Adenylosuccinate synthetase regulation

The enzymatic activity of amido phosphoribosyltransferase (P-Rib-PP— PR A) is low and flux through the de novo pathway in vivo is regulated by the end-products, AMP, IMP and GMP. Inhibition of reaction 1 by dihydrofolate polyglutamates would signal the unavailability of /V1()-formyl tetrahydrofolate, required as a substrate at reactions 3 and 9 of the pathway. The purine pathway is subject to further regulation at the branch point from IMP XMP is a potent inhibitor of IMP cyclohydrolase (FAICAR—> IMP), AMP inhibits adenylosuccinate synthetase (IMP—> sAMP) and GMP inhibits IMP dehydrogenase (IMP— XMP). 

AMP is a competitive inhibitor (see "Enzymes Catalysis and Kinetics" Lecture) of Adenylosuccinate Synthetase, GMP competitively inhibits IMP Dehydrogenase. Note GTP is required for AMP syndesis and ATP is required for GMP synthesis, hence there is coordinated regulation of these nucleotides. 

Feedback regulation of the de novo pathway of purine biosynthesis. Solid lines represent metabolic pathways, and broken lines represent sites of feedback regulation. , Stimulatory effect , inhibitory effect. Regulatory enzymes A, PRPP synthetase B, amidophosphoribosyltransferase C, adenylosuccinate synthetase D, IMP dehydrogenase. 

The enzymes that convert IMP to XMP and adenylosuccinate are both regulated. GMP inhibits the activity of IMP dehydrogenase, and AMP inhibits adenylosuccinate synthetase. Note that the synthesis of AMP is dependent on GTP (of which GMP is a precursor), whereas the synthesis of GMP is dependent on ATP (which is made from AMP). This serves as a type of positive regulatory mechanism to balance the pools of these precursors when the levels of ATP are high, GMP will be... 

Fig. 41.9. The regulation of purine synthesis. PRPP synthetase has two distinct allosteric sites, one for ADP, the other for GDP. Glutamine phosphoribosyl amidotransferase contains adenine nucleotide and guanine nucleotide binding sites the monophosphates are the most important, although the di- and tri-phosphates will also bind to and inhibit the enzyme. Adenylosuccinate synthetase is inhibited by AMP IMP dehydrogenase is inhibited by GMP.

Regulation, Genetics, and Properties of Adenylosuccinate Synthetase A Review... 

Since the two isozymes of adenylosuccinate synthetase differ so markedly, changes in the relative amounts of the two could drastically affect the regulation of the reaction they catalyze, and therefore the direction of purine nucleotide metabolism. Determination of this ratio could be a useful indicator of the relative importance of the biosynthetic and the cyclic aspects of the adenine nucleotide interconversion pathway in different tissues or under different metabolic conditions. 

IX. Regulation and Metabolic Roles of Eukaryotic Adenylosuccinate Synthetase... 

The regulation of mammalian adenylosuccinate synthetase is complicated. It is dependent on the isozyme content and levels in a given tissue as well as the effects of substrate and product levels. The two isozymes may have different metabolic roles either in AMP biosynthesis and interconversion, or in the functions of the purine nucleotide cycle. Most studies have considered kinetic parameters for the isolated enzyme and in only a few instances has regulation been studied in vivo. Sufficient information is available concerning the regulation of the basic isozyme in muscle to consider that enzyme in detail. Factors controlling the acidic isozyme are less clearly defined. 

A. Regulation of Adenylosuccinate Synthetase with Respect to AMP Synthesis and Interconversion... 

Matsuda et al. (27) showed that the adenylosuccinate synthetase basic isozyme has a lower Km for aspartate, is more sensitive to inhibition by fructose 1,6-bisphosphate, and less sensitive to inhibition by nucleotides than the acidic isozyme. These properties could indicate that the basic isozyme is regulated coordinately with glycolysis (or gluconeogenesis) as proposed for the operation of the purine nucleotide cycle in skeletal muscle. The enzyme could also be affected by the availability of aspartate, as was found in Ehrlich ascites cells. The increase in basic isozyme activity, under conditions used in this study where the animal must rely on protein for most of its energy, is consistent with the idea that it is involved in the purine nucleotide cycle. This probably is not as an alternative to glutamate dehydrogenase in urea synthesis but is simply in amino acid catabolism. The small... 

A. The Role of S. cereWs/ee Adenylosuccinate Synthetase in the Regulation of de Novo Purine Biosynthesis... 

To date, no biochemical evidence exists to support this notion of a bifunctional role for adenylosuccinate synthetase. In particular, the synthesis of specific enzymes has not been correlated with the loss of regulation. Instead, the loss of regulation has been followed by measuring purine excretion or by observing adenine-insensitive accumulation and polymerization of aminoimidazole ribotide (red color formation) in cells blocked at adel or add2. In addition, adenylosuccinate synthetase from S. cerevisiae has not been purified or characterized and the... 

The dual role of adenylosuccinate synthetase in purine nucleotide interconversions and in de novo AMP biosynthesis complicates studies of its regulation. There is evidence suggesting that in wild-type cells, both the de novo and the salvage pathways play an active role in the maintenance of appropriate ATP/GTP ratios (78). 

GMP inhibited both adenine and guanine nucleotide synthesis (Table II compare assays (ii) and (v) (iii) and (vi) (vii) and (ix)). The branch point regulation i.e. inhibition of adenylosuccinate synthetase (EC 6.3.4.4) and IMP dehydrogenase (EC 1.2.1.13) may be more apparent than real due to the extremely low rate of IMP synthesis secondary to proximal pathway inhibition. GTP apparently also inhibited IMP dehydrogenase as the synthesis of guanine nucleotides was significantly decreased in the assay for simultaneous adenine and guanine nucleotide synthesis (Table II compare assays (iii) and (vii)). 

Fonne-Pfister, R. et al. (1996) The mode of action and the structure of a herbicide in complex with its target binding of activated hydantocidin to the feedback regulation site of adenylosuccinate synthetase. Proc. Natl Acad. Sci. USA 93, 9431-9436... 

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