GMP from IMP

Up until 2003, evidence for the presence of adenylosuccinate synthetase, which catalyses the condensation of IMP with aspartate to form adenylosuccinate and is the first committed step in the synthesis of AMP or GMP from IMP, has relied on the inhibitory action of hadacidin, an aspartate analogue (Webster et al., 1984a). But, in 2002, the dimeric... 

ANIMATED FIGURE 23.21 The synthesis of AMP and GMP from IMP. Sign in at www.thomsonedu.com/login to explore an animated version of this figure. 

Describe the synthesis of adenylate (AMP) and guanylate (GMP) from IMP. List the cofactors and intermediates of the reactions. 

In the most important degradative pathway for adenosine monophosphate (AMP), it is the nucleotide that deaminated, and inosine monophosphate (IMP) arises. In the same way as in GMP, the purine base hypoxanthine is released from IMP. A single enzyme, xanthine oxidase [3], then both converts hypoxanthine into xanthine and xanthine into uric acid. An 0X0 group is introduced into the substrate in each of these reaction steps. The oxo group is derived from molecular oxygen another reaction product is hydrogen peroxide (H2O2), which is toxic and has to be removed by peroxidases. 

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. 

IMP does not accumulate in the cell but is converted to AMP, GMP, and the corresponding diphosphates and triphosphates. The two steps of the pathway from IMP to AMP (fig. 23.11) are typical reactions by which the amino group from aspartate is introduced into a product. The 6-hydroxyl group of IMP (tautomeric with the 6-keto group) is first displaced by the amino of aspartate to give adenylosuccinate, and the latter is then cleaved nonhydrolytically by adenylosuccinate lyase to yield fumarate and AMP. In the condensation of aspartate with IMP, cleavage of GTP to GDP and phosphate provides energy to drive the reaction. 

In mammals specific enzymes for converting purine bases to nucleotides are present in many organs, and in heart muscle this may be the main source of purine nucleotides. The most important of these enzymes is hypoxanthine-guanine phosphoribosyltransferase, which catalyzes the formation of IMP from hypoxanthine and GMP from guanine ... 

The second important level of regulation of purine nu-cleotide synthesis is in the branch pathways from IMP to AMP and to GMP (see figs. 23.11 and 23.24). The first of the two reactions leading from IMP to AMP is the irreversible synthesis of adenylosuccinate. This requires GTP as a source of energy and is inhibited by AMP. Of the two reactions required to convert IMP to GMP, the first is irreversible and is inhibited by GMP, and the second requires ATP as a source of energy. Thus, two types of regulation occur at this level of purine nucleotide synthesis (1) a forward ... 

Synthesis is regulated at several other places as well. After IMP is made the pathway splits to build either AMP or GMP. Enzyme XII, which catalyzes the first step from IMP to AMP, is itself slowed down by excess amounts of AMP. Similarly, the catalysis of the first step from IMP to GMP is inhibited by excess GMP. (Unlike King Midas, the enzymes can tell when they have too much of a good thing.) Finally, Enzyme XII uses GTP as an energy pellet because, if a lot of GTP is around, more A nucleotides (AMR, ADP and ATP) are needed to keep the supply in balance. The final step in the synthesis of GMP uses ATP as an energy source for similar reasons. ... 

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). 

Figure 10.6 HPLC elution profiles of an incubation mixture made up of. 2 nmol of hypoxanthine/guanine phosphoribosyltransferase, SO fiM guanine (G), SO /xM hypo-xanthine (H), 100 fiM PRibPP, and 1 mM MgCl2 in potassium phosphate (pH 7.4). At time intervals of 0 to 5 minutes, aliquots of the mixture were injected onto the HPLC ion-exchange column and eluted. Inset Time-dependent utilization of H and G and formation of GMP and IMP as determined by the absorbance of each peak at 254 nm. (From Ali and Sloan, 1982.)...

The results of an experiment are shown in Figure 10.6. The formation of GMP and IMP from guanine and hypoxanthine, respectively, can be followed. With this method it was possible to track the initial rates of formation of IMP and GMP separately, the initial rates of both determined simultaneously, and the rate of PRibPP utilization with a fixed ratio of hypoxanthine and guanine. 

HGPRT Hypoxanthine-guanine phosphoribosyltransferase the enzyme that catalyzes the synthesis of inosine monophosphate (IMP) and guano-sine monophosphate (GMP) from hypoxanthine and guanine, respectively. It makes up part of the purine salvage pathway, a way of recycling purine bases back to the nucleotides. 

Multifunctional enzymes in vertebrate cells contain several of these actvities in the same molecule. Pathway from IMP to GMP and AMP ... 

The de novo pathway of purine synthesis is complex, consisting of 11 steps, and requiring 6 molecules of ATP for every purine synthesized. The precursors that donate components to produce purine nucleotides include glycine, ribose 5-phosphate, glutamine, aspartate, carbon dioxide, and N -formyl FH4 (Fig. 41.1). Purines are synthesized as ribonucleotides, with the initial purine synthesized being inosine monophosphate (IMP). Adenosine monophosphate (AMP) and guano sine monophosphate (GMP) are each derived from IMP in two-step reaction pathways. 

GMP is also synthesized from IMP in two steps (Fig. 41.8). In the first step, the hypoxanthine base is oxidized by IMP dehydrogenase to produce the base xanthine and the nucleotide xanthosine monophosphate (XMP). Glutamine then donates the amide nitrogen to XMP to form GMP in a reaction catalyzed by GMP synthetase. This second reaction requires energy, in the form of ATP. 

Recall How many high-energy phosphate bonds must be hydrolyzed in the pathway that produces GMP from guanine and PRPP by the PRPP salvage reaction, compared with the number of such bonds hydrolyzed in the pathway leading to IMP and then to GMP ... 

In practice, plots of kohs versus [Nu] were linear (20,21,23) as shown typically for the reaction of [Ru (hedtra)(H20] with GMP and IMP in Fig. 4 underlying the validity of the proposed scheme. The values of and k i determined from the slopes and intercepts of such plots, respectively, are summarized in Table I. 

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