Ribose l-phosphate

The first step in this pathway involves SN2 displacement by fluoride on S-adenosine-L-methionine (SAM) catalyzed by the newly discovered enzyme fluor-inase (905-910), which also can function as a chlorinase (912). Fluorinase has been isolated and characterized, and the gene has been cloned (916). Both 5 -fluoro-5 -deoxyadenosine (847) and 5 -fluoro-5 -deoxy-D-ribose-l-phosphate (848) have been identified as intermediates (905-908). Fluoroacetaldehyde (850) is the immediate precursor, presumably via fluororibulose-1-phosphate (849) (915), to both fluoroacetate and 4-fluorothreonine (837) (901). The requisite enzymes fluoroacetaldehyde dehydrogenase (902) and L-threonine transaldolase-PLP (903) have been isolated and purified. The steps from 848 to 850 remain to be established but are based on known biochemistry. The pronounced toxicity of fluoroacetic acid... 

Fluoro-5-deoxy-D-ribose-l-phosphate as an Intermediate in Fluorometabolite Biosynthesis in Streptomyces cattleya. Chem Commun 592... 

Free bases can be salvaged using either PRPP or ribose-l-phosphate. The products are nucleoside monophosphates in the former case and nucleosides... 

Nucleosides are formed by the reaction of a purine or pyrimidine base with ribose-l-phosphate, and in mammals, this provides the most effective salvage pathway for pyrimidines ... 

A close look at this reaction reveals that in the opposite direction, the reaction is of the phosphorolysis type. For this reason, the enzymes catalyzing the reaction with ribose-l-phosphate are called phosphorylases, and they also participate in nucleic acid degradation pathways. Purine nucleoside phosphorylases thus convert hypoxanthine and guanine to either inosine and guanosine if ribose-l-phosphate is the substrate or to deoxyinosine and deoxyguanosine if deoxyribose-1-phosphate is the substrate. Uridine phosphorylase converts uracil to uridine in the presence of ribose-l-phosphate, and thymidine is formed from thymine and deoxyribose-l-phosphate through the action of thymidine phosphorylase. 

F8. Friedkin, M., Desoxyribose-l-phosphate. II. The isolation of crystalline desoxy-ribose-l-phosphate. J. Biol. Cherti. 184, 449-459 (1950). 

X-ray crystal structures of the beef purine nucleoside phosphorylase enzyme with a substrate-analogue complex (inosine and the non-nucleophilic sulfate), a transition state-analogue complex (immucillin H and phosphate) and a product complex (inosine and ribose-l-phosphate) led to the conclusion that most atomic motion was in the anomeric centre, which swung from attachment to... 

Purine nucleoside + phosphate = purine + alpha-D-ribose l -phosphate. 

Purine nucleoside phosphorylase (PNP). In contrast to lU-NH, PNP appears to use extensive contacts with the purine ring to promote catalysis and relatively few contacts with the ribosyl ring. The crystal structure of PNP has been determined in a complex with an iminosugar inhibitor, immucillin H, which was developed based on TS analyses of PNP-catalyzed hydrolysis and arsenolysis reactions. TS analysis revealed that the enzyme catalyzes a dissociative A Dn mechanism. The crystal structure was compared with the structures determined by Ealick and coworkers ° of PNP in a Michaelis complex analogue, PNP-inosine-sulfate, and a product complex, PNP-a-D-ribose-l-phosphate-hypoxanthine. 

Phosphorylases (see Figure 22.2), which use phosphate to separate the base from ribose, forming free bases and ribose-l-phosphate. 

Ribose-l-phosphate is a product of catabolism of nucleosides by phosphorylases (Figure 22.2). An example reaction is shown here and as follows ... 

Note that the reaction (shown here) is reversible, providing a way for a cell to rebuild a nucleoside from ribose-l-phosphate and a base. Some cells have enzymes called nucleoside kinases which, in the presence of ATP, convert the nucleoside to a nucleotide. 

Much of the motivation for the study of XO electrochemistry is the development of amperometric biosensors. Commercially available bovine XO is invariably the enzyme of choice used in these applications. High levels of hypoxanthine are linked with asphyxia in newborns, SIDS and hypoxia in general. Coupled with other enzymes, such as purine nucleoside phos-phorylase, XO can be used to determine phosphate coneentrations in clinical, food and waste samples. Purine nueleoside phosphorylase catalyzes the phosphorylation of inosine liberating hypoxanthine and ribose-l-phosphate (Scheme 5.5). In the presence of XO, hypoxanthine is oxidized to xanthine and produces one equivalent of H2O2. Thus due to the stoichiometry of the reaction one equivalent of H2O2 is produced for every phosphate anion present. ... 

