylate Irom IMP Thus, blockage rather than enhancement ol IMP metabolism to AMP and GMP effectively inhibits purine biosynthesis. 

Some negative regulators act in the opposite way. The free enzyme shows Michaelis-Menten kinetics and the regulator shifts it to positive cooperativity so that the [S]0.5 is too high for much activity. An example of this behavior is given by amidophosphoribosyltransferase. This enzyme is the initial step in purine synthesis in which control by negative feedback occurs. The... 

Pathways that use nitrogen to make amino acids, purines, and pyrimidines are controlled by feedback inhibition. The final product, such as CTP, inhibits the first or an early step in its synthesis. 

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