It is noteworthy that for the Lesch-Nyhan syndrome fibroblasts, the assay in the presence of TTP furnished the bulk of the radioactivity in the nucleoside fraction rather than in the nucleotide fraction, in contrast to normal cell extracts in which the bulk of the radioactivity is found in the nucleotide fraction. Two possible explanations may be entertained to explain this observation. First, in the HGPRT-deficient system the nucleotidase activity is far in excess over the HGPRT activity, and presumably the residual nucleotidase activity, not inhibited by TTP, is sufficient to degrade the relatively small amount of nucleotide formed. Secondly, the possibility should be considered that the accumulation of radioactivity in the nucleoside fraction reflects the anabolic activity of nucleoside phospho-rylase reacting the purine base with ribose-l-phosphate to form the nucleotide by an alternative pathway. However, this latter explanation seems to be invalid in view of the absence of the suitable substrate, ribose-l-phosphate, from the incubation system, and by the linearity... 

Scheme 14.13. A representation of a salvage reaction in which a purine (adenine, A) undergoes reaction with a-D-ribose-l-phosphate in what is represented as an SN2-type process but which almost certainly involves participation by the ring oxygen as well as purine nucleoside phosphorylase (EC 2.4.2.1). NB The name of the enzyme as a phosphorylase refers to its potential to catalyze the reverse of the reaction shown.

Pyridine Nucleotides. Nucleoside phosphorylase is capable of using the base nicotinamide. The product of the reaction of this base with ribose-l-phosphate is nicotinamide riboside. The formation of the corresponding nicotinamide mononucleotide is catalyzed by a typical kinase, using ATP. A specific enzyme purified from human erythrocytes has been shown to form nicotinamide mononucleotide by a second mechanism in which PRPP and nicotinamide react to form inorganic pyrophosphate and... 

Variation in Purine Synthesis. An important development in purine biochemistry came from studies on sulfonamide inhibition of bacteria. A compound was found to accumulate in cultures of E. colt incubated with sulfadiazine. This was identified as 4-aminoimidazole-5-car-boxamide. In the present concept, this compound represents a degradation product of the corresponding ribotide, but it can also be used as a purine precursor. The free imidazole reacts with ribose-l-phosphate in the presence of nucleoside phosphorylase to form a nucleoside which can be phosphorylated by a kinase. Thus, variations of the basic scheme can and do occur. 

It was further found that inosine was not an intermediate in the conversion of hypoxanthine to inosinic acid, but that hypoxanthine must have reacted directly with some ribose phosphate derivative to form inosinic acid. The inhibitory effect of purified nucleoside phosphorylase may be offered as further evidence that inosine was not involved as an intermediate in the conversion of hypoxanthine to inosinic acid. The nature of the ribose phosphate compound which reacted with hypoxanthine to yield inosinic acid is unknown at present. The nucleoside phosphorylase reaction, hypoxanthine plus ribose-l-phosphate to form inosine and inorganic phosphate, suggests that hypoxanthine may react with ribose-l,5-diphosphate to yield inosinic acid and inorganic phosphate in an analogous fashion. Klenow and Larsen have recently reported the isolation of ribose-l,5-diphosphate, which might conceivably be formed in the above system by phosphorylation of ribose-l-phosphate by ATP. 

Ribose-l-phosphate -f- hypoxanthine ribose-l-hypoxanthine -f phosphate... 

Starting with equimolar amounts of ribose-l-phosphate and hypoxanthine, the equilibrium mixture consists of 80% ribose-l-hypoxanthine (inosine) and 20% ribose-l-phosphate. 

The reaction is essentially an exchange of hypoxanthine for phosphate, and is analogous to the phosphorolysis of glycogen and sucrose. For this reason, the enzyme has been termed nucleoside phosphorylase. It can also catalyze an exchange between ribose-l-phosphate and inorganic phosphate. This was shown by incubating ribose-l-phosphate with P in the presence of the enzyme and subsequently isolating radioactive ribose-l-phosphate. With arsenate, an irreversible arsenolysis of the nucleoside results. 

The furanose form of D-ribose is required for the enzymatic phosphorolysis reaction. Pyranose-ribose-l-phosphate will not react with hypoxanthine to liberate inorganic phosphate. On the other hand, desoxyribose nucleosides are quite reactive ... 

Desoxyribose-l-phosphate has been obtained as the crystalline cyclo-hexylamine salt. It is even more acid-labile than ribose-l-phosphate. Thus, 50% of the phosphorus is released as inorganic phosphate within 10 to 15 minutes upon hydrolysis at pH 4, at 23°C. The equilibrium favors synthesis. It is felt that the same enzyme catalyzes the phosphorolysis of hypoxanthine riboside and hypoxanthine desoxyriboside, because the Michaelis-Menten constants are nearly identical and there is no summation of rates with both substrates present at optimal concentrations. Furthermore, nucleoside phosphorylase purified from horse liver is active with both nucleosides and desoxynucleosides. ... 

Inosine (or guanosine) - - phosphate ribose-l-phosphate + hypoxanthine... 

Nicotinamide can be formed by animal cells from tryptophan but is usually present in the diet. Starting from ribose-l-phosphate and niacin (nicotinamide), DPN is probably synthesized as shown in Fig. 76. TPN results from the phosphorylation of DPN in the presence of a specific phosphokinase. TPN can be converted to DPN by a phosphatase. 

